When a Relay driver circuit is used in Sensor based electronic circuits, false triggering may occurs due to unwanted relay clicking. The input signal level is not sharp in most sensor circuits and the transition stages are slow. So the relay driver transistor partially turns on/off . The relay coil will not become fully energized leading to vibration of the relay contacts and heating. A Snap Acting relay driver with Schmitt trigger action can eliminates this.
The diagram below is the relay driver circuit with Snap Acting for the clean switching of the relay. When the input signal at the base of T1 decreases, it turns off allowing T2 to conduct by giving base current through R3.When T2 conducts, its emitter current increases and this raises the voltage at the emitter of T1. This rapidly reduces the base current of T1 and it switches off rapidly. When T1 turns off rapidly, T2 turns on faster and relay energize. This snap action reduces the time gap between the working of T1 and T2 and prevents relay chattering.
Note: The value of R2 must be higher than the resistance of the Relay coil to give more current through R5 when T2 turns on and less flow when T1 turns on.