All of us Makers like to tinker with stuff, and in this process, we may find ourselves thinking about how to connect device A to my Arduino… Device A may operate at a different voltage from the Arduino, and may thus damage it badly….
Many different solutions exist to do this, but, many of them, like relays, can be quite bulky, increasing the overall size of your project, as well as putting bigger demands onto your power supply unit.
Having worked in the Industrial Automation sector for a few years, I remember that we used to have dedicated hardware to protect our sensitive controllers from the harsh outside signals that we needed to monitor. These devices were called isolators, and today I will show you how to construct your own version of this essential device.
But some theory is needed first…
What does it mean to isolate a signal? In the electronics world, you might have seen that you usually have to use a common ground between all your devices to make them work together properly. While this is definitely true, let us look at another example…
Let us say you have some device, that will send you a voltage signal when it switches on, and another voltage signal when it is switched off. This device runs on 24 volts, so some of the more informed of us will immediately say you need a level converter, meaning a device that changes the 24v signal into a 5v signal… Others will try to use a relay to convert the signal ( A relay is also a type of isolation device ). A much more elegant way of doing this will be by using an Optic Isolator chip.
A simple Optic Isolator Chip
This chip provides complete isolation between your device and the Arduino or other microprocessor. It does that by using infrared light to transmit the signal. Light, as we all know, does not conduct electricity 🙂
Whereas a relay will only give you a on or an off state, the Opto-coupler or Optic Isolator can also do linear current transfer, meaning that the more IR light it transmits, the more current the photo-transistor will allow to pass as well.
A good tutorial on Opto-Couplers can be found here
Opto Isolator Circuit
In my circuit, I made use of the following circuit…
Two Optic Isolator Level converter Circuits
As we can see in the two circuits above, there is no common ground between the input and output sides of the circuit. This is ideal, as noise and other undesirable signals will not be transferred from one circuit to the other. It also allows you to use a very high input voltage, at a frequency of up to 2kHz.
I have also decided to combine this with the PCF8574 I2C Port Extender. That way, I can cascade up to 64 inputs on the I2C bus. In a later version, I will also do an Opto-Isolated Output module.
The Shield is only slightly bigger than the standard Arduino Uno, and all Arduino pins are broken out on headers. It is important to remember that A4 and A5 should not be used for any other purpose (They provide access to the I2C bus). Likewise, the interrupt pin of the PCF8574 can be connected to either D2 or D3 with a jumper, or left disconnected by completely removing the jumper. Device addressing can be set with the 3-way DIP switch on the board.
8 DI Optically Isolated I2C Arduino Shield
This device is currently being manufactured. In Part 2 of this article, I will show you the completed PCB, as well as give you access to the Gerber design files if you want to manufacture your own. I will also make a limited amount of these boards available for sale from my website ( this site ) as well as from https://www.facebook.com/makeriot2020
Every single project that we build needs an On/Off switch. Hardware switches are big and clumsy and can be quite expensive. Most of them also don’t look very good.
This post will be about an experiment that I recently performed, building a reliable soft start circuit, or latching circuit.
Full disclosure: I did not design this circuit. Full credit to the designer, who shall be mentioned later.
As such, This shall be about my experiences with this great little circuit, and also to show off the neat little prototype PCB that I designed and had manufactured for this experiment. It only took a few minutes to design, and even less to assemble.
To answer this, let us look at what components are in the circuit first. Q1 is a P-Channel Mosfet. I chose the SI2301, because that is what I had lying around. T1 and T2 are NPN BJT transistors, I chose S8050’s here, and also tested 2n2222a’s with great success. Other components are 3 100k pullup resistors, 1 1M pullup and an 1k dropper resistor for the indicator LED.
In the off state, R1 (100k) keeps the gate of Q1 tied to the supply voltage, thus keeping the MOSFET switched off. T1’s collector, also connected to the MOSFET Gate, are thus pulled High, with the Base of T1, although pulled High, connected to the Drain of Q1.
Q1 is still off, so that base will be low, for now at least.
The collector of T2, is pulled up to the supply voltage via R4, and also, through the push button, to the base of T1.
The base of T2, is pulled up via a 1M resistor to the drain of Q1, and a 22uf to 47uf electrolytic capacitor to ground.
