Arduino - First Light

[Susquatch]

Quite some time ago, I got introduced to the Arduino by reading a thread about @YYCHM adding an RPM Readout to his mill. Craig inspired me.

Microcontrollers are not new to me. I've spent a large part of my career using them and their sensors in the automotive world. I also spent time with PICS, the 8748, and the Basic Stamp. But the Arduino was new to me - I'd heard of it but never played with one.

On the strength of that thread and my desire for an RPM Readout on my mill, I bought an Arduino Nano and an LCD display. Sadly, I never actually used them. When I bought the DRO for my mill, it had a built in RPM display. So that was that. The Arduino started to gather dust just like so many other projects of mine.

Recently, I got covid and ended up in bed - bored out of my mind. Hey, says I to myself, what a great time to read up on the Arduino! And so I did.

Yesterday, I installed the new 2.0 IDE and connected my Arduino Nano to my desktop PC in the house and fired it up! Sooooo many problems. Can't connect, upload failed, etc etc. But one-by-one I clubbed them to death. Turns out the default comm port is wrong - easy fix. But still no joy. It turns out my Nano came with the old bootloader. Switched to the nano with original bootloader and problem solved. Last night I successfully uploaded my first program - blinking LED. Not exactly a huge program, but it worked! Then I played around with the blink timing and did a small program to play an "SOS" sequence of blinking.

Now I am hooked! Just picture an old man smiling over a blinking LED..... I know, it's pathetic but hey, I'm not ashamed to admit it!

I think I will do an RPM display for my lathe..... This will be more challenging. Fortunately, @whydontu has already written some first rate code for that.

I could also plan to improve on whydontu's code by playing with direct processor and register access coding. But before I run, I gotta walk. So I'm gunna set that goal aside for a little while.

Perhaps first I should play around with a few other simpler projects. First some programs that just exercise the device without add-on electronics, and then maybe dig out my breadboards and fiddle around with some additional electronics and maybe even some relays for motor and device control.

I welcome fun suggestions.

Some good can even come from Covid.

 

[phaxtris]

Do you remember when stamps were like 100$ +, and now you can buy a nano from China for 3$, oh the times we live in right!

OLED displays are pretty neat, I would buy one or two of them

Check out Ali, they have all of the break out boards for the popular chips on there for dirt, temp sensors, gyros, magnetometers, accelerometers, relay boards, etc etc etc

 

[Susquatch]

OLED displays are pretty neat, I would buy one or two of them

I like OLED. Will do!

Check out Ali, they have all of the break out boards for the popular chips on there for dirt, temp sensors, gyros, magnetometers, accelerometers, relay boards, etc etc etc

Ya, exciting times. I have a drawer in my electronics loft full of unused stamps. I wonder what I could get for them now.

I recently saw an Arduino board with a 3D accelerometer sensor and relays right on the main board!

 

[phaxtris]

I like OLED. Will do!



Ya, exciting times. I have a drawer in my electronics loft full of unused stamps. I wonder what I could get for them now.

I recently saw an Arduino board with a 3D accelerometer sensor and relays right on the main board!

Do they still make stamps ? I don't know if you could even give them away, there has been so much development behind the Arduino ide, even schools use Arduino's

Seems like the have a board for just about anything you could imagine! and the code library's are free and easy to get, it was not this easy when I was in school that's for sure

But it's exciting how accessible this stuff is, then you add YouTube to the mix and the possibilities are endless

 

[gerritv]

Sadly, using the Arduino IDE as a teaching tool leads to some truly terrible coding and sw design skills. Doesn't belong in schools besides tinkering.

 

[whydontu]

Things I’ve learned using Arduino:

> My coding skills are stuck at 1972 BASIC
> some arduino boards are 3.3v. they die easily when connected to real-world stuff
> one set of the Arduino Uno socket connectors are offset by 0.05”, so they won’t plug into a regular strip board.
> 0.1” pin headers suck in a vibration/heat/mist environment. Use the screw terminal adapters for gear intended for use in your shop.

 

[Susquatch]

Sadly, using the Arduino IDE as a teaching tool leads to some truly terrible coding and sw design skills. Doesn't belong in schools besides tinkering.

This sounds like the voice of experience.....

Can you provide some examples?

 

[phaxtris]

Sadly, using the Arduino IDE as a teaching tool leads to some truly terrible coding and sw design skills. Doesn't belong in schools besides tinkering.

