ESP8266 Community Forum

I am attempting to read a 10 K ohm Type 3 Thermistor.
I have it wired up like this:

[Ground] ---- [10k-Resistor] -------|------- [Thermistor] ---- [+5v]
|
Analog Pin 0

I realize I need to reduce the voltage to 1Vdc max for the ESP8266

I do not understand how to size the resistor, can someone please help?

f4u5t wrote:I am attempting to read a 10 K ohm Type 3 Thermistor.
I have it wired up like this:

[Ground] ---- [10k-Resistor] -------|------- [Thermistor] ---- [+5v]
|
Analog Pin 0

I realize I need to reduce the voltage to 1Vdc max for the ESP8266

I do not understand how to size the resistor, can someone please help?

It's a negative temperature coefficient one?

If you read my post about Internal ADC and Voltage Divider calculations you will find all the info that you need.

You must use voltage divider calculation with lowest temperature in your environnement.
for instance , if coldest temperature is 20°F and Vss is 5V :
top resistor is 4 x 50 KOhm = 200k Ohm

I really think you want to use this configuration:

[Vcc] ---- [Resistor Rdiv] -------|------- [Thermistor] ---- [GND]
|
[Analog Pin 0]

For Vcc = 5V (why 5V and not 3.3V?)
If is a NTC then (just example values, check NTC table for precise values for your specific Thermistor):
Rntc/25C = 10kohm
Rntc/125C = 300 ohm

- At 25C
ADC Input Voltage = 0.98 V -> Rdiv = 41k / Rntc =10k

- At 125C
ADC Input Voltage = 0.036 V <- Rdiv = 41k / Rntc =300

This will be your scale, from 0.036V to 0.98V.

To calculate Temperature, you can use table lookup technique or the Steinhart–Hart equation.

For more details read also : http://en.wikipedia.org/wiki/Thermistor