The above figure shows the circuit diagram of a 555 adjustable timer. The LM555 has a maximum typical supply voltage of 16V while the relay's luminaire coil is activated at 12V. Therefore, a 12 V power supply is used to minimize the number of components, such as linear voltage regulators. When pin 2 on the LM555 is triggered (by short-circuiting it to the ground) through the torque switch S1, the timer starts.
The timer generates an output pulse with an ON time period determined by the RC network, ie t = 1.1 RC. In this case, the fixed value of the capacitor is 100uF. The value of R consists of a 10 kΩ resistor in series with a 1 MΩ potentiometer. We can vary the potentiometer to change the time period of the output pulse.
If the potentiometer is set to 0Ω, the value of R is equal to 1
But if the pot is set to 1MΩ, the value of R is 1MΩ + 10KΩ = 1010KΩ. Therefore, t = 1.1 x 1010K x 100u = 100 seconds.
When pin 4 of the LM555 is triggered (by short circuiting it to the ground) through the torque switch S2, the timer is reset.
When the timer starts, the relay is switched on. Therefore, the common (COM) terminal is shortened to the relay to the normally open (NO) terminal. A high power load can be connected to this terminal, e.g. a light bulb or water pump. A transistor Q1 acts as a switch and ensures that sufficient drive current is supplied to the relay. The diode D1 acts as a return diode which protects the transistor Q1 from voltage tips caused by the relay coil.
LED2 lights up to indicate when the relay is switched on. LED1 indicates that the circuit is switched on. An SPDT switch S3 is used to turn on the power. Capacitors C2 and C4 are used to filter sound in the supply line.