Shop Learn P1 Docs P2 Docs Events
Cheap Temp. sensor — Parallax Forums

Cheap Temp. sensor

This a temp. sensor using a 2N2222A metal can transistor.


  • jmgjmg Posts: 15,059
    edited 2022-04-06 23:40

    There are IC's that are designed to be used with transistors connected as temp sense diodes.
    The smarter ones use multiple current checks, to eliminate the forward offset by manufacturer, and they correct for linearity too.

    below is an example of the LTC2986 with an external 2N3904 transistor

    Of course, you can buy a calibrated i2c temp sense, for under 20c these days, so external transistor use is less common, usually used when that sense element is on a die, or needs to be external.

  • Thanks for the info. The 2n2222 is for my low budget analog thermocouple circuit, not using any PIC controller. I solder it to the terminals as a cold junction compensator.

  • ercoerco Posts: 20,224

    Good info. I have a collection of cheap thermistors which work well with a simple ADC. Prices are rising but you can still get 50 pcs for ~$5 shipped.

  • One bit troubles me about the circuit in the pdf. It seems to me the polarity on the voltmeter is wrong. The circuit is driven by a -10V supply, so ground is the most positive node. I do agree that the transistor or even a 1N4148 diode makes good sense at minimum cents for a nearly linear temperature sensor.

  • The polarity of the meter is correct. The higher the temperature the more positive the meter reads. Example: at room temp. the reading is -0.645 Vdc, now increase the temp. to the 600-700 F range and the reading is about -0.300 Vdc. Rising temp., rising voltage. At least that's how it works in my circuit.

Sign In or Register to comment.