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An
infra-red or wireless remote control has the disadvantage that the
small, handy, remote transmitter is often misplaced. The sound
operated switch has the advantage that the transmitter is always
with you. This project offers a way to control up to four latching
switches with two claps of your hand. These switches may be used
to control lights or fans – or anything else that does not produce
too loud a sound. To prevent an occasional loud sound from causing
malfunction, the circuit is normally quiescent. The first clap
takes it out of standby state and starts a scan of eight
panel-mounted LEDs. Each of the four switches are accompanied with
two LEDs – one for indicating the ‘on’ and the other for
indicating the ‘off’ state. A second clap, while the appropriate
LED is lit, activates that function. For example, if you clap
while LED10 used in conjunction with Lamp 1 is lit then the lamp
turns on. (If it is already on, nothing happens and it remains
on.) A condenser microphone, as used in tape recorders, is used
here to pick up the sound of the claps. The signal is then
amplified and shaped into a pulse by three inverters (N1 through
N3) contained in CMOS hex inverter IC CD4069. A clock generator
built from two of the inverter gates (N5 and N6) supplies clock
pulses to a decade counter CD4017 (IC2). Eight outputs of this IC
drive LEDs (1 through 8). These outputs also go to the J and K
inputs of four flip-flops in two type CD4027 ICs (IC3 and IC4).
The clock inputs of these flip-flops are connected to the pulse
shaped sound signal (available at the output of gate N3).
Additional circuitry around the CD4017 counter ensures that it is
in the reset state, after reaching count 9, and that the reset is
removed when a sound signal is received. Outputs of the four
flip-flops are buffered by transistors and fed via LEDs to the
gates of four triacs. These triacs switch the mains supply to four
loads, usually lamps. If small lamps are to be controlled, these
may be directly driven by the transistors. If this circuit is to
be active, i.e. scanning all the time, some components around
CD4017 IC could be omitted and some connections changed. But then
it would no longer be immune to an occasional, spurious loud
sound. The condenser microphone usually available in the market
has two terminals. It has to be supplied with power for it to
function. Any interference on this supply line will be passed on
to the output. So the supply for the microphone is smoothed by
resistor-capacitor combination of R2, C1 and fed to it via
resistor R1. CD4069, a hex unbuffered inverter, contains six
similar inverters. When the output and input of such an inverter
is bridged by a resistor, it functions as an inverting amplifier.
Capacitor C2 couples the signal developed by the microphone to N1
inverter in this IC, which is configured as an amplifier. The
output of gate N1 is directly connected to the input of next gate
N2. Capacitor C3 couples the output of this inverter to N3
inverter, which is connected as an adjustable level comparator.
Inverter N4 is connected as an LED (9) driver to help in setting
the sensitivity. Preset VR1 supplies a variable bias to U3. If the
wiper of VR1 is set towards the negative supply end, the circuit
becomes relatively insensitive (i.e. requires a thunderous clap to
operate). As the wiper is turned towards resistor R4, the circuit
becomes progressively more sensitive. The sound signal supplied by
gate N2 is added to the voltage set by preset VR1 and applied to
the input of gate N3. When this voltage crosses half supply
voltage, the output of gate N3 goes low. This output is normally
high since the input is held low by adjustment of preset VR1. This
output is used for two things: First, it releases the reset state
of IC2 via diode D1. Second, it feeds the clock inputs to the four
flip-flops contained in IC3 and IC4. In the quiescent state, IC2
is reset and its ‘Q0’ output is high. Capacitor C4 is charged
positively and it holds this charge due to the connection from R5
to this output (Q0). IC2 is a decade counter with fully decoded
outputs. It has ten outputs labelled Q0 to Q9 which go
successively high, one at a time, when the clock in put is fed
with pulses. IC3 and IC4 are dual JK flip-flops. In this circuit
they store (latch) the state of the four switches and control the
output through transistors and triacs. At the first clap, the
output of gate N3 goes low. Diode D1 is forward biased and it
conducts, discharging capacitor C4. The reset input of IC2 goes
low, releasing its reset state. All the J and K inputs of the four
flip-flops are low and so these do not change state, even though
their clock inputs receive pulses. When the reset input of IC2 is
low, each clock pulse causes IC2 to advance by one count and its
outputs go high successively, lighting up the corresponding LEDs
and pulling high the J and K inputs of the four flip-flops, one
after the other. Resistor R8 limits the current through LEDs 1
through 8 to about 2 mA. Larger current might cause malfunction
due to the outputs of IC2 being pulled down below the logic 1
state input voltage. If a second clap is detected while the J
input of a particular flip-flop is high, its Q output will go
high, regardless of what state it was in previously. Similarly, if
its K input was high, the output will go low. (If both J and K are
high, the output will change state at each clock pulse.) Thus
although all flip-flops receive the clap signal at their clock
inputs, only the one selected by the active output of IC2 will
change state. Resistor R9 and capacitor C6 ensure that the
flip-flops start in the off state when power to the circuit is
switched on, by providing a positive power-on-reset pulse to the
reset input pins when power is applied. The preset input pins are
not used and are therefore connected directly to ground. When,
after eight clock pulses, output Q8 of IC2 becomes high, diode D2
conducts, charging capacitor C4, thereby resetting IC2 and making
its Q0 output high. And there it stays, awaiting the next clap.
The four Q outputs of IC3 and IC4 are buffered by npn transistors,
fed through current limiting resistors and LEDs (to indicate the
on/off state of the loads) to the gates of four triacs. Four lamps
operating on the mains may thus be controlled. For demonstrations,
it might be better to drive small lamps (drawing less than 100 mA
at 12V) directly from the emitters of the transistors. In this
case the triacs, LEDs and their associated current limiting
resistors may be omitted. It has to be noted that one side of the
mains has to be connected to the negative supply line of this
circuit when mains loads are to be controlled. This necessitates
safe construction of the circuit such that no part of it is liable
to be touched. The advantage is that it may be mounted out of
reach of curious hands since it does not need to be handled during
normal operation. It is advisable to start with the low voltage
version and then upgrade to mains operation, once you are sure
everything else is working satisfactorily. CMOS ICs are used in
this circuit for implementing the amplifying and logic functions.
Use of a dedicated supply is recommended because the integrated
circuits will be damaged if the supply voltage is too high, or is
of wrong polarity. An external power supply may get connected up
the wrong way around, or be inadvertently set to too high a
voltage. Therefore it is a good idea to start by constructing the
power supply section and then add the other components of the
circuit. If the clock is working, you may turn your attention to
the amplifier. LED9 should be off, and should flash when the
terminals of capacitor C2 are touched with a wet finger (the
classic wet finger test). Preset VR1 may need to be adjusted until
LED9 just turns off. The output of gate N2 will be at about half
the supply voltage. The output of gate N3 would normally be high.
The voltage at the input of gate N3 should vary when preset VR1 is
varied. High-efficiency LEDs should preferably be used in this
circuit. The microphone has two terminals, one of which is
connected to its body. This terminal has to be connected to
circuit ground, and the other to the junction of resistor R2 and
capacitor C2. These wires are preferably kept short (one or two
centimetres) to avoid noise pickup. With the microphone connected,
a loud sound (a clap) should result in LED9 blinking. Adjust
preset VR1 so that LED9 stays off on the loudest of background
noises but starts glowing when you clap. If the clap-to-start
feature is not required, it may be disabled by omitting components
D1, D2, R5, C4 and connecting a wire link in place of diode D2.
Then IC2 will be alive and kicking all the time.
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