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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.
Click on Drawing Below to view PDF version of Schematic
