ESP8266 Community Forum

- Thu Jan 03, 2019 9:32 am #79844 Hi All,

I have built a thermometer using a voltage divider (10k thermistor and 1 k resistor). At the moment it is powered by a 3.3v breadboard power supply and goes into a loop of deep sleep for 5 mins, wakes, sends the temp and then goes back to sleep . There is also a 10k resistor between the ADC pin and the divider to stabilize the values.

I am now thinking of changing it to be battery powered and realised that although the esp is in low powered deep sleep for most of the time, the voltage divider is always on. Given the resistors involved, even if the thermistor is at theoretical 0 omhs, it is only drawing 3 mA (if my calcs are correct - I have not included the 10k resistor).

So.... should I worry about this? One option would be to power it off one of the GPIO pins rather than Vcc so when the ESP wakes up it sets the pin high and this is used to power the voltage divider.

Any thoughts on best way to approach this to extend battery life?

thanks

Lee.

- Thu Jan 03, 2019 1:12 pm #79847

One option would be to power it off one of the GPIO pins

That is what I would do.

The one concern I have is that powering the voltage divider from Vcc will produce different reading as Vcc changes. Also the A/D on the ESP8366 is noisy and it can be difficult to get consistent results without taking special precautions.

For these reasons I prefer the DS18B20 sensor if it is used close to the ESP. For longer distances I might choose a thermistor and an I2C A/D.

- Sat Jan 05, 2019 5:18 am #79877

For these reasons I prefer the DS18B20 sensor if it is used close to the ESP. For longer distances I might choose a thermistor and an I2C A/D.[/quote]

The challenge i have is that the thermistor is for a food temp probe so has to be thin and protected. At the moment I have bought a couple of digital food thermometers with a probe and wire which terminates in a headphone type plug. I am reusing the probe from these in my project. I couldn't find anywhere to buy the probes on their own at a sensible price

- Sat Jan 05, 2019 12:55 pm #79881 Looking into this further, it seems that the max current from a GPIO is 12mA so assume this might be an issue? Another thought was to use a transistor controlled by the GPIO to act as a switch. What do you think?

thanks

Lee.