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Beeper &
Buzzer Circuits
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Star Trek Doorbell
In the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of
a crew member makes a particular “beep-boop” sound. The 3v battery powered circuit below
tries to simulate this sound. The circuit uses one 74HCT74 dual D flip/flop IC, wired as
two one-shot circuits. Both are designed to produce a pulse about one half second long.
The first pulse turns on a 555 timer to form the beep sound. The second flip/flop one
shot turns the 555 timer circuit and turns on an n-channel FET, which switches in an extra
capacitor, to make the “boop” sound. To my ears, I think the beep sound is right around
2KHz, while the boop sound is about 10% lower at about 1.8KHz. A variable resistor sets
the beep sound frequency. The output of the 555 timer feeds a low power 8 ohm speaker
using a buffer n-channel FET. A diode “or” gate and another FET enables the speaker and
time circuit. Each time the pushbutton switch is pressed, the circuit should produce the
beep-boop sound. I have observed that some 74HCT74 ICs do not always operate correctly in
this circuit. Some devices have a poor Schmitt trigger action on their inputs. Let me
know if you have problems. |
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Beeper &
Buzzer Circuits
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