Relay(s) Interfacing with AT89C51


Relay is an Electromechanical Device which consists of coil (inductor) & a physical switch. It has 5 pins namely Coil 1, Coil2, COM (Common), NO (Normally Open) and NC (Normally Closed). When the relay in OFF state; COM to NC points are shorted. To turn the relay ON two Coil pins need the current to flow through it for switching to happen from NC to NO. The COM pin is common in this switching. A small spring is attached between the switching viper and the body of the relay. When typical current passes through the relay coil, the strong magnetic field is generated around the coil which attracts the iron type viper towards it and hence switching from NC to NO happen. The relay can be driven using transistors, ULN2003, ULN2803 or Optocouplers. Normally the relays are used to drive the high voltage devices like bulbs, Motor, Air Conditioner etc.

Leave a Reply