The emitters of both T1 and T2 are grounded.
When you press the switch, the base of T1 goes high, and this switches T1 on, pulling the gate of Q1 to ground, switching it on. This now pulls the base of T1 high through R2, latching it on, and keeping Q1 switched on. This also pulls the base of T2 high through R3. As T2 is now also switched on, the junction between the collector of T2 and the switch is now for all purposes a ground.
Now, when you push the switch again, the base of T1 is pulled down to ground via T2. This switched off Q1. As the capacitor on the base of T2 can now discharge, the circuit is reset, and the next press of the switch will turn it on again.
How big a load can be switched?
Obviously this depends on the rating of the MOSFET at Q1. In my experiments, a load of about 150mA was easily controlled, but more than that, started to somehow drain the cap at C1 too fast, and the circuit would reset.
This is not a problem to me, as the circuit is ideal to switch a relay, which in return can switch the main load.
What other issues did I encounter?
The SI2301 seems to have a small amount of leakage, which allowed a few millivolts to activate T1 or T2. I solved that with the addition of a 10k pulldown to ground, on the base of T1.
This does not seem to be the case with other MOSFETS, and I believe changing the value of R1 to provide a stronger pullup, might solve this issue.
Where does the circuit come from?
I found the circuit, and explanation on the excellent Youtube channel EEVBlog. The owner, Dave, does an excellent job of explaining how this works. All credit to Dave for this excellent circuit!
It has zero current consumption in the off-state, and very little when on.
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
Conclusion
I believe this is a great little circuit to keep around, with many excellent use scenarios. It does need a bit of fiddling to get to work perfectly every time ( as you can use many different P Channel Mosfets and NPN BJT transistors)
It provides an excellent alternative to a mechanical switch, and for my own use, the PCB Module is small enough to be mounted into an enclosure with another project with ease.
As a companion module to my recently published 8Ch PMOD breakout board, I decided to do a similar PCB, but with NMOS devices instead. This opens up more possibilities for proper testing and prototyping, as PMOS and NMOS devices has different use applications, and most importantly, can sometimes even be combined for a particular purpose, like an H-Bridge motor driver, for example.
8Ch NMOS Breakout
What is on the PCB?
As NMOS devices function quite differently from their PMOS counterparts, it did not make sense to reuse the PMOS board, and just change the devices… although some people may be tempted to think you could…
The N Channel Mosfet basically “works in mirrored mode” from a P Channel one, and is used to do so-called ” LOW Side switching” which means that your load connects to the positive power rail, and then to the DRAIN pin of the MOSFET, with the source being connected to ground… ( It can sometimes also be used the other way around… but lets not go there now….
The current prototype PCB contains 8 BSS138 NMOS Mosfets, in my case, with is capable of about 800mA of current… All source pins are internally connected to ground. This forces you to use this module as a low side switch…
Two 10-way 2.54mm headers are provided, with a ground pin on Pin 1 and 10 of each of these.
The Drain pins of each NMOS device is available on the top header, labeled D1 through D8, and the Gate pins of each respective NMOS device is available on the bottom header, labelled G1 through G8.
Each gate has a pull-down resistor to ground, to keep it from flapping around, as well as a gate resistor. In my case, I selected to use a 10k pulldown, and a 1k gate resistor, as that is sufficient for my general needs…
Each NMOS device also has a LED signal indicator, to assist in visual confirmation of a specific channel’s state.
PCB Top Side
The Schematic
Schematic
Using the breakout
The module is very easy to use, and as briefly mentioned above, you are only required to connect one side of your load to the positive supply rail, and the other side to the drain pin of your choice.
Connect the ground pins of the module to your ground rail.
The Gate pin, with a corresponding number to the drain you have selected, can now be connected to your GPIO of choice on a microcontroller.
Drive the pin High to switch on the load, drive it log to switch off. Easy.
Please note: While the NMOS devices used on the board can handle quite a lot of current, (800mA in the case of the BSS138), it is not recommended to try and pull too much current through a single channel. The PCB traces can safely handle about a maximum of 300 to 400mA per channel.
