It is a simple timer circuit which uses Basic Electronics Components. In this we are using Transistor, Resistors, Capacitor, LED and Switch. This circuit is used to perform Light Dimming acting with the help of timer which is formed by Capacitor C1 and Resistor R1. Depending upon values of C1 and R1 time delay is produced.

When we press the switch the supply of +9v reaches to the Base terminal of transistor Q1, through which capacitor C1 starts charging. and LED  turns ON. When we released switch in ideal condition LED should be immediately turn OFF but rather than turning OFF suddenly it will slowly turns OFF. This is because capacitor starts discharging through base of Q1 and continuously provides proper supply to the LED. When C1 is completely discharged LED turns OFF.

Circuit Diagram:

Component Values:

  • Resistor : R1 = 47 Kilo Ohm
  • Color code for 47 Kilo Ohm: Yellow-Violet-Orange-Golden
  • R2 = 220 Ohm
  • Color Code for 220 ohm: Red-Red-Brown-Golden
  • Electrolyte Capacitor: C1 = 100 uf
  • Tactile Switch : SW1
  • Transistor : BC 547
  • Light Emitting Diode :LED1
  • Battery: 9 Volts


Circuit Building Steps:

Step 1:

Mount Transistor Q1 on Bread Board.

Step 2:

Connect R1 of 47K ohm to the base terminal of Q1.

Step 3:

Connect negative terminal of capacitor C1  to one end of R1.

Step 4:

Connect positive terminal of C1 to Ground.

Step 5:

Connect Emitter terminal of Q1 to Ground through Black wire.

Step 6:

Mount tactile switch on Bread Board.

Step 7:

Connect one terminal of switch SW1 to negative terminal of C1 and  connect second terminal to VCC.

Step 8:

Mount resistor R2 of 220 Ohm on Bread Board, Connect one terminal of R2 to VCC.

Step 9:

Mount LED on Bread Board.

Step 10:

Connect anode of LED to R2 and cathode to Collector terminal of transistor Q1.

Step 11:

Connect battery terminals to the Bread Board as shown below:

Press switch SW1 and observe the light dimming effect produced by the circuit.