As you may know, there are three types of timer circuits: ON delay timer, OFF delay timer and the PERIODIC timer. Here is the circuit of OFF delay timer using IC 555.
When supply is connected to the circuit keeping the push-to-on switch in OFF condition, the output of the IC remains LOW and the circuit remains in this state, until you press the switch Sw1. Also the timing capacitor C1 remains discharged through pin-7.
When the switch is pressed, pin-2 is momentarily grounded and the internal trigger comparator triggers the flip-flop inside the IC and thus the output at pin-3 becomes HIGH. The relay is ACTIVATED and so the load connected to its n/o contacts is switched ON.
Now the capacitor C1 starts charging through timing resistor R1. When its voltage is slightly greater than 2/3 of Vcc, the internal threshold comparator triggers and the flip-flop inside the IC is SET. So the output at pin-3 becomes LOW. Due to this, the relay is DEACTIVATED and the load turns OFF.
When the output of the IC becomes LOW, at that instant the capacitor C1 quickly discharges into pin-7 directly and the circuit waits for the next timing cycle. Whenever, the switch is pressed again, the same process repeats to turn on the load for a predetermined time.
The time for which the load will remain ON can be calculated with the help of simple formula:
T = 1.1.R1.C1
where, T is in seconds, R1 is in ohms and C1 is in Farad
For more information about internal structure of IC 555, refer this popular article.
Important points to note
- The resistor R2 is used to pull up pin-2 to positive potential. This avoids accidental triggering of the circuit.
- The capacitor C2 across pin-5 and ground absorbs external noise and avoids abrupt change in time length when the switch is pressed.
With the given values of R1 = 1M and C1 = 100uF, the circuit will have a time period of about 110sec. However, you can change the time period as per your requirement.
Can be used to control the bathroom lights or store room lights which we generally forget to switch OFF. This circuit will ensure automatic switch off of these lights avoiding wastage of electricity.