PCB Bottom
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
Example code for using the breakout (Arduino)
// Example code for 8Ch NMOS breakout
int Gate1 9;
int Gate2 10;
void setup() {
// drive the two gate pins low to ensure NMOS devices
// are in a positively known state at startup
digitalWrite(Gate1,LOW);
digitalWrite(Gate2,LOW);
// Set gpio to output mode
pinMode(Gate1,OUTPUT);
pinMode(Gate2,OUTPUT);
}
void loop() {
// Toggle the two channels in an alternating pattern
digitalWrite(Gate1,!digitalRead(Gate1));
digitalWrite(Gate2,!digitalRead(Gate1));
delay(1000);
}
While prototyping our projects, we Makers often need to interface devices with a higher current draw, like motors, or RGB lights, to our microcontrollers. These typically are unsuitable for connecting directly to an Arduino, ESP32 or Raspberry Pi’s GPIO pins. This is usually the time when we start grabbing transistors or MOSFETs.
While I normally keep a few leaded transistors and MOSFETs in the lab, These are not always convenient to use, as they may be in big packages or have the wrong specifications for the task that we are trying to perform.
SMD versions are more common in my lab, but they come with the problem of being small, and also completely unfriendly to the breadboard environment.
I have thus been playing with an idea to make a series of dedicated breakout boards for just this purpose. Having an easy way to test a specific MOSFET for a design, and having more than one of them handy, without all the wiring issues, and using the bare minimum of those DuPont wires!
I came up with the following prototype, which, while not completely optimised yet, already makes things easier. The breakout board provides 8 P-Channel Mosfets, with a single source connection, and individually broken-out Drain and Gate pins.
LED indicators on each channel provide a visual indication of the status of each P-Mos device, and the breakout can also be mounted directly into an enclosure if needed.
What is on the PCB?
Each channel comprises a P-Channel Mosfet, in this case, a SI2301, which has a suitably low gate voltage, a pullup resistor on the gate, to keep it from floating, a status-indicating LED and a current-limiting resistor for the LED.
No gate resistor was added, as this would change depending on the actual MOSFET, as well as the microcontroller that you use. The Gate pullup resistor can also be left unpopulated, in case you need to do something specific there.
Two rows of 10-way, 2.54 header pins are at the top and bottom of the PCB, to make using the breakout on a breadboard possible.
The Pinouts are as follows
H2 – Top V+ D1 D2 D3 D4 D5 D6 D7 D8 GND with Dx corresponding to the Drain pin of each MOSFET. All the Source pins are internally connected together, as I assumed that I will use the same source voltage on each channel anyway.
H1 – Bottom V+ G1 G2 G3 G4 G5 G6 G7 G8 GND with Gx corresponding to the gate pin of each MOSFET.
V+ and GND for each header is internally connected, to make it possible to supply V+ and Gnd on any of the two headers.
PCB Top Layer
The Schematic
Schematic
Using the Breakout
Using the breakout is straightforward. Connect a source voltage to either of the V+ pins and Ground to either of the GND pins. ( the ground is used internally for the status LEDs)
Connect your load, with the positive to a drain pin, let us say D1, and the load ground to your breadboard, or power supply ground. Connect the corresponding gate pin, in our case G1, to the microcontroller pin of your choice, through a suitable gate resistor, and pull it high at setup, to ensure that the MOSFET stays off. Pull low to activate as needed.
Please note that you should not try to switch excessively large currents through a single MOSFET Channel, as the PCB traces can realistically only handle approximately 300 to 400mA per channel.
Note 2: If you are driving an inductive load, it is considered good practice to add a flywheel diode on the load as well. This will protect the MOSFET from back EMF when the MOSFET is switched off.
PCB Back
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
Example code for using the breakout (Arduino)
// Declare Gate driving GPIO pins
int gate1 = 10;
int gate2 = 11;
void setup() {
// Set the GPIO pins as outputs and drive them HIGH
// This keeps the channels switched "OFF"
digitalWrite(gate1,HIGH);
digitalWrite(gate2,HIGH);
pinMode(gate1,OUTPUT);
pinMode(gate2,OUTPUT);
// Writing to the GPIO's before setting their pin Mode,ensures that the
// GPIO's are in fact initiated in a know correct state.