I was referring to highschool, I don't think anyone learns how to do anything well there...but it is exposure

 

[whydontu]

This sounds like the voice of experience.....

Can you provide some examples?

My code would be a good example…

 

[jcdammeyer]

Decades ago Niklaus Wirth stated that people who learned Fortran as their first language were brain damaged for life. I always found that quite funny even if it was a bit rude.
He is the author of Pascal and other algorithmic languages on which even the Arduino is based.

Niklaus E. Wirth - A.M. Turing Award Laureate

amturing.acm.org

And I do use the Arduino for a small SD card logging application where I've had to modify it for my clients specific purposes. Here you can see the sparkfun log card in red along with a wiznet Ethernet card.

I've also attempted a few other projects with the Arduino but eventually always go to something that is more robust.
I used some strip LEDs and an Ardunio based library with Wifi to set the blinking sequences. After about 6 hours it would crash. There's nothing in the Arduino infrastructure that makes that easy (or maybe even possible) to debug. My guess is that there is a memory leak and eventually the stack is corrupted.

And there's the problem with the Arduinio and Python world. Random memory garbage collection that can adversely affect the application either for speed or reliability.

Having said that I have most of the parts to build an electric Kiln controller. (Project #42). I bought an add on board with CAN bus and was able to log messages from my car. But I found that without a decent list of what the messages mean it was pretty difficult to diagnose the problem or clear the faults. Eventually just bought a high end code reader.

I thought about using the Arduino for my Power Draw bar and coolant controller but in the end I used one of my dsPIC33F5011 modules since I could actually set break points and look at what was happening under the coveres.

I now have a couple of Leonardo Arduinos on the way from China. They can apparently really easily become USB keyboard emulators so the idea of a specialty Pendant is really interesting.

And there's the rub. The infrastructure and the number of libraries for the Arduino is staggering. I can't see not using it when the application begs for something that simple.

Would I use it in a system where peoples lives are at risk? Not a chance.

So you will see me reporting on things like the kiln controller or other projects that make things move.

 

[Susquatch]

My code would be a good example…

Mine too. Which is why I asked.

FWIW, I learned to throw away the programming rules very early on. It enabled me to write code that worked despite the fact that disciplined coders couldn't make their code work.

I am a firm believer in truly understanding the hardware first and ditching the coding rules. I know lots on here will argue (correctly) with that, but I argue that if it works it works. I'll take one good programmer who ignores the rules and writes code that works over 10 who follow the rules but can't make it work.

For example, a human modifies his thinking based on results. But it is considered horrid programming to mix data with code. Yet I used to routinely write programs that treated the program like data and actually changed the machine language program instructions themselves in memory to react to results. Of course, the reason for that was to be independent of compilers. So I would simply write code that could determine where it was so it could be modified on the fly. Often (but not always) this was way faster and more flexible than saving and restoring CPU status in subroutines. If you don't need it, why do it?

My guess is that there is a memory leak and eventually the stack is corrupted.

You might be right. My guess would be a timer or counter overflow or an interrupt gone AWOL. The AWOL interrupt might corrupt the stack too. Especially if it doesn't honour other Interrupts!

I say that because my brief investigation of the Arduino language / ide provides no way to conveniently access the timers, counters, registers, or the Interrupts. I see this as a major bottleneck to efficient fast code and a potential memory leak. Any program that has to sit around doing nothing while it waits for a complete count is doomed to run at a snails pace. Good code should be able to go do other things while it waits.

My guess is that some of the libraries out there don't do this correctly and as a result counter overflows are probably common. Same goes for Interrupts which are not even supported in the IDE let alone in any predefined way like the counters are.

Hence my earlier stated desire to bypass that part of the IDE and directly control the hardware.

For now, that's all just a lot of empty talk till I actually do it though! LOL!

 

[gerritv]

To be honest, I started with Assembler on NCR315 using paper tape in 1965, then went to Fortran. More assmebler, Cobol, some APL and then C and php. I spent a bunch of years architecting medical imaging connectivity stuff at ISG.

One misconception though, Arduino is not a language, it is an eco system that is mostly written in C and C++ and an IDE that hid everything from the user while providing little to no contextextual help in writing code. As in one step above a Notepaad++ editor. To the point where you too often have one file (.ino) with global variables, hundreds/thousands of lines of code, no structure, no isolation and typically no design. The libraries are buried deep in your file system. Nothing in your code is reusable. The RELS code I am using in my electronic leadscrew is typical of that mess. Totally unportable to new hw.

