Common electromagnetic relays with coil voltage of 5V, 6V or 12V can operate power circuits in the range of 250 to 300 volts and up to 20A. The actuating current of these relays is in the range of 50 to 100 mA. In many applications, such as temperature-controlled equipment, energisation/de-energisation of 5V, 6V or 12V relays for ‘on’/‘off’ control of the load is to be achieved by a few millivolts. Circuit diagram : Simple Relay Actuating Circuit Diagram Here is a simple relay actuating circuit that can be used to energise relays with a few millivolts as the input. As shown in the figure, the circuit is built around op-amp µA741(IC1) followed by an emitter follower using transistor 2N3440 (T1) and a few discrete components. You can change the sensitivity required (when the input is in millivolts range) to energise the relay by changing the value of resistors R1, R2 and R3. In all the cases, values of R4, R5 and R6 remain unchanged. Resistor R8 is basically a current-limiting res...