Serial.begin(115200);
}
void loop() {
// In the loop, we just toggle the GPIOs, thus
// alternatively switching the channels on or off
digitalWrite(gate1,!digitalRead(gate1));
digitalWrite(gate2,!digitalRead(gate1));
delay(1000);
}
In a previous post, I designed a breadboard-friendly MCP23017 breakout module. A few months have passed, and after using the module for a while, some issues came to light…
In this post, I will show you how I have fixed those issues, and then I can continue testing/using the new generation prototype, and hopefully, it can become the final revision of this project.
Old Versus New
Let us start by looking at the old and new PCB designs…
Old style MCP23017 Breakout – Top viewNew style MCP23017 Breakout – Top view
There will not be a lot of obvious differences at first, but if we look closely, here are the changes: – In the old version, due to the size of the SOIC28 footprint, I had to place the bypass capacitors, as well as I2C pullup resistors on the bottom layer of the PCB.
The new design, using the more readily available ( at least where I live) SSOP28 footprint, leaves enough space for these components on the top layer, thus resulting in a mostly single-layer layout, with only a few tracks on the bottom layer.
I2C pull-up resistors can now be controlled by a jumper, enabling or disabling them completely… This helps when adding a few devices to the I2C bus, and rather having the pull-up’s close to the MCU ( as is generally recommended anyway )
Other cosmetic changes involve the separation of the data ports (A and B) from the interrupts, reset, Vcc and ground pins. On initial testing of this on a breadboard, it makes using the device a bit easier, and access to the io pins faster. ( In my biased opinion anyway )
MCP23017 Breakout – New version
Pinouts and connections
I have tried to make all the connections easy to find and use, with the IO ports ( A and B) on opposite sides of the breakout, Numbered A7 to A0 on the top, and B7 to B0 on the bottom. VCC, GND, SCL and SDA are on a separate 4-way header pin, with the two interrupt pins (I-A and I-B) together with the reset (RST) pin on a 3-way header opposite the power and signal header..
Addressing pins are in the centre of the PCB, marked with a 2 1 0 ( for AD2, AD1, and AD0 respectively), Jumpers to the bottom ( towards port B) pull the pins to ground, where the opposite side will pull the address pins high.
To the right of that, another 3-way jumper enables or disables the I2C pull-resistors on the module, which in this case is set at 4k7.
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
Some More Pictures
Bottom LayerTop LayerTop LayerPopulated PCB3D Render of PCBOld style breakout, for reference
In a recent post, I looked at a single-channel version of this module. While it may be a repetitive post, I will continue, as it shows how easy it is to double up on this circuit to provide more than one latching switch on a single circuit board. The current drawn by this module is so little, even when energised, that it compares favourably with even a microprocessor-controlled solution.
The real advantage will obviously be the cost, as the hand full of discrete components needed for this is way cheaper than a microprocessor alone, and the fact that it doesn’t need any coding makes for an attractive solution.
It is however worth noting that the circuit is quite sensitive to external interference, sometimes resulting in unwanted operation. This does not concern me too much, as 1) This is still a prototype and 2) While it does work as intended, and surely is quite useful, I do not intend using it to switch any high current load, or control any expensive or important equipment.
Since the previous post looked at the base circuit in detail already, I think it will be a good idea to talk a bit about electrical isolation, tracking and keeping the AC and DC sides of a circuit separated completely
In the picture above, we can clearly see that the DC side (near my hand, at the top is contained completely on the right side (top in this case) of the PCB. The hashed copper pour also stops clear of the two relays. There are only four tracks going to the relay coils, and they are all on the same layer of the PCB.
Also note the square cut-out slot around the common terminal of each of the relays. This provides additional isolation to the relay, as well as the DC side of the circuit, as air is a very good insulator ( at least for 220v at no more than 10A — or so I was taught …) These cutouts will prevent any mains voltage of tracking, think burning towards, towards any other tracks in this area.
The entire left-side top layer ( underneath the relays) are also completely free of copper, to make tracking even more difficult.
If we now look at the bottom layer of this same PCB, we will see that the DC side and its ground-plane are once again completely separated from the relay contact terminals. Also note that the tracks connecting the screw-type connector and the relay terminals are very thick (100mil), straight and as short as possible. All copper around these tracks has also been etched away, further reducing the chances of tracking.
