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Sound Controlled Filp Flop
Click here for the circuit diagram
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
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