Help with digital thermometer circuit

Discussion in 'The Projects Forum' started by Silver87, Nov 1, 2008.

  1. Silver87

    Thread Starter New Member

    Nov 1, 2008
    I have a digital thermometer that I use to measure outdoor temp. The probe wiring broke down due to being outside for many years. I have no way to ohm out the probe to determine its resistance value. I tried another probe and measured it at around 30k ohms at 75'F. It works, but my reading is off by about 100'F I am fortunate that the unit has a trim pot that I can fine tune it, but I don't know how to determine what resistor I will need to get my temp reading down 100'F. I'm sure this is probably easy to figure out, but I don't know where to start. Help please.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    If your probe is based on a thermistor, then you can use some device like a trim pot to find the desired resistance at 75 deg. (most thermistors are rated at 25 C, but close is probably close enough). Put it in place of the probe set up as a variable resistor and adjust until you get a reading that agrees with ambient. Use your meter to find the "right" thermistor. Try a 20K trimmer to start.