These on off intervals can be adjusted by varying the 555 timer output and number of counter outputs. This circuit uses very basic components like 555 timer and 4017 counter. In this circuit a timer with cyclic on off operations is designed. Adjustable ON OFF Timer(using 555 astable mode) Some 555 versions may get damaged because of this.To prevent 555 timer from flyback current in the relay use a diode before the relay.This circuit is used in such applications where the load is switched on for sometime and is off for rest of the time.Thus relay will be on for required amount of time set by the user using pot and then it is switched of automatically.A 12v relay is used to drive the ac load connected at the output. Hence time T=120k*470uf=6 2 seconds~1 minute (approximately).For minimum time place the pot in least position.Then R= 120k.T= 1.2M*470uF = 620 seconds≈10 minutes.This is the maximum time. When Pot is maximum R is 120K+1.1M ≈ 1.2M (approximately) and C1=470uf.Time period can be calculated using formula.The above proposed circuit is a 1-10 minute timer.When Pot is minimum it gives 1 minute delay,where maximum value of pot can produce 10 minutes.This pulse width depends on R and c values. When the trigger input is applied,555 timer produces a pulse.Here 555 timer is operated in monostable mode. This circuit is flexible to adjust required time. Using simple 555 timer we can design an adjustable timer switch. Simple Adjustable Timer Circuit with 555 IC
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