In a production PCB, Warning labels would also be present in the bottom silkscreen of the PCB in this area, warning the user of the possibility of mains voltage in this area. As this is a prototype, and to make the above-mentioned points easier to see, I have not added these labelling on these boards.
Important Disclaimer: Electricity is NEVER “SAFE”. There are only safe practices and procedures. It is always the responsibility of the user to ensure their own safety. While the design shown above is considered “SAFE” by myself, I only consider it “SAFE” because I am aware of the risks involved in using such a circuit to switch mains voltage, at a certain current, and under a specific use scenario. DO NOT BE FOOLED into simply replicating this circuit, or parts of it, and believing it is “SAFE”. Every use case of a circuit is different, and the devices connected and controlled by it will always differ. Make sure that you ACTUALLY know what you are doing BEFORE using any High voltage/Current and switching it with any electronic device.
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
The humble 555 timer IC has been around for a very long time. It can be configured to do a lot of timer based functions, the most common know being to flash LED’s at a given frequency.
A slightly more unknown function of this versatile chip is the capability to be configured as a latching switch, -meaning a press on press off switch-.
In this short two part series, I will show two such latching switch modules that I have designed around the 555 timer. In the first part, we will look as a single latching switch, with an attached relay output to switch higher current and voltage loads safely.
The PCB
Latching switch Prototype
With only 11 components ( excluding the relay and connectors or course) this is an extremely easy and cheap circuit to build. It can also quite easily be built on a breadboard, or strip board, if you do not want to use a custom PCB.
Schematic
Operation of the Circuit
The operation of this circuit is quite easy. The PCB is powered by a 5v supply, in this case, but the 555 can allow for a supply voltage of up to 15v DC ( Please note that the Relay needs to be capable of accepting the input voltage without damaging its coil… you would thus have to select a suitable model)
When you press and release the push button, pin 3 of the 555 will go high, lighting the indicator LED, as well as pulling the gate of the BSS138 Mosfet High, allowing current to flow through the relay coil, thus energising the contacts.
The relay will stay energised until you press and release the button again, or power is removed from the circuit.
Possible uses
This type of circuit has many uses, like switching a light on and off with a single press. It is obviously cheaper and easier to just use a toggle switch, but it is also interesting to explore the possibilities of a discrete component solution, without a microprocessor, to achieve a result similar to that of a toggle or rocker switch.
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
`Ai-Thinker (#notsponsored) should be quite well known to many makers as a company that manufactured and designs many of the modules that we use in our projects. We, MakerIoT2020, definitely make use of quite a few of their products, like the RA-02, as well as their ESP32-S module.
A few months ago, I got the opportunity to play with one of their newest projects, the AI-WB2, which is based on the BL602 Risc-V Chip. After a very very bumpy ride, mainly due to the chip being quite new, and documentation being virtually nonexistent in the English language, I decided to take a step back, and stop trying to reinvent the wheel 🙂 Afterall, I don’t want to use Apache NuttX or a similar RTOS for every project, as the thought of having to write almost all of the different required components from scratch, does not really appeal to me. especially as the SDK is in Chinese, and the English version of it is a bit patchy, to say the least…
This made me quite a bit frustrated, at least until I decided to change my thinking, and take a look at the stock AT command set that comes shipped on the modules from the factory… While excellent for use as a WiFi modem, it did not seem to allow any access to any of the GPIO on the WB-AI2 module… But wait… is that really a problem? No… Let me tell you why…
A few very quick tests later, It was clear that the AI-WB2 will be a very compact WiFi as well as BTLE connectivity solution for these XIAO modules, and, If I design with the future in mind, the GPIO pins of the AI-WB2 module can also become useable to me as well… once the firmware and SDK gets more accessible..
What followed from this is a very basic prototype PCB, with the XIAO RP2040 as the main processor, and the AI-WB2-12F as a “connectivity co-processor”, meaning that all communications functions will be offloaded to the AI-WB2 and the results of those, sent back to the XIAO for processing…
This in itself presents quite a few challenges, especially on the communications handling, and using the second UART port, which is currently not possible with the official Arduino Core for the RP2040… Luckily, the XIAO RP2040 uses an alternative core, that supports the second UART port quite well …
What is on the PCB?
