Request made by Mr. Aparajit:
I want to interface a Relay with 8051 microcontroller and its
operating voltage is 5v.
A single BC547 (in Common emitter) Transistor driving a 12v Relay.
At initial Power-on all output of microcontroller are logic high(+5v).
Even I initialize it's output to logic Low(0v) it takes a fraction of
second to change state. Which resulting a fluctuation at Relay and its
So, I want to design a inverted Circuit to drive that relay. i.e Input
logic 0v to ON, logic high(5v) to OFF the Relay.
I have used NOT-GATE like 74hc04, results are perfect, but i need a
small transistor based solution.
Thanks for responding.
My Reply to Mr. Aparajit
Either you can use a PNP transistor like a BC557
in place of BC547 and connect the relay across its collector and
ground, or,.... connect another BC547 with the existing one in the
The relay driver BC547s base resistor end which
was previously connected to the microcontroller o/p now gets connected
to the collector of the new BC547. This junction also gets connected to
the positive via a 2K2 or nearby value resistor.
The emitters of both the BC547 are commonly connected to ground.
The base of the new BC547 gets connected to the microcontroller o/p via a suitable resistor,
may be of the order of 10K or so.
Any of the above inverting options may be selected for the desired functions.
Do not forget to connect the flyback diode across the relay coil for the above cases.
I think the first option which uses a BC557 transistor is much straight forward.