Continuous Sound Output Timer (ART114)

Time delays can be adjusted in a range from minutes to half an hour. A compact unit can be built and housed in a small plastic box.

You can fit this unit in your pocket for portability and convenience.

The circuit works like many projects described in this section, so the only explanation necessary is about the output stages.

IC1-b acts as a control LED audio oscillator, running only at the end of the adjusted time delay This oscillator supplies the output signal to a drive stage, and afterward to a piezoelectric transducer.

Time delay is adjusted by R2, and the output tone is adjusted by R3. C1 determines the time range. With a 1,000 µF capacitor and a 4.7 M ohm potentiometer, we can get time delays of up to half an hour.

The circuit can be powered from AA cells or 9 V battery.

A schematic diagram of the timer is shown in Fig. 1.

Figure 1 - Schematic diagram of the timer

R2 and R3 can be replaced by trimmer potentiometers or common resistors if you want a definite time delay and output tone. S2 is required to discharge Cl after each period of use as the capacitor maintains its charge during long time delays.

X1 is a piezoelectric transducer or a crystal earpiece.

To use the timer, adjust the time delay via R2 and turn on the power switch, closing S1. After the desired time delay, the buzzer will produce a continuous sound with a tone given by R3 adjustment. R3 and R4 can be replaced by a 27 or 33 k ohm common resistor.