Sensors - Tranducers CircuitsVoltage Regulators Circuit Diagrams


The circuit depicted in the diagram is connected via terminals X3-1 and X3-2. A voltage input ranging from 11-15V enters through this input and is rectified for the circuit’s half-wave supply using components D6, C8, and C9. The circuit features two temperature sensors, KTY10, which are linked to terminals TH1, X1-1, and X1-2, as well as TH2, which connects to terminals X2-1 and X2-2. The power supply for these sensors is facilitated by resistors R1, R2, capacitors C1, C2, C3, and C4. The temperature variations sensed by TH1 and TH2 alter the internal resistance of these sensors, leading to changes in the values of P1, C1, and R5. These values then enter the low-power dual-voltage comparator.


  1. KTY10 Temperature Sensor:
    • Connect the KTY10 sensor to a power supply and ground.
    • Connect the output of the sensor to one of the inputs (non-inverting) of the LM2903 comparator.
  2. LM2903 Comparator:
    • Connect the other input (inverting) of the LM2903 comparator to a reference voltage source, like a potentiometer, to set the desired temperature threshold.
    • Connect the output of the LM2903 comparator to the control input of a relay module.
  3. Relay Module:
    • Connect the relay module to an external power supply and ground.
    • Connect the normally open (NO) or normally closed (NC) contacts of the relay to your desired load (like a fan, heater, or any other electrical device).

Circuit Description:

  1. The KTY10 temperature sensor measures the ambient temperature and provides an analog voltage output proportional to the temperature.
  2. The LM2903 comparator compares the voltage from the temperature sensor with a reference voltage. When the temperature exceeds the set threshold, the output of the comparator changes state.
  3. The relay module is controlled by the output of the comparator. When the comparator output changes, the relay switches, controlling the connected load.


LM2903 I01A and I01B comprise two low-power comparators. R3 is connected to the inverting input of comparator C6 I01A, while R4, R6, and C7 are connected to the non-inverting input of comparator I01B. The resistance of R7 is dependent on the inverting output of comparator I01A, and R8 is associated with the non-inverting output of comparator I01B. P2 potentiometer regulates these two comparator groups. If the comparator outputs are negative, diodes D3 and D4 remain inactive, and current flows through D1 over R9 and D2 over R10.

Additionally, resistor R12 is grounded at the base end of T1 darlington transistor, keeping the transistor in the off state. When the outputs turn positive, LEDs D1 and D2 turn off, and current flows through diodes D3 and D4. This activates the T1 transistor through R11.

Caution: LM2903 Op amp feed legs on the diagram are not shown, leg 8 is no + plus 4 is the leg – minus in the diagram out of the places where vcc feed connection on these legs.


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