AI-WB2-12F XIAO Combo
The Top Section of the PCB is dedicated to the AI-WB2-12F and its supporting components, including a flash and reset button. The GPIO for the WB2-12F is broken out onto H1.
At the right, below H1, is a series of jumpers, connecting the Xiao RP2040 and WB2-12-F Uart ports, or, alternatively, connecting the XIAO Rp2040 to the pin headers at the side of the PCB.
The rest of the PCB is dedicated to the Xiao RP2040 or Xiao SAMD21 module, with its supporting circuitry, and a dedicated Reset button for the SAMD21 module ( also works for the RP2040)
The board is powered with 5v DC through a dedicated header at the left bottom. This directly powers the Xiao and indirectly powers the WB2-12-F through a 3.3v LDO Regulator. Please note that although the Xiao is powered via 5v, the GPIO pins are all 3.3v logic!
For the Xiao RP2040, like I used, it is possible to use the second UART to connect to the AI-WB2 chip.
As I am still not completely done with my development, I will not release the full code at this moment.
I have also been informed by AI-Thinker that a new version of the AT-command firmware is available that will allow using the GPIO on the AI-WB2 via AT Commands. I am currently investigating that new version, and that is also a big reason for not releasing any code at this stage.
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
These days, Makers have access to quite a few different microprocessors for use in their projects. Most of them can be found on development boards of some sort, but not all are in a convenient size. The reason for this is that most of these development boards were designed with a breadboard in mind, and then after your prototype is done, you are required to design a PCB and place the specific processor and its supporting components onto this custom PCB…
Many makers choose to skip this step, either choosing to keep the project on the breadboard, or place the entire development module onto a piece of stripboard or similar, and then place their supporting components and sensors around that.
This is where the SEEED XIAO is different. It comes in a thumb-nail-sized package and can be used on the breadboard or directly placed onto a PCB via either pin headers or if you want access to all of its features, SMD pads.
I have also included a small prototype area on the PCB, so that Makers can easily transfer their existing XIAO projects onto a semi-custom PCB, without having to design their own.
I have also addressed a problem area, especially with the XIAO SAMD21, it has no onboard reset button, only two tiny pads, by including a reset button for ease of use.
The PCB is in Arduino Uno form factor, and also provides headers to power it from an external 5v DC supply. Please note that the prototyping area has a 3.3v power rail, – due to the fact that all of the XIAO GPIO are limited to 3.3v anyway -. This power rail is powered directly from the XIAO 3.3v output, and the current is limited as per the specifications of the XIAO module that you are using.
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
In answer to quite a few requests for a prototype shield, similar to my ESP32-S Dev Prototype shield, but for use with the ESP-12E DEV board, I have decided to do a quick design, and make it available publicly
This is the MakerIOT2020 ESP12E-DEV Prototype Shield. It is similar in purpose to the above-mentioned ESP32-S Dev Prototype shield, but I have also added some additional cosmetic changes to make it a little easier to use as well.
With many of my prototype designs, I tend to sometimes leave out something, as I usually use it for my own purposes only, but with this design, as many people specifically asked for it, I took a bit more care, as it is no longer just a prototype, right?
What has changed?
The most obvious is the increased prototyping area. The initial ESP32-S version had a 60-hole breadboard-style prototyping area. The new design has 128 prototype holes.
There is also a dedicated power input header, something that I somehow left out on the ESP32-S version… The Flash and Reset push-buttons were also moved inline, and to the bottom of the shield, making it more comfortable to use.
The design retains the plated through-hole design on the prototype area with connecting tracks on both sides of the PCB to allow for a bit more current.
The big ground plane on both sides of the PCB has also been retained.
PCB Design and Schematic
Top Layer LayoutBottom Layer Layout
The prototype shield is for all purposes a breadboard. I did thus not bother with a formal schematic. I believe that it is easy enough to understand the connections by just looking at the two images above.
