Appliance Timer cum Clap Switch

Pankaj D. Choudhury


Two AA-size batteries supply 3V DC to the clock and maintain a positive voltage to the collectors of T6 and T7 through diode D7. This keeps the circuit active during power failures also. A step-down transformer supplies 12V DC to the relay coil and sound amplifier section. Diodes D5 and D6 are rectifier diodes and C5 is the ripple filter capacitor. Diode D4 prevents the 3V battery from draining out into the rest of the circuit.

The digital clock is a commonly available digital calendar with at least one alarm setting and one countdown timer setting. The digital calendar, being cheap, keeps the total cost of the project low and allows for precise settings of the alarm times. The alarm can be set 24 hours in advance, while a second alarm can be selected in the countdown timer mode, which allows for setting of the time 100 hours (99:59:59 hours to be precise) in advance. Availability of more than one alarm setting in the clock will give the added advantage of setting multiple switching times.

Component alternatives

Instead of the digital calendar, any other digital clock or battery-operated quartz clock (with alarm) can also be used as the basic timing device, though the alarm time setting is less precise in case of the latter. Instead of one clock, multiple clocks can be wired by connecting diodes parallel to D8.

Note that once set in the clock mode, the alarm operates daily at the same time. But in the countdown mode, it operates only once. So if an appliance is to be turned on and off daily at the same time without human intervention, at least two digital clocks have to be wired (if the clock does not have two alarm settings apart from the countdown timer).

Construction & testing

The appliance timer can be assembled on a general purpose PCB. The clock, battery, switches, relay, transformer, etc are wired with the PCB (not shown in the circuit). A plastic switchboard (available in electrical shops) can be used as the cabinet for assembling the unit. Holes can be drilled easily on the plastic cabinet. House the PCB, transformer, relay, etc inside the cabinet. Fix the plug socket, switches and external connector on the rear side of the cabinet. Indicator LEDs (fixed on LED sockets) on the front panel show ‘on’ or ‘off’ condition of the output plug.

Glue the condenser microphone inside the front or side wall with small holes drilled in front of it to receive external sound. The battery chamber housing two pencil cells can be fixed inside the cabinet or on the rear of the cabinet as per convenience. The clock is glued on top of the cabinet. Before fixing the clock on the cabinet, open it carefully to disconnect its piezoelectric buzzer. The terminal that shows pulsating voltage during an alarm operation (detected with a multimeter) is connected to the base of T10 through D8 and R19. The internal battery is replaced and the terminals are connected to the external battery chamber with proper polarity.

Operating modes

The operation of the circuit can be divided into two parts: clap mode and timer mode.

The timer can be put in clap mode by turning on the clap switch (S2). The connected appliance can now be turned on/off by clapping with an audible intensity. The clock timer will function as usual in this mode.

While clapping, leave a gap of a few seconds between two successive claps. Thus, the gadget will show better response because it has been designed to consider two overlapping claps as one, ignoring the second one.

For timer mode, switch S2 is turned off. The alarm is set at the time when switch over is required. The second switch over time can be set in the countdown timer. For that, the time difference between the present time and the time at which switching is required is calculated and this time is set in the countdown timer. When setting is done, set the output plug as ‘on’ or ‘off’ (as desired) by pressing reset switch S3.

While setting the alarm, ensure a delay of at least three minutes between two successive alarm times (on/ off) to allow for the first alarm to subside completely. Otherwise,the unit may malfunction (ignore the second alarm).

The article was first published in November 2006 and has recently been updated.


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