We can not Directly drive the Relay through Microcontroller. Because the Microcontroller Gives only Regulating Pulse ( Drive Pulse to ON/OFF ) (0 or 1). Which is not capable to drive the Relay coil directly. The Relay coil (12 volt) can energies at +12volt supply, at this time the coil take dc current approx 20-25 ma.
Thus the total Power required by Relay coil for energies is,
Power = Volt * Current
= 12 * 0.020
= 0.240 mw
Microcontroller does not Provide sufficient Current for this Operation. It provide only a few micro-ampere or Milli-ampere current for such any application control like as LED, 16*2 LCD display, Transistor drive,etc.
Thus the Relay Operate with 240 milli-watt power. The Microcontroller give only a few micro-watt or milli-watt power which is only Describe as Signal.
Thus we require a second part which can deliver the power as per requirement of Relay coil, and operate by Microcontroller Signal pulse.
ULN2003 ic is introduce for this purpose. it is CMOS ic, which is specially design for relay drive with including free welling Diode and Buffer Circuit. The ic UlN2003 is capable for operate seven no of Relay with seperately operation.
It has also seven separate Input for drive the seven Relay separately.
Circuit Diagram:
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| circuit diagram of UlN2003 Interfacing with 8051 Microcontroller |