The reason for no debug is a combination of no hw support on Atmel 328 chips (on which the early Arduino hw is based) and a penchant for *nix types to use printf's instead of an actual debugger. Printf's almost universally block execution while its output is spewed out over a slow serial interface, holding up you code from doing anything timed. To the point where you get something working, turn off the printf spewing oand now you code no longer works because the timing changed. AES Data in 1985 had ahw designer in Mtl who couldn't understand why I wanted dma driven serial IO to mitigate that on a 68k based board he was designing!

If you write your sw using Visual Studio Code and use Platformio as a build system then you can get much of this under control. And pio provides a way to use at least a simulator to test/trace/debug your code. Better chips such as SAM D21 etc support hw level tracing, which of course is even better.

 

[gerritv]

Coding rules exist for a reason, you will know that when trying to fix someone else's code. Testable code is even more important. Tracelable code and design to requirements and regulatory stipulations are important.

I am a firm believer in truly understanding the hardware first and ditching the coding rules. I know lots on here will argue (correctly) with that, but I argue that if it works it works. I'll take one good programmer who ignores the rules and writes code that works over 10 who follow the rules but can't make it work.

That would get you fired by me in a heart beat. This has no place in sw development for an actual use, e.g. medical imaging. If its not maintanable it has no place in anything important. In fact I fired some 'expert/guru' like that who refused to let his team use source level debugging, insisted on printf's.

A major phone company in Canada got them selves in a heap of grief on one of their main offering's billing systems because a) no design, b) no design docs, c) no idea how subsytems were used, d) poor/terrible code. It took >2 years to upgrade to a new version due to this. Very costly.

 

[jcdammeyer]

One misconception though, Arduino is not a language, it is an eco system that is mostly written in C and C++ and an IDE that hid everything from the user while providing little to no contextextual help in writing code. As in one step above a Notepaad++ editor. To the point where you too often have one file (.ino) with global variables, hundreds/thousands of lines of code, no structure, no isolation and typically no design. The libraries are buried deep in your file system. Nothing in your code is reusable. The RELS code I am using in my electronic leadscrew is typical of that mess. Totally unportable to new hw.

Which is why I tend to still use the PIC family with the ICD-3/4 debuggers.

That photo I posted above is an M9S12DP512 based on the original 6809 but expanded to be closer to a 16 bit processor but really still just 8 bit. All the code is C with a few places assembler because the compiler screws up with the paging.

My ELS uses a PIC18F series processor and can be debugged/programmed with the Microchip tools. Even the LCD display code is set up as a Tree like data structure with 4 possible branches based on the soft keys in the row right under the display. So I've used the platform for other projects just by changing that data structure. And the code is all open source.

Still there are places where the Arduino is appropriate.

 

[jcdammeyer]

Coding rules exist for a reason, you will know that when trying to fix someone else's code. Testable code is even more important. Tracelable code and design to requirements and regulatory stipulations are important.

A few decades ago I designed a product that could have been the forerunner of the Arduino. The board had I/O, A/D, Serial. But the technology at the time was EPROMs so debugging had to be done with print statements. Integrated IDEs didn't really exist yet. But I wrote an RTOS for that processor and a colleague wrote the cross assembler and a Tiny Pascal Interpreter (the idea of Python).

The product was used in a few places but the EPROM need killed it. We had 7 Segment displays, 3 axis quadrature encoder boards, D/A boards. Serial was RS485 so we could network them. The RTOS found all the modules in ROM which could be written in either assembler or Pascal.

Shortly after I moved to The Netherlands and worked on OS9 based embedded systems with EPROMs but firm rules on how to code so others could read it. Again C code. But now State Machine Based. In the long run easier to debug than using an RTOS and for 99% of the control system fast enough.

 

[jcdammeyer]

As I mentioned above the approach I use now is state machine based which allows both serial port debugging and even CAN bus debugging. Back in 2000 I used a small module called a CANDIP which had an ATMEL processor that supported CAN bus. I made a board for it that drives transistors for the spark coil and the PWM out to the furnace fan along with gas solenoid.

