THE circuit shown in Fig.4.1 is designed to switch on a relay, RLA, when microphone MIC1, senses a single clap of the hands, and switch off the relay on the next hand-clap, and so on.
How it works
The circuit comprises four main building blocks. The
first is centered on NPN transistor TR1, which switches on when the sound
of a handclap is sensed by the microphone. Current fl owing through the
collector (C) and emitter
(E) terminals of TR1 causes a fall in the voltage at its
collector, which triggers the second building block.
The second building block is based on IC1, a 555 timer
wired as a monostable. On receiving the sharply falling voltage from the
collector of TR1, IC1 produces a short pulse of about 20ms, which is fed into
the clock terminal of IC2, a JK fl ip-fl op, which makes up the third building
block.
If IC2 pin 14, the Q output, is logic low, transistor
TR2 is off and the relay is not energized. The pulse from the monostable
changes the state of the fl ip-flop and the Q pin goes logic high, switching on
TR2, hence energising the relay. Thus TR2 switches on or off, energising or
de-energising the relay on subsequent claps, each clap ‘toggling’ the fl ip-flop.
LED1 is optional, but is useful in monitoring the change of the Q output of the
fl ip-fl op.
The relay connected in parallel with resistor R8 and
LED1 is energized when LED1 lights and its normally-open contact is used to
switch external circuits on and off. Note that this circuit must not be used
to control mains-powered circuits.
Breadboard
The Protobloc component layout for the Clap Switch is
shown in Fig. 4.2.
When soldering the leads to the electret microphone
insert connecting pads be as quick as possible as it does not take kindly to
excessive heat. Note that one pad is also connected to the case of the mic, so
make sure this lead is connected to the board 0V line.
Notes
! Do not use the relay to control power from the
mains supply. If you want to control mains-operated devices you should seek the
help of a qualified electrician.
! Use the Circuit Tester described in Project 1 to
identify the base leads of TR1 and TR2 to confirm that they are both NPN
transistors.
! Once assembled on Protobloc, you will find that LED1 is
either on or off, but a sharp clap of the hands will change this by either
switching the relay on (LED1 lit) or off (LED1 out).
! Diode D1 is used to protect the semiconductors, ie the
transistors and integrated circuits, from possible damage by the sharp surge
of voltage known as back EMF, which is generated as the relay switches off.
! No adjustments are necessary to the
circuit, but you might like to fashion a small paper cup, a curved reflector,
and fit it around the microphone to enhance the directional sensitivity of the
circuit to the sound of a clap.
Dr: Malcolm Plant
Everyday Practical Electronics, January 2009
image quality of circuit diagram is not good. i cant see the values of components and pin no of ic clearly and properly.change the image with better better one.
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