You can power the device with four AA cells or a 9 V battery. Current drain is very low, extending battery life to several months.
Time delays are in the range of a few seconds to more than half an hour.
A piezoelectric transducer or crystal earphone produces a high-level intermittent tone. You can house all the components in a small plastic box to obtain a portable, easy-to-use unit.
The device has only one adjustment, R1, for the time delay. A schematic diagram for the Pulsed-Tone Turn-Off Timer is shown in Fig. 1.
Proper positioning of the electrolytic capacitors must be observed.
R3 determines the tone, and R4 the interruption rate. Values of these components can be varied to change the sound.
S2 should be pressed to use the unit again. This switch discharges Cl after a time delay. With a 1,000 µF capacitor and a 4.7 M ohm potentiometer, time delays are up to 45 minutes.
IC1 - 4093 CMOS integrated circuit
X1 - Piezoelectric transducer or crystal earpiece, Radio Shack 273-07 3 or equivalent
S1 - SPST toggle or slide switch
S2 - SPST momentary switch
R1 - 2,200,000 ohm or 4,700,000 ohm potentiometer
R2 - 100,000 ohm, 1/4 W, 5% resistor
R3 - 39,000 ohm, 1/4 W, 5% resistor
R4 - 2,200,000 ohm, 1/4 W, 5% resistor
C1 - 10 µF to 1,000 µF, 12 WVDC electrolytic capacitor (see text)
C2 - 0.022 µF ceramic or metal film capacitor
C3 - 0.47 µF ceramic or metal film capacitor
C4 - 100 µF, 12 WVDC electrolytic capacitor