Manufacturing
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
This post will look at my prototype Analog front-end for the Scoppy RP2040 Oscilloscope. It is important to state right from the beginning that this circuit is one of the 5 recommended designs from the Scoppy Website. I have only moved it from the breadboard design as published, onto a PCB.
The entire circuit, with all of the original designer’s writeups, is available here
So, why use someone else’s circuit? Well, the reason for this is two-fold. 1) The circuit designer also designed the firmware, so it stands to reason that his circuit will be optimised for use with the firmware. 2) Using his circuit provides a solid reference, making it possible to test the firmware for correct operation, and later on, providing a base for my own design – if and when I do decide it is worthwhile to actually design my own.
As I already have a proper oscilloscope as well as a logic analyser, this entire exercise is purely academic, I find the Scoppy project interesting, and as such, I would like to see how it compares with my commercial products ( while also knowing that it won’t be a very fair comparison ).
With all the limitations, I am however still quite impressed at the level of use that you can get out of this very simple device. It is definitely quite useful for a beginner.
What is on the PCB, and what did I change?
The PCB is a dual-layer shield that is designed to be used with the MakerIOT2020 Raspberry Pi Pico Carrier board. The shield is directly powered by the carrier board.
The original Analog Front-End #3 circuit featured a single channel input, capable of accepting a -18.0v to 18.0v signal input.
My changes were limited to doubling up on that circuit, to provide two channels.
I have redrawn the original schematic, partly to make it easier to understand for myself, as well as to help me with the design of the PCB.
Lets take a look at the schematic ( by using text from the original designer) The author makes no warranty, representation or guarantees regarding the suitability of this design for any particular purpose. Nor does the author assume any liability arising out its use and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
This design builds on Design 2 and adds over and under voltage protection to the analog front end. After all, we won’t have a cheap oscilloscope if we keep frying our components!
I’m assuming here that the minimum and maximum voltages that will be applied to the input of the scope will be -18V and +18V respectively. It has been tested from -18.5V to 18.5V (two of my 9V batteries in series) but of course If you decide to use this design you are doing so at your own risk. I personally wouldn’t use Scoppy with an expensive phone/tablet just in case something unexpected goes wrong (better to use an old, obsolete phone that is no longer used for anything else) – especially when dealing with higher voltages – but of course you can do what you like.
Protecting the Op-Amp input(s)
First of all we need to protect the op-amp. In this design we’ll be using an LM324 op-amp, which is very similar to the LM358 but contains four individual op-amps rather than two. We’ll be using three of these op-amps. The reason for this will be explained later.
According to the datasheet for the LM324 the allowed input voltage range goes from -0.3V to 32V. Of course 32V is above the maximum expected voltage (18V) and so we don’t need to worry about over-voltage protection. However we do need to ensure that the voltage at the input pins don’t go below -0.3V. A schottky diode can be used to clamp the voltage to something above -0.3V (D1 in the schematic).
One thing that needs to be considered when selecting the diode is its reverse current. The 1N5817 has a very low forward voltage but high reverse current and this results in a voltage drop at the input of the op-amp (in the order of 100mV). Presumably this is because it draws current through the high value input resistor (Rg1). The 1N5711 has a much lower reverse current specification and I couldn’t discern any voltage drop when this was inserted into the circuit. However, its forward voltage (at the current expected in this part of the circuit) is very close to the minimum allowed voltage of -0.3V. To be safer I prefer to use something like a BAT46. It does result in a voltage drop of a few millivolts but the clamped voltage is more like -0.23V.
Protecting the Pico/RP2040
The Pico datasheet states that:
the ADC capable GPIO26-29 have an internal reverse diode to the VDDIO (3V3) rail and so the input voltage must not exceed VDDIO plus about 300mV
The obvious way to protect the ADC inputs (GPIO26-29) then is to simply insert a schottky diode between the ADC input and VDDIO. However, the RP2040 datasheet says that:
the voltage on the ADC analogue inputs must not exceed IOVDD ...<snip>... Voltages greater than IOVDD will result in leakage currents through the ESD protection diodes
That suggests to me that we shouldn’t be allowing current to pass (leak) through our clamping diode and into IOVDD. I could be completely wrong here – if you think so then please share your thoughts in the forum (Discussions).
