Relay operation using Darlington Pair

Summary
Description
Circuit Diagram
Components

Summary

Two transistors with a common collector configuration and connected in a cascaded form constitute a Darlington Pair. While Darlington pairs can be made on a breadboard using a few electronic components, there are IC available that work as them and thus aid in making a circuit simple and easy to be operated.

A darlington pair has various applications associated with it and one of them is to use it as a switch. As the transistor's collector current is proportionally limited by its base current, it can be used as a current-controlled switch. This simple project can be used in various application like alarm circuits, in electronics control circuit to on and off the device.

Description

The working of this circuit is based on well-known Darlington phenomenon of transistor. A Darlington pair behaves like a single transistor with a very high current gain. It has three leads which are equivalent to the leads of a standard individual transistor. The two transistors are connected in such a way that the current amplified by the First transistor (T1) is amplified by second transistor (T2). By connecting the two transistors in such a way will provide more gain then the individual transistor will provide. The idea behind the logic is that the both transistor will share the common collector.

In this project, the first transistor’s emitter feeds into the second transistor’s base and as a result the input signal gets amplified by the time it reaches the output. When light falls on the LDR, its resistance reduces. The bias voltage is supplied to the transistor and this voltage is enough to make the transistor and relay work. A variable resistor is also connected on the base of transistor to adjust the sensitivity.

Circuit Diagram

Components


12,348-Reads	Resistor Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits. ...
25,475-Reads	LDR An LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained...
5,394-Reads	Potentiometer Potentiometer (Pot) is another class of variable resistors and is used as an adjustable voltage divider. It consists of a fixed resistance track having connections at both ends and a sliding contact, called wiper, which moves along this track by turning the spindle. If only one of the connections and wiper are used, it behaves as a variable resistor or rheostat. In case wiper is not used, it will offer fixed resistance across th...
27,825-Reads	DPDT relay DPDT stands for double pole double throw relay. Relay is an electromagnetic device used to separate two circuits electrically and connect them magnetically. They are often used to interface an electronic circ...