Latch Switch Using Transistors

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A latch circuit is a circuit that locks its output when an input trigger signal is applied and retains that state, even after the input signal is removed. This State will remain for an indefinite time until the power is reset or some external signal is applied to turn it off. The latch circuit is very much similar to SCR (Silicon Controlled Rectifier) and can be in use at times when SCR is not available. It can be very useful in alarm circuits, where if triggered once, the sirens will flash indefinitely or the buzzer will sound indefinitely until someone manually removed the power.

Latch switches can be designed through various techniques. In this project we have used transistors to perform the latch function; the circuit is using two transistors 2N4401 and 2N4403 and a few resistors. A relay switch is also used at the output, from which any desired electronic application or device can be controlled.

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Hardware Components

The following components are required to make Latch Switch Circuit

S.no	Component	Value	Qty
1.	Battery	5V-12V	1
2.	Transistor	2N4401, 2N4403	1
3.	Relay	5V or 12V	1
4.	Diode	1N4007	1
5.	Resistors	4.7K, 10K, 1K	2, 1, 1

2N4403 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N4403

2N4401 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N4401

Latch Switch Circuit

Working Explanation

This simple latch circuit can be operated using a 5V-12 battery. The circuit uses two transistors. One is an NPN 2N4401 transistor named Q1. Other is a PNP 2n4403 transistor named Q2. When a signal or a pulse is applied at the base of the Q1, the Q1 starts conducting and thus connects the base of the Q2 to the ground, as a result, Q2 also starts conducting which activates the relay connected with the collector of the Q2. A 4.7K resistor is also connected from the collector of the Q2 to the base of the Q1. This provides a feedback voltage at the base of Q1, which makes Q1 conduct continuously, till the power supply is disconnected. A diode 1N4007 is used for protection from voltage sparks and reverse current.