The main line looks like this and the key rule is no function is allowed to block preventing other threads or device drivers from running.

void
main(void) {

    // Init all variables and global hardware.
    InitMachine();

    while (1) {
        // First Run all device drivers
        Timer();
        SparkDevice();
        SerialDevice();
        MotorDevice();
        FlameDevice();
        KeySwitchDevice();
        MAPCAN_Device();
        
        switch (MachineState) {

        case INIT :
            ThreadsInitialized |= BurnerThreadInit();
            if (ThreadsInitialized == INITIALIZED_THREADS) {
                puts("READY");
                 MachineState = READY;
            }
            break;

        case READY :
            // Check all existing threads again.
            ThreadsRdy = 0;
            ThreadsRdy |= BurnerThreadRdy();
            ReportThread();
            if (ThreadsRdy == READY_THREADS) {
                puts("STARTING");
                MachineState = STARTING;
            }
            break;
            
        case STARTING :
            ThreadsRunning |= BurnerThreadRdy();
            if (ThreadsRunning == RUNNING_THREADS) {
                puts("RUNNING");
                MachineState = RUNNING;                     
            }
            break;
            
        case RUNNING :
            BurnerControl();
            ReportThread();
            if (ThreadsRunning == 0) {   // All threads stopped?
                puts("STOPPING");
                MachineState = STOPPING;
            }
            break;
        
        case STOPPING :
            if (fERROR) {
                fERROR = FALSE;
                puts("ERROR");
                MachineState = ERROR;
            }
            else {
                puts("READY");
                MachineState = READY;
            }
            break;

        case ERROR :
            ReportThread();
            break;
        }
    }
}

One of the key devices is the timer() device. Here you can see the timer functions used in the make spark device. The states return if the timer isn't done. The timer counts ticks via an interrupt routine so it doesn't actually delay other than the time spent checking if a timer is finished.

// Spark Device for Crucible Furnace

//  By     John Dammeyer
//      Automation Artisans Inc,
//        PO Box 20002 Sidney BC
//        CANADA - V8L 5C9

//        1-205-544-4950
//        [email protected]
//
// Initial Version:

// Include constants and function definitions
#include <io8515v.h>
#include <stdio.h>
#include <eeprom.h>


#include "general.h"
#include "furnace.h"

//#include "WiCAN.h"

#include "Timer.h"

#include "mapcan_avr.h"
#include "ObEEROM.h"

#include "SparkDevice.h"

enum SPARK_STATES {
     SPARK_INIT,
     SPARK_WAIT,
     SPARK_ON,
     SPARK_OFF
};

BYTE SparkCmd = MAPCAN_DEV_INIT;     // Semaphore for control of device.
BYTE SparkMsg;

void SparkDevice(void) {
  static enum SPARK_STATES SparkState = SPARK_INIT;
    switch (SparkState) {

    case SPARK_INIT :
        SparkMsg = SPARK_STOP;
        SparkState = SPARK_WAIT;
        break;
        
    case SPARK_WAIT :
        if (SparkMsg == SPARK_START) {
            StartTimer( SPARK_TIMER, GetEEROM_Word(EE_SPARK_DWELL));
            PORT_ON(PORTB, SPARK_COIL);
            SparkState = SPARK_ON;
        }
        break;
        
    case SPARK_ON :
        if (TimerDone( SPARK_TIMER ) ) {
            PORT_OFF(PORTB, SPARK_COIL);
            StartTimer( SPARK_TIMER, GetEEROM_Word(EE_SPARK_DELAY) );
            SparkState = SPARK_OFF;
        }
        break;
        
    case SPARK_OFF :
        if (TimerDone( SPARK_TIMER ) ) {
           SparkState = SPARK_WAIT;
        }
        break;
    
    default :
        SparkState = SPARK_INIT;
        break;
    }
}

The above routine just pulses the ignition coil while the Spark_Msg remains set to SPARK_START.

Either with the serial port, the CAN bus or from another thread the spark can be started. I fake out the air flow detection here. Just assume that if I get here the fan is spinning and there is airflow.

        // FAN blowing at Ignition Speed or Manual control speed.
        case BURNER_FAN_ON :
            // Check Air Velocity Sensor for Blower Air.
            // MAPCAN_GetMessage()
            // if (Air is moving)
            if ( TimerDone(BURNER_TIMER) ) {
                // Cancel Start request.
                RunFlag &= ~fSTARTING;
                SendDeviceMsg( LAMP_DEVICE, MAPCAN_DEV_CMD, WARN_LAMP, LAMP_FLASH ); 
                if ( TRUE /*(PINB & AIR_FLOW) == 0*/ ) {    // Airflow exists
                    SendDeviceMsg( SPARK_DEVICE, MAPCAN_DEV_CMD, SPARK_START, 0 );
                    FlameThreshold = GetEEROM_Word(UV_SPARK_THRESHOLD);             // Low threshold for spark plug.
                    StartTimer(BURNER_TIMER, GetEEROM_Word(EE_SPARK_CHECK_DELAY));
//                    putchar('2');
                    puts("B:SPARK");
                    BurnerState = BURNER_SPARK;
                }
                else {
                    fERROR = TRUE;
//                    putchar('8');
                    puts("B:FAN_OFF");
                    BurnerState = BURNER_FAN_OFF;
                }
            }
            break;

The other states are similar. We test parameters or fake them out if I didn't finish the hardware. Wait the time interval and then speed up the fan from ignition speed to full speed and shut off the sparkplug.

