After successfully building a small circuit for 3-pin fan PWM control, I decided to add a temperature sensor. Now I have the option to make it smarter about adjusting speed (or stopping entirely) based on temperature. There are many temperatures sensor options, I decided to start simply and inexpensively with a sensor that returns an analog voltage representing temperature. A batch of TMP36 (*) seemed like a good starting point.

According to the Analog Devices datasheet, TMP36 output pin voltage is 0.75V at 25 degrees Celsius. For every degree of temperature rise, voltage increases 0.01V. For my first draft I wired it to my Wemos D1 Mini module's analog input pin. But I had to adjust the scaling because a D1 Mini includes a voltage divider to scale input of 0-3.3V down to ESP8266 ADC range of 0-1V. This voltage divider (and math necessary to convert it back) added error to the reading. Since I intend to use this sensor for measuring room temperature, I do not expect to measure above 50 degrees Celsius (122 Farenheit) which corresponded to 1 Volt. Thus, I soldered a wire to connect TMP36 signal directly to ESP8266 module analog input, bypassing the voltage divider.

I noticed that there appears to be a startup time period where the temperature reading is 2-3 degrees too high but, after about 5-10 minutes, it will drop to a steady state temperature and stay there. I'm not sure if this startup behavior is from the TMP36 sensor or from the ESP8266 ADC. Either way, it meant I could not use this sensor in a sleep/read/sleep/read cycle because such a quick read will always result in a too-high value from this startup behavior.

With the initial breadboard test complete, I built a dedicated temperature sensor node with just an ESP8266 with a TMP36. In the interest of compactness, I decided to solder the sensor directly to ESP8266 module pins.

Upon startup, I saw that it reported temperature that was a few degrees too high, but I thought that was just the startup behavior I already noticed. But instead of dropping, it had kept going up. I thought I had a bad TMP36 until I realized it was accurately reading heat generated by a running ESP8266. According to my USB power meter, it consumed less than a third of a Watt, but that's plenty of heat for a directly-mounted TMP36 to pick up.

If I wanted to measure a room's air temperature and not temperature of a running ESP8266, I needed to give the sensor some distance. But even then, the readings weren't reliable.

A little web research taught me that the ESP8266 ADC isn't very precise nor is it calibrated. For most applications, being off by a few hundredth of a volt is a negligible error, but here every hundredth of a volt represents an entire degree of temperature which is decidedly not negligible. Taking multiple values and averaging them did help with the precision, but not accuracy. Knowing what I know now, in hindsight I should have done this with an ESP32. Those chips (or at least newer units) have their ADCs calibrated at Espressif factory. Though it is more likely that I will try a different temperature sensor in a future project. Either way, right now I have TMP36 on hand with a circuit board suitable for controlling 3-wire PC cooling fans. Time to put them together to do something useful.

Even though it doesn't work very well, here's an ESPHome YAML excerpt anyway. This will read a TMP36 every second and report average value every five minutes. TMP36 signal is assumed to have been soldered directly to ESP8266 analog input, bypassing Wemos D1 Mini voltage divider.

sensor:
  - platform: adc
    pin: A0
    name: "Mobile Node Temperature"
    unit_of_measurement: "°C"
    update_interval: 1s
    accuracy_decimals: 2
    filters:
      - multiply: 100
      - offset: -50
      - sliding_window_moving_average:
          window_size: 450
          send_every: 300
          send_first_at: 15

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