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Described
here is a very inexpensive solution to many phono-controlled
applications like remote switching on, for instance, or activating
a camera, tape recorder, burglar alarms, toys, etc. The circuit
given here employs a condenser microphone as the pick-up. A
two-stage amplifier built around a quad op-amp IC LM324 offers a
good gain to enable sound pick-up upto four metres. The third
op-amp is configured as a level detector whose non-inverting
terminal is fed with the amplified and filtered signal available
at the output of the second op-amp. The inverting input of the
third op-amp is given a reference voltage from a potential divider
consisting of a 10k resistor and a 4.7k preset. The 100-ohm
resistance in series with the potential divider ensures against
the mis-triggering of the circuit by noise. Thus by adjusting the
preset one can control the sensitivity (threshold) of the circuit.
The sensitivity control thus helps in rejecting any external
unwanted sounds which may be picked up by the amplifier. The
output of the level detector are square pulses which are used to
trigger a flip-flop. The 100mF capacitor connected across the
supply also helps in bypassing noise.
A well regulated supply is recommended for proper functioning of
the circuit because an unregulated supply can cause noise pulses
to appear in the supply rails when the circuit changes-over state
(especially when a load is connected to the circuit). These pulses
can be picked up by the sensitive amplifier which will cause the
circuit to again switch-over states, resulting into motor-boating
noise.
Since the circuit operates at 4.5V, it can be easily incorporated
in digital circuits. Fig. (b) shows how the circuit can be
employed to control the direction of a DC motor. The circuit
employs four npn transistors. Transistors T1 and T4 have their
bases tied together and they switch-on simultaneously when Q
output is logic 1. Similarly T2 and T3 conduct when Q output is
logic 1. Thus current through the motor changes direction when the
flip-flop toggles. Filters connected in the circuit and tuned to
different bands of audio frequencies will enable the same circuit
to control more than one device. For instance, a high frequency
sound (such as whistle) can switch on device 1 and a low frequency
sound (such as clapping) can control device 2.
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