As the name suggest 555 timer is basically a “Timer”, which create an oscillating pulse. It means for some time output pin 3 is HIGH and for some time it remains LOW, that will create a oscillating output. We can use this property of 555 timer to create various timer circuits like 1 minute timer circuit, 5 minute timer circuit, 10 minute timer circuit, 15 minute timer circuit, etc. All we need to change the value of Resistor R1 and/or Capacitor C1. We need to set 555 timer in Monostable mode to build Timer. In monostable mode, the duration for which the PIN 3 would remain HIGH, is given by the below formulae:
T = 1.1 * R1*C1
So to build 1 minute (60 seconds) timer we need resistor of value 55k ohm and capacitor of 1000uF:
(1.1*55*1000*1000)/1000000 = 60.5 ~ 60 seconds.
A variable resistor of 1M is used here and set on 55k ohm (measured by multimeter). We can easily calculate the resistor value for 5 minute, 10 minute and 15 minute timer circuit:
5 Minute Timer Circuit
5*60 = 1.1 * R1 * 1000 uF
R1 = 272.7 k ohm
So to build a 5 minute timer circuit, we would be simply changing the resitor value to 272.7k ohm in above given 1 minute timer circuit.
10 Minute Timer Circuit
10*60 = 1.1*R1*1000 uF
R1 = 545.4 k ohm
Similarly to create a 10 minute timer we would be changing the resistor value to 545.4 k ohm.
15 Minute Timer Circuit
15*60 = 1.1*R1*1000 uF
R1 = 818.2 k ohm
As per above calculations, for a 15 minute timer circuit, we need the value of resistor to 818.2k ohm.
We should note here that we have used LED at reverse logic, means when OUTPUT pin 3 is low, LED will be ON, and when OUTPUT is HIGH then LED will be OFF. So we have calculated OFF time above, means after the calculated time LED will be turned ON. LED will be ON initially (OUTPUT PIN 3 LOW), as soon as we press push button (trigger the 555 via TRIGGER PIN 2), the timer will start, and LED will become OFF (OUTPUT PIN 3 LOW), after the calculated time duration, PIN 3 will again become LOW, and LED get turned ON.