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Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:08 PM
List of Dave's Circuit Designs
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More
Piezoelectric Circuits, Buzzer Circuits,
Switch Circuits |
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Piezoelectric Tap Switch |
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This circuit uses a flat piezoelectric wafer, glued inside a plastic box, as a
finger tap sensor. With each tap of a finger to the box holding the wafer,
the circuit turns on and off AC or DC power to an external device. The
circuit is powered by a 9v battery and drives a single coil 5v latching relay with
a 10 Amp contact rating. The relay can switch AC or DC power. In
standby mode the circuit draws a low 1ua. |
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A firm finger tap to the plastic box cover causes the piezoelectric wafer
to produce multiple voltage pulses. The pulses are fed to a low power voltage
comparator. The comparator converts the low voltage pulses into 9v pulses.
Those pulses are fed to the clock input of one half of a CD4013 dual D flip/flop.
One section is configured as a half second one shot. The second half is wired as a
standard flip flop. With this circuit, the flip/flop changes state with each finger
tap to the box, holding the circuit. The output of the flip/flop is connected to a
dual FET push-pull driver circuit, which feeds power to a pair of capacitors. The
capacitors are wired in series with parallel diodes. This forms a 235uF
non-polarized capacitor. With this charge pump configuration, the power pulses from
the FETs feed either a positive pulse to the relay coil, to latch it, or a negative pulse
to unlatch it. The pulse lasts long enough to fully open or close the relay
contacts. |
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Click
on Drawing Below to view PDF version of Schematic |
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Piezoelectric Circuits, Buzzer Circuits,
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eMail David A. Johnson,
P.E. about this circuit |
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