2N3904 NPN Transistors to Make a Simple Time Circuit

A simple timer circuit is a key part of any electronic circuit that introduces a set time delay in various switching devices. This delay of several seconds or minutes becomes a key requirement to ensure the correct operation of the circuit. Without the specified delay introduced by this circuit, the device may malfunction or even be damaged. Therefore, in this project, we will use 3 2N3904 NPN transistors to design a simple timer circuit. The 2N3904 is an NPN transistor, so when the base pin is grounded, the collector and emitter will remain open (reverse biased), and shorted (forward biased) when a signal is supplied to the base pin. The gain value of 2N3904 is 300. The maximum amount of current that can flow through the collector pin is 200mA. Therefore, we cannot use this transistor to connect loads that consume more than 200mA. In order to bias the transistor, we must provide current to the base pin. The current (IB) should be limited to 5mA.

Here are the electronic component you need in the project:
1. Breadboard 1

2. Connecting Wires 1

3. 9v Battery 1

4. 2N3904 NPN Transistor 3

5. Relay DC 1

6. Push button 1

7. Diode 1N4007 1

8. LM7805 IC 1

9. Capacitor (470uF) 3
10. Resistors (470k, 47k) 3,3

Working Principle

This circuit is basically an enhanced version of a simple single-transistor delay circuit. When the button (S1) is momentarily pressed, the transistor Q1 receives a control signal at its base through a 47K resistor. This triggers the Q1 switch and charges the capacitors C1 and C2. Now, when the button is released to disconnect the circuit from the power source. Capacitor C2 is discharged after the delay, and then transistor Q2 is triggered to charge capacitor C3, thereby introducing a further delay. Thereafter, capacitor C3 discharges and triggers transistor Q3. Subsequently, the relay coil is energized and the SPDT is switched to power the AC or DC load

Applications
It is usually used as a protection circuit to protect any electrical or electronic equipment and appliances from sudden high or unstable voltages, such as voltage stabilizers for refrigerators, UPS and PCs.