Mar 092015

Time Delay Light switch Circuit

This Time Delay Light switch circuit is useful to illuminate locations where a light source of low power is needed. This circuit is ideal for small or temporary facility locations. Examples of places to use this circuit are: a drawer, a closet, a small room, the car trunk, etc.. Its main feature is that it turns off automatically, avoiding unnecessary energy consumption.

This circuit activates a light source, and stays ON for a set time. To activate the light source again, you must press the momentary contact button. See SW1 on the diagram.

Time Delay Light Switch Circuit

How a Time Delay Light Switch works?

The time in which the light source remains ON, depends on the values of resistor R2 and capacitor C1.

To increase this time, you can change the value of R2 using a bigger one. R2 can also be replaced by a potentiometer, to avoid the need to change the current resistor with another of different value.

When the SW1 button is pressed, the electrolytic capacitor is charged through resistor R5. The capacitor is charged to the same voltage of the power supply, and the transistor Q1 is saturated. This in turn saturates transistor Q2.

Thus, we get the collector of Q2 with voltage slightly lower, than the source voltage, and so the bulb lights. This circuit can operate with a 9V battery or 12VDC voltage source

List of circuit components

  • Q1: 2N2222A NPN transistor
  • Q2: TIP2955 PNP transistor
  • R1: 47, 1/4 W resistor
  • R2: 4.7M, 1/4 W resistor
  • R3: 100 ohms 1/4 W resistor
  • R4: 2.2K, 1/4 W resistor
  • R5: 100 ohms, 1W resistor
  • C1: 470uF electrolytic capacitor
  • L1: 12V, 2W Bulb
  • SW1: momentary contact switch, single pole, NO.

You can increase the amount of light that radiates the Time Delay Light Switch Circuit, changing the bulb for the larger one. (3 Watts, 5 Watts)

You can also change the light bulb, with 3 high intensity white LEDs, in series.

Note: NO = Normally Open


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