 

[jcdammeyer]

Connect a serial port on a laptop to the furnace controller and this is what I see.

Crucible Furnace Version 0.22a

READY
STARTING
RUNNING
B:ACTVE
B:FAN_ON
B:SPARK
B:GAS_ON
B:FLAME_ON

Now furnace is running and heating. If I switch the run switch back to Center Position (OFF) then this is what happens. A change of state to FLAME_OFF and the GAS Solenoid is switched off

B:FLAME_OFF
B:GAS_OFF
B:FAN_OFF

Here I switch the switch from OFF to FAN ONLY while it's coasting down from high speed.

B:BURNER_INIT
B:ACTVE
B:FAN_IDLE

And then Switch if OFF again and then once it's stopped switch it on again.

B:INIT
STOPPING
READY
STARTING
RUNNING
B:ACTVE
B:FAN_IDLE
B:INIT
STOPPING
READY

The fan idle state happens when the main switch is moved from the RUN position to the FAN position. It sets the PWM slower than the initial slow fan speed for lighting the flame. I have that to just move air through the furnace after it's been used for melting to cool it and spread the warm dry air around the shop in the winter..

the "B:" is the burner thread. The interrupt driven output Q must be large enough to hold the max size message so the system doesn't hang waiting to fill the Queue. There are ways around that with the co-operative multi-threading systems.

I did not include functions to set EEROM to new values or make it so EEROM values could enable/disable the air flow and flame detector modules. And now it's unlikely I have the tools to reprogram this processor module without a lot of work.

Hence the idea of using the Arduino instead.

 

[Susquatch]

To be honest, I started with Assembler on NCR315 using paper tape in 1965, then went to Fortran. More assmebler, Cobol, some APL and then C and php. I spent a bunch of years architecting medical imaging connectivity stuff at ISG.

I started with machine language on a 4004 and then an 8080. There was no assembler yet because there were no computers (other than mainframes) to run them on! At first code had to be entered one byte at a time each time a program was run. No tape, no cassettes, no key punch yet for microprocessors. But I quickly adopted assembly language as soon as it arrived.

I too learned Fortran on a mainframe with punch cards.

My early years were spent designing electronic control systems for cars and trucks and then microprocessor based control systems for engine, body, and transmission control.

On my own time I designed and built many different kinds of what we call personal computers today. I did several from my own boards and chips, wrote my own OS, and then wrote user type programs from Word processors to spreadsheets. Then along came CP/M and it was love at first sight. After that I built mainly specialty boards like ADA and I focussed on writing drivers and applications at the assembler level. I didn't like the higher level compiler based languages of the time. They were way too slow (I think they still are). I wrote and sold quite a few programs like that because mine were always so much faster than my competition could write.

That would get you fired by me in a heart beat. This has no place in sw development for an actual use, e.g. medical imaging. If its not maintanable it has no place in anything important. In fact I fired some 'expert/guru' like that who refused to let his team use source level debugging, insisted on printf's.

So we are even then Gerrit! LOL!

Adherence to rules like you describe would get you fired in the auto industry! Everyone documented and everyone worked in a team. Everyone knew what everyone else was doing and anybody could be (and was) replaced. But anybody spending time adhering to rules got canned. We broke rules and did it intentionally. It was encouraged. Very few problems occurred because of software. Those tended to get worked out during prototype testing. The primary problems we encountered after production were related to humans messing where they shouldn't. It was a constant problem - and as far as I know it still is today. I used to get frequently challenged by the so-called high tech sector who criticized the industry for being slow to adopt technology. My standard response was to suggest they take their latest gizmo, pound it down a gravel road at every temperature and precipitation mother nature can throw at it, cover it in salt spray, blast it with high powered RFI, and then let's see how well their gizmo still works. It always shut them up. Even so called industrial equipment can't stand up to what a car endures day in and day out. Although I am not positive of the outcomes, I did participate in various military exchanges and I have good reason to believe that they are heavily based on automotive methods.

