One of the serious problems in relay operated circuits is the relay clicking or chattering during the on/off of the relay driver transistor. This problem is severe if the input circuit is a light/temperature sensor. During the transition of light/temperature levels, the relay clicks which may cause sparking of contacts. By using a simple tip, this problem can be avoided.
Below is the circuit of a relay driver using the NPN transistor BC 548. The relay is connected between the positive rail and the collector of the transistor. When the input signal passes through the I K resistor to the base of the transistor, it conducts and pulls the relay. By adding a 470 uF electrolytic capacitor at the base of the relay driver transistor, a short lag can be induced so that the transistor switches on only if the input signal is persisting. Again,even if the input signal ceases, the transistor remains conducting till the capacitor discharges completely. This avoids relay clicking and the offers clean switching of the relay.
Another 470uF capacitor is added parallel to the relay coil which maintains steady current through the relay coil so that relay clicking can be avoided if the power supply varies momentarily.
IN 4007 diode eliminates back e.m.f when the relay switches off and protects the transistor. LED indicates the on status of the relay