54 Everyday Practical Electronics, January 2009
by Dr Malcolm Plant
A beginner’s guide to simple, solder-free circuit prototyping
Part 4: Clap Switch and Low-Budget Intercom
HE 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.
This month, in Part 4, we present a
couple of more interesting circuits
for building on breadboard – a clap
switch and a simple intercom.
Fig.4.1: Complete circuit diagram for the Clap Switch
Integrated circuits, IC1 and IC2: type 555 timer (IC1); type 4027 CMOS dual JK
master/slave ﬂip-ﬂop (IC2)
Transistors, TR1 and TR2: both type BC108 or similar in a TO18 style package
Electret microphone insert, MIC1: sub- or ultra-miniature omni-directional
Light emitting diode, LED1: suggest red
Diode, D1: type 1N4148 signal diode
Relay, RLA: low voltage 6V type, single-pole changeover contacts
Resistors, R1 to R8: values 4.7kΩ (R1,R7), 2.2MΩ (R2), 220kΩ (R3,R6), 2.2kΩ (R4),
10kΩ (R5) and 220Ω (R8), all 0.25W carbon ﬁlm
Capacitors, C1 to C3: values 10nF polyester (C1), 100nF polyester (C2, C3)
Switch, S1 (On/Off): single-pole, single-throw (SPST)
Battery, B1: 9V and connecting leads
Protobloc and wire links
Everyday Practical Electronics, January 2009 55
Seen from below, the
emitter lead is next to the
small metal tag. Clock-
wise from the emitter are
the base, and
IC2, type 4027 JK ﬂip-ﬂop
Viewed from above, an indented dot
and a ‘half-moon’ shape at one end
indicates pin one. Once pin 1 has
been identiﬁed pins are numbered
1 to 16 going anticlockwise ending
up at pin 16 opposite pin 1.
TR1 and TR2, type BC108
You need to solder short lengths
of 0.6mm diameter insulated
wire to the solder pads. One pad
is connected to the case of the
microphone, so make sure this
lead is connected to 0V.
MIC 1, electret microphone
RLA, relay 6V ener-
This has single-pole
contacts for switching
on and off a separate
circuit from the electronic one.
How it works
The circuit comprises four main
building blocks. The ﬁrst is centred on
by the microphone. Current ﬂowing
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
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
ﬂip-ﬂop and the Q pin goes logic high,
switching on TR2, hence energising
the relay. Thus TR2 switches on or off,
subsequent claps, each clap ‘toggling’
the ﬂip-ﬂop. LED1 is optional, but is
useful in monitoring the change of the
Q output of the ﬂip-ﬂop.
The relay connected in parallel with
tact is used to switch external circuits
on and off. Note that this circuit must
not be used to control mains-powered
The Protobloc component layout for
the Clap Switch is shown in Fig. 4.2.
microphone insert connecting pads
be as quick as possible as it does not
one pad is also connected to the case
of the mic, so make sure this lead is
connected to the board 0V line.
Do not use the relay to control
power from the mains supply. If you
you should seek the help of a qualiﬁed
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 dam-
age by the sharp surge of voltage known
as back EMF, which is generated as the
relay switches off.
circuit, but you might like to fashion a
small paper cup, a curved reﬂector,and
the directional sensitivity of the circuit
to the sound of a clap.
Fig.4.2: Assembly of the Clap Switch on the Protobloc breadboard
Viewed from the
top, an indented
dot and a ‘half-
moon’ shape at
one end indicates
pin one. The pins
are numbered anti-
clockwise ending at pin
8 opposite pin 1.
IC1, type 555 timer IC