Almost all sensors in my smarthome are based on the ESP8266, most of the time a NodeMCU flavor.
At the typical sensor places there is a 5 V voltage source around or a mains outlet.
But there are some places I want temperature sensors at, that have no power connection or at least it would be very ugly to put wires there.
ESP-01
The first solution I thought of, was an ESP-01 with some step down converter.
In normal operation the ESP-01 draws around 73 mA at 3.3 V.
Taking 4 AA batteries and 5.32 V this would be 45 mA resulting in a rumtime of 42 hours using 1900 mAh battery capacity.
Not very impressive...
In theory the ESP can spent its idle time in deep sleep with disabled CPU and WiFi.
To wake up from this slumber, the ESP needs a reset signal.
On a ESP-01 the reset pin is one of the eight pins, but unfortunately the corresponding pin to sent the wake up signal is not.
Therefor a small wire has to be soldered to the CPU package (pin 8 - GPIO16) and the reset pin (RST). ESP package pin out
This then looks like this (with some hot glue to keep everything in place...and yes, this is indeed a bit to much):
Now, using deep sleep the ESP draws only 20 µA.
If we take a measurement every 10 minutes and assume 10 seconds for every wake up, this results in a runtime of 83 days.
The loss of the voltage regulator is not yet accounted for in this case.
To save some extra current, the power indicator LED could also be removed from the board.
NodeMCU
Unfortunately using the ESP-01 has proven to be somewhat cumbersome. Soldering the wire is a pain and on top of that the ESP-01 has only two really usable IO pins. A similar sized alternative would probably be the ESP-12.
Enter NodeMCU, again!
Supplying a NodeMCU with 5 V from a batteries is not very effective as the on board voltage regulator (AMS1117) has a considerable quiescent current (5 mA).
However the NodeMCU can be powered with 3.3 V directly, but the AMS1117 has to be removed though as even in this mode it causes some loss.
1N4007
A prototype board using 3 AA batteries and a 1N4007 diode to lower the voltage down by 0.7 V and into the acceptable range of the ESP (yes, this is ugly (; )
- Normal operation: 75 mA at 3.72 V
- Deep sleep: 46 µA
Runtime: 49 days
MP1584EN
Here a protoype board usign a MP1584EN buck converter to lower the voltage to stable 3.3 V. Input voltage from the 4 AA batteries is around 5.56 V.
- Normal operation: 48 mA at 5.56 V before the converter
- Deep sleep: 204 µA
- Loss at buck converter: 187 µA (91% :-/)
Runtime: 64 days
Additional there is a voltage divider to measure the direct battery voltage for low battery notifications and a socket for a DS18B20 temperature sensor.
