Time Switch

This circuit is
especially designed for those who often need to wake up early in the
morning. Ordinary alarms in electronic watches are not loud enough and
very often they fail to wake up. The switch circuit described here will
come handy; it can be used to switch on a TV, radio or tape recorder
etc, which will not allow even the laziest amongst us to ignore their
sound for too long. Besides, this time switch can also be used to switch
on/off any other electric or electronic gadget at any time. What you
need is a simple analogue electronic clock with alarm facility and a
small circuit to implement the time switch. This time switch has two
modes. One is ‘time-on’ mode and the other is ‘time-off ’ mode. In
time-on mode, you set up the alarm in your clock as per normal procedure
and at the set time this switch turns on the gadget connected at the
output socket-1.

In time-off mode, it turns your gadget off at the set time. The
optional output socket-2 is wired in such a way that when you use this
socket, the mode changes without having to flip the mode switch
(i.e.mode switch can be omitted). Please refer to the back panel diagram
of a typical analogue clock and the audio jack, to see how the existing
buzzer of the clock is required to be wired to the audio output from
the clock. This will ensure that when plug is inserted in the audio
jack, the clock’s buzzer will remain off and not consume any power
unnecessarily. The audio alarm output from the clock is coupled to the
AF detector built around low-power switching transistor T1. During
alarm, the collector of transistor T1 will fluctuate around ground level
and Vcc.


Circuit diagram

During absence of audio alarm input, the collector of transistor T1
is held at Vcc potential. The next stage consists of an S-R latch built
around NAND gates N1 and N2. Capacitor C2 and
resistor R4 are used for power-on-reset. On switching the power supply,
gate N2 output will acquire logic 1 and that of gate N1 logic 0. This is
the initial state, irrespective of the position of mode switch. At the
time of alarm, when point A connected to collector of transistor T1
passes through logic 0 state, the output logic state of both the gates
will toggle. Assuming that mode switch is flipped to ‘Mode Off’ position
at power-on-reset (when point D is at logic 1), initially diode D1
would be in blocking state and transistor T2 would be forward biased via
resistor R5 and diodes D2 and D3. As a result, the relay is in
energised state, which makes output power available at output socket1
and cuts it off from socket-2.

At alarm time, the audio signal toggles logic output states of both
gates N1 and N2. As a result, point D goes to logic 0 state. Diode D1
conducts, taking the voltage at junction of diodes D1 and D2 to near
about 1 volt. Diode D3 ensures that its series combination with diode D2
puts them in blocking mode. Capacitor C3 meanwhile discharges via
resistor R6 and the voltage at base of transistor T2 approaches towards
ground level, cutting off transistor T2 and de-energising relay RL1. Now
the power at output socket-1 would be cut off while it becomes
available in socket-2. If the above operation is repeated with switch S1
in ‘Mode On,’ the power would initially not be available in socket-1
(but available in socket-2). But after the alarm, the power would become
available in socket-1 and not in socket-2.

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