Using ESP32-C3 Super Mini, which GPIO pin would be best for reading a fan tachometer signal?

[Nodi-Rubrum]

Using ESP32-C3 Super Mini, which GPIO pin would be best for reading a fan tachometer signal? This is what I believe is valid?

The fan is a typical 12v, 3pin or 4pin case fan that is cooling a small wifi access point that is in an extreme environment. Yes, the ESP32-C3 is going to suffering a bit, but replacing a super-mini worse case once or so during the summer extremes, a lot less hassle than replacing the entire wifi access point. I may add a temperature sensor (via i2c) as well, but maybe that is next generation. I have no plans to implement any fan control via PWM. Fan always on, at maximum RPM, is fine.

1. ESP32-C3 Super Mini lacks a PCNT (hardware counter)? So I am limited to an interrupt based scenario, for this use case, not a big deal if I have some CPU lag on the tachometer accuracy. This is better much just validating the fan is spinning, if it basically accurate, a plus

2. I figure I can use D3 (GPIO 5) usually is the top left most pin, if looking at the board with USB port north, and WiFi antenna south.

3. Can I use ADC-1, i.e. GPIO 1? I think... it may transmit 'garage' during boot, but not sure I care? Or do I? A second or two, should have valid tachometer values, right?

4. I plan to use mqtt_as as the core of the solution so I can send the tachometer results via MQTT, as will as request read of tachometer value on demand, as long as I initialize the interrupt, before the main asyncio loop, I just need to read the 'counter' on each publish or subscribe request. Oh, and reset the counter as applicable of course.

5. Oh, as I recall most GPIO pins on ESP32 have software resistor pull up, need that for the interrupt, believe GPIO 1 and GPIO 5 have such.

6. I might add a separate LED to flash as fan is spinning... but that is just shooting for the moon... :) Or I just flash on-board LED (GPIO 2) of course.

This seem feasible, fair approach or am I missing something?

[2dof]

to read tacho from fan i recommed to use:

a) omp-amp or some simpler solution ( zener diore , etc) for forming impulses see also : Computer Cooling Fan Tachometer Wire ,or:
b) Optocoupler separation for impulse forming , for example Pico_RPM

Ad1.
I guess that (correct me if frong) You want use 12 V PC fan? - if yes then speed of fan usually has max 1800-2200rpm some 3300 -> max 30- 60 Hz.

So You can use aby ISR input to count impulses. And since You propably want measure high speed of Fan - just simple code for frequency meter will be sufficient .
I do often projects for 50 Hz zerocrossing - any ESP32 with micropython is sufficient for any control power algorithms - so for Your fan will be Ok.

If You want just turn on/off fan (without speed control) then You can use just ADC for current measurement (voltage on shunt resistor) ,
Here reading cirrent (voltage) during working WIFI will cause noise

ad2
there is 13 user-accessible GPIO pins for ISR but of cource some GPIO are aleready taken, and some will be used by Your program so choose any left convenient for You like Your GPIO 5.

Ad5.
from my experience noise and interference occure mostly in cases :

• using PWM near i2c/ISP line can cause proplem
• in ESP32 using built in ADC Where WIFI/ BT is active is a nightmare
• there is no power or opto separation for power elements.

But all this problem are always easy to solve ( ussualy shielded wires or magnetic screens for insulation or propet use ground line layering on PCB'develepment board).

Tacho signal amplitide is ussaly much higher than noise, but if impulse noise occure - then simple R-C filter will help ( like in ad1. a) link
it is easy combine R-C filter into impulse reading.

Ad3 - for What purpose You want validate tacho -impulses witch reading ADC ?
Even if You want to control speed ( close loop control) ussaly any fault detection - in moment when You start fan and then start reading tacho - You will ne know that is a problem ( fault detection)

AD6. For any control solutoin it is good practice to indicate( by led or other visual/sound mechanism) state of the process like
green led - all work ok, red - Alarm = somethong wrong, or just use one diode and blinking visualisation.

[peterhinch]

I have no plans to implement any fan control via PWM. Fan always on, at maximum RPM, is fine.

In which case why bother reading the fan RPM? The only reason I can think of is to check for the case where the fan has failed. I would simply use two temperature sensors, one on the outside air intake and the other on the equipment being cooled. Switch the fan on when the inside temperature exceeds a threshold. Sound an alarm if either the inside temperature exceeds a higher threshold or if the temperature diference becomes excessive.

This avoids any hassle with signal quality and voltage levels.

There are failure modes that measuring rotation will miss such as blockages to inlet or outlet vents. The fan will happily spin while achieving nothing. Measuring temperatures is safer.

[Nodi-Rubrum]

@peterhinch , yes you raise a valid point. In the past I have had fans fail, and yes I do want to know if or when it does, for this project. The RPM results will going to my home automation I already have, i.e. MQTT based, and I have a custom Node Red dashboard (Yes, I could use something else, but I enjoyed learning Node Red). Never mind the fact that this would, IMHO be a fun project to do. I happen to have an ESP32-C3 Super Mini, and why not use it. :) There is no reason why I cannot add a couple of i2c based temperature sensors.

@2dof, I think I noted, I have not plans for speed control, or any PWM use at this point. The possible use of ADC, of which I have done little with ADC, was more of sanity check... in that I have done interrupt signal processing before in MP and otherwise, so to some modest extent familiar with falling or raising 'digital' signaling. But figured I would ask, in case someone could see 'better' use case for using ADC. From what I gather given all the responses, use of ADC has some complexities that I would rather not tackle for this specific project. But, as a lab, or learning effort, I do want to explore more ADC use.

I designed a simple prototype layout board last night, which includes a WS2812, so I can provide 'LED' status color coding, as visual indicator. This will of course also be telemetry to the Node Red dashboard for virtual LED indicator.

Guys, again, thank you, off and on over the years, I have interacted with MP forum, and later here in the discussions, always wonderful.