Latching relays require only a single pulse of control power to operate the switch persistently. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays. The traditional form of a relay uses an electromagnet to close or open the contacts, but other operating principles have been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

Relays were first used in long-distance telegraph circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit.

Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The same happens when any person/objects will pass again through that door, the LED beam will be interrupted again and the relay switch will turn off switching any appliance connected to it off too.An automotive-style miniature relay with the dust cover taken offĪ relay is an electrically operated switch. Any appliance connected with the relay will switch on. Toggle Switch CircuitĪfter the activation of transistors second part of the circuit activates, decade counter IC CD4017 will now receive input and the output of this IC will activate the relay switch. When any person or object will pass through the door and blocks the light LDR’s resistance will be maximum and all the voltage will pass to transistors. In the first part, LED1 is continuously giving light to the LDR and its resistance is minimum therefore it sends all the voltage to the ground and keeps the circuit OFF. Let’s take a look at the working of both parts. This circuit is further divided into two parts, the first part contains a light/dark sensor circuit and the second part has a CMOS decade counter IC which works as a toggle switch.

Auto turn off relay circuit