Short Delay Timer Circuit

Short Delay Timer Circuit  is useful for providing controlled ‘ON’ times for such equipment as small heaters, incandescent lamps and many time operated devices. The working of the Short Delay Timer is based on the operation of timer IC555 (IC1). As IC555 is very reliable, it is the most popularly used circuit. Short Delay Timer has settable timings between 1sec to 100sec.  
About the Circuit :
Here IC1 operates as a mono stable multi vibrator. In this mode of operation, the timer functions as a one shot. The capacitor C1 is initially held discharged by a transistor inside the timer. On pushing switch S1, a negative trigger pulse of voltage less than 1/3Vcc is applied to trigger pin2 of IC1. Hence, IC1’s output at pin3 goes high and relay connected across it, thus it is energized. The voltage across C1increases gradually. At the end of time ‘t’, when its voltage becomes 2/3Vcc, it discharges and drives the output to its low state. As a result, relay becomes off. The timing interval between 1sec to 100sec is set by preset P1. It is independent of power supply. During the timing cycle, when the output is high, the further application of a trigger pulse will not affect the circuit. The output remains in low state until the trigger pulse is again applied.