There are many reasons why the automotive software was done so differently. Here are three of the primary reasons: The first is corporate espionage - most code was written so it couldn't be reverse engineered. The second reason was performance competition in terms of function, cost, and manufacturability. Do more with less was always the order of the day. The third was the need for speed. A lot of things happen in a car engine in just a few CPU clock cycles. Just three or 4 functions would be easy to do. But it gets complicated real fast as additional mapping covered more dimensions. It doesn't take long and that CPU can't keep up. So we were always optimizing software and rules had to be broken to do it.

In many ways, the auto industry is the mirror image of what most of us assume it is.

Ya, you would fire me and I would fire you. But man would we ever have fun debating it!

Btw, I had a buddy who worked in information systems. We spent many awesome hours together arguing the pros and cons of the two paradigms. We always ended up agreeing to disagree. I expect that is where you and I will land too.

I wrote diagnostic imaging software too! But not connectivity software. I wrote diagnostic imaging software. We had a partnership with a university to develop 3D ultrasonic imaging hardware. We used it for non-destructive testing of car parts and they used it for medical diagnostics. That was a long time ago, but we used the same kind of software techniques for that as we did for car controllers and nobody suggested firing me.

I suspect our different approaches come from different segments of software development. You said you wrote connectivity software. I wrote device software. Connectivity and information systems software is totally different in many ways. Your world requires portability, upgradability, constant change, and even global conpatability. The product world is only rarely subject to those requirements. In fact as described above, it's often deliberately different.

Which is why I tend to still use the PIC family with the ICD-3/4 debuggers.

Boy do we agree on that! But I'm all done designing circuit boards. If I never do that again for the rest of my life it will be too soon! And I never did learn to like any of the complete pic based systems out there. WAY too expensive for personal use. They might be better now though. It's been 15 years since I last looked at what was available. Lots of things change in 15 years.

Still there are places where the Arduino is appropriate.

We agree on that too or I wouldn't be trying to learn how to use one. There is something to be said for a complete standalone controller system that can be programmed in a few days that only costs a few dollars with so much freeware available. I'm also planning to have some fun breaking the rules!

About debuggers. I've seen the odd article that mentions a replaceable boot strap with integrated debugging capabilities. But I can't find any solid links. Know anything about that?

 

[jcdammeyer]

We agree on that too or I wouldn't be trying to learn how to use one. There is something to be said for a complete standalone controller system that can be programmed in a few days that only costs a few dollars with so much freeware available. I'm also planning to have some fun breaking the rules!

About debuggers. I've seen the odd article that mentions a replaceable boot strap with integrated debugging capabilities. But I can't find any solid links. Know anything about that?

I recall reading about that. Apparently you have to do something to prevent the IDE from deleting files. Apparently while it's compiling it puts all the compiled C and assembler files in one spot and then once the final code is complete for downloading erases the works. For the TRS80 basic type programmers (Or Arduino or Python) this is probably a good thing because over time their system would have tons of files with no structure or trackability.

But for embedded systems where it's important to look at what the compiler did the Arduino fails miserably.

And for small modules to do simple things (sometimes even complex) it succeeds tremendously.

 

[jcdammeyer]

So just to hijack this thread a bit more. Here's a photo of the electronics that runs my foundry furnace. To be precise only the middle board with the rust and corrosion is in use. The little DIP board with the two crystals is called a CANDIP and has an Atmel 8515 processor and a CAN chip on board. The software written in C does PWM to a transistor that runs the PA surplus motor which turns a fan. It does that spark coil on/off pulsing. (There's a story behind that for another time). And it opens/closes the gas solenoid.

Now the other board interfaces to that little glass tube driven with about 300V to detect ultraviolet light. If the flame is on there is UV. If the flame goes off it might be white hot inside the furnace (which would relight the gas) but no UV. And if the sensor was connected and the software enabled it would close the gas valve.

The other sensor, not visible because I moved the ribbon cable over it, is a hot wire anemometer. Two Thermocouples, one heated by a resistor and one in ambient air. The difference in temperature tells if there is air flowing since the resistor beside the thermocouple warms it up unless there is air flowing. Simple and we don't really care for an absolute air speed. Just did the fan stop turning?

I also painted the back of the fan with black and white paint and have a reflective sensor to determine fan speed but never even wrote software nor have pins for it.