Multimeter as temperature control (schematic diagram included)

Discussion in 'The Completed Projects Collection' started by Jackson Mwangili, Feb 11, 2018.

  1. Jackson Mwangili

    Thread Starter New Member

    Jun 17, 2017

    -Temperature Control Device (TCD) is single bit digital temperature control device which mechanically turns the relay ON and OFF as the result of temperature change.

    -It uses normal digital multi meter as temperature reading screen.

    - This circuit designed to control temperature in areas where the constant temperature is required throughout, or where the temperature must not exceed or decrease certain temperatures range, such as in incubator machines, machine, refrigerators, rooms, drying machine, etc

    -The circuit above can switch road up to 1000 Wattage of 220/110 VAC ,but you can increase the road driving ability by replacing relay with the big one (high current switching relay)

    -This circuit can also be used to switch very high voltage and high current devices if it connected to high current conductors which will be switched on by relay. (as I will show on part 2)


    A. Select multi meter to 200mV and switch on the circuit.

    B. Hold TC within your armpit for about 10 minutes, then twist VR1 until the Multi meter reads to 37.2 (this will be the real voltage of the world setting).

    C. Release the TC from your armpit and leave it to the surrounding, now the reading will start to copy with the surroundings temperature, the readings to your multi meter will either increase or decrease depending the surrounding temperature.

    D. First move TC close to any other source of temperature(if it is warm the reading will increase and vice versa) where it cut across your desiring set

    E. When the reading reach the point of your desiring set twist VR2 until the relay switches(repeat the action until you get closer to the point of set.

    F. When you get cutting point exactly of what you want Eg. 37.7 centigrade for chicken eggs incubator, just glue the VR2,

    G. VR3 will be used to set cutting point to the exactly point if VR2 didn’t get to exactly point (VR3 is use for further fine setting)

    H. Temperature switching range of this device is between 0 up to 100 degrees of centigrade (depending on temperature set on the VR2 and VR3)

    I. The difference between ON and OFF is 0.9 degree of centigrade of certain range,to reduce this just shift connection of R5 and C1 to gain point (GP)

    For example if you limit temperature to 38.5,the device will turn OFF when it reaches 38.5 centigrade and turn ON when temperature drop to 37.6 centigrade

    J. You can vary R5,to increase or decrease the interval between ON and OFF


    1. When the circuit turns on

    - if the surrounding temperature where TC is fixed is below the set temperature), Relay switches ON and allow 220/110 voltage to flow to the output (or any device connected to the output side)

    - When surrounding temperature where TC is fixed goes higher than the set temperature), Relay switches OFF and cut 220/110 voltage to flow to the output (or any device connected to the output side)

    -The processes is endless.


    1. IC =LM 741

    2. THEMORCOUPLE = Any TC which reads between 30kilo ohms to 50 kilo ohms in 28Centigrade.


    4. MULTIMETER=Any multimeter with 200milivolts selection

    5. TRANSISTOR = BC 547

    6. R1=1K






    7. VR1=10K

    VR2= 50K


    8. D1 =4007

    9. Z1 =Zener diode 3.2v

    10. C1 =16/10uf

    GP=Gain point refer to above description

    Last edited: Feb 11, 2018
  2. dendad

    Well-Known Member

    Feb 20, 2016
    Years ago I did a similar thing in a bore water pressure meter design as back then, displays were very expensive.
    It used an auto ranging multimeter set to volts and glued inside the front panel with just its LCD showing. I made an isolated power supply using 2 x LEDs as zeners supplied the +/- 1.5V to run it. One of the LEDs was the front power LED.
    The other part of the supply was controlled via a multiturn pot to excite the strain gauge.
    The meter could be switched to read the excitation and the signal volts.
    I'd almost forgot about that job. AH, memories :)