Anyway, to be safe we’re going to avoid that situation by sending the current to the output of one of our op-amps (LM324-sink on the schematic). The LM324 is able to sink up to ~10mA so this should work fine if we limit the current from the main op-amp (LM324-amp in the schematic). Given that the maximum voltage expected at the output of LM324-amp is around 4.5V (Vcc – 1V) then we need a resistor of at least 120R to limit the current to 10mA (4.5-3.3 / 0.010 = 120). A 220R resistor should do fine (Rout).
And of course the reason we are using an LM324 rather than the LM358 of the previous designs is that three op-amps are required.
A 1N5817 diode (D2) is used here (rather than a BAT46 – used on the input of LM354-amp) because at the expected maximum current of 10mA the forward voltage drop of the BAT46 is higher than 300mV. The high reverse current of the 1N5817 is not such an issue here because Rout has a low value and so there will only be a small voltage drop across Rout when D2 is reverse biased.
Construction
Here are some instructions for assembling this front end on a breadboard. The pin numbers refer to the LM324 PDIP package. Refer to the schematic and breadboard image above. NB. The rail labelled 5V on the schematic is actually VSYS which of course is not necessarily 5V because it depends on how charged the battery is on your Android device.
Connect 3V3 of the Pico to the top red power rail of the breadboard. Connect VSYS to the bottom red power rail. Connect both ground rails of the breadboard to one of the GND pins of the Pico. The fuse as shown in the breadboard image is optional.
Connect the Vcc pin of the LM324 to the VSYS rail. Connect the GND pin of the LM324 to the GND rail. Don’t connect anything to the ADC pin(s) of the Pico yet.
Now we’ll configure each of the 4 op-amps of the LM324 in turn.
The voltage at the non-inverting input should be approximately VSYS/2 and the output should be the same.
Op-amp 4 – Vref Wire up this op-amp as shown in the schematic. The voltage at the output should be approximately 1.65V.
Op-amp 3 – sink Wire up this op-amp as shown in the schematic. The voltage at the output should be 3.3V.
Op-amp 1 – amp Wire up this op-amp as shown in the schematic, including the under-voltage protection diode on the input (D1) and the current limiting resistor (Rout) and over-voltage protection diode (D2) on the output. Don’t connect the output to the Pico yet.
Testing and initial operation
You should now be able to safely apply any voltage at Vin1/Vin2 of between -18V and +18V. Test that the voltage at the input of the LM324 (Vampin) doesn’t go below -.3V and the voltage at the output of op-amp 2 after Rout (Vadc) doesn’t go above 3.6V.
Once you’ve confirmed that all of the op-amps have been wired correctly you can connect the output of Rout to the ADC pin of the Pico.
The author makes no warranty, representation or guarantees regarding the suitability of this design for any particular purpose. Nor does the author assume any liability arising out its use and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
Designing the PCB
PCB Design and Layout
Manufacturing the PCB
The PCB for this project has been manufactured at PCBWay. Please consider supporting them if you would like your own copy of this PCB, or if you have any PCB of your own that you need to have manufactured.
Assembly
The PCB was assembled with help of a stencil to ease and speed up the solder paste application. The components were then hot-air soldered. As this is only a prototype, I chose to only place 2.54mm header pins on pins that are required for operation, as well as all the ground pins, to ensure a proper ground plane.
SMD Stencil to speed up assemblyAfter AssemblyAFE with Raspberry Pi Pico Carrier boardTop viewStacked, side viewStacked, top front viewStackedBack view
Conclusion
This was quite an interesting project. While everything works as expected, resolution and frequency are limited. ( of course, it is…) The project is however still useful, and will definitely give you some useful results in a pinch.
The logic analyser is by far more useful, but once again, a commercial device will be way more accurate and useful for professional use. Hopefully, the designer will add some protocol filters etc in future.
This device will not replace a proper oscilloscope or logic analyser, but it will definitely give enough accuracy and resolution on low-frequency applications to satisfy some of the basic needs of a beginner or student just starting out with electronics.
It is also important to note that you should be safe, and not try to connect this to high voltages etc. Also, don’t connect this to your expensive phone or tablet, use an old one instead, as accidents may happen, and we don’t want to damage our valuable handheld devices…
