The circuit can be powered by four AA cells or a 9 V battery, and current requirements are very low, extending battery life to several weeks. The tone is determined by C3 and R5.
To get an adjustable tone, you can replace R5 with a trimmer potentiometer in series with a 10 k ohm resistor.
A 100 k ohm trimmer is suitable.
A schematic diagram of the Dual Mini Timer is shown in Fig. 1.
Special care must be taken with the positions of the polarized components (electrolytic capacitors and battery).
With 1,000 µF capacitors for C1 and C2 and 4.7 M ohm potentiometers for R1 and R3, you can get time delays up to an hour and a half.
Operation is as the same with the other timers described in this section.
IC1 - 4093 CMOS integrated circuit
X1 - Piezoelectric transducer or crystal earphone, Radio Shack 279-073 or
equivalent
S1 - SPST toggle or slide switch
S2 - SPST momentary switch
B1 - 6 or 9 V (four AA cells or battery)
R1, R3 - 1,000,000 ohm to 4,700,000 ohm potentiometers or trimmer potentiometers
R2, R4 - 100,000 ohm, 1/4 W, 5% resistors
R5 - 47,000 ohm, 1/4 W, 5% resistor
C1, C2 - 10 µF to 1,000 11F, 12 WVDC electrolytic capacitors
C3 - 0.022 µF ceramic or metal film capacitor
C4 - 100 µF, 12 WVDC electrolytic capacitor
