(Published in 'Electronics For You' magazine and 'Electronics Projects Vol 3')
(Article first published in November, 1982)
Many a time when a clock
rings the alarm early in the morning, set by you the previous night before
going to bed, you just want to sleep undisturbed for a few more minutes. But if
you shut off the alarm in your mechanical clock, there is all the possibility
that you may over-sleep and curse yourself when you finally get up - too late!
Here is a solution for you - a ‘snooze’ switch for your clock which will let
you sleep for just a few minutes more, and ring the alarm again and again after
pre-determined intervals.
This switch will also help you to switch on or switch
off an external gadget, such as an electric geyser or porch lights, at any
desired time. And, using your imagination, you can think up of numerous other
applications for this gadget.
Mechanical arrangement
At the time set for the alarm to ring, the alarm key
starts unwinding. This rotary motion of the alarm key can be used to activate
the electronic circuit for the snooze switch through a switch in a volume
control used in radio receivers.
Since the torque provided by the alarm key is not
enough to activate the switch in the volume control directly, the arrangement
shown in Fig. 1 may be used.
The volume control with the built-in on /off switch is
fitted on a metal sheet. A tuning drum of the kind normally used in radio receivers
is mounted on the volume control shaft. A tuning spindle is also mounted on the
metal sheet at some distance from the large tuning drum. However, while mounting
the spindle it should be ensured that the height of the alarm key and the
spindle from ground are the same (refer Fig, 1). A thread is now wrapped on the
drum and the spindle so that if the spindle is rotated clockwise, the tuning
drum rotates anticlockwise. A spring may be attached to the thread so that it
firmly grips the spindle and prevents slippage of the thread.
Next, take a piece of aluminum sheet measuring about
2.5cm x 2.5cm, cut it into an hexagonal shape as per Fig. 2 and fold
it along the dotted lines. Mount this piece on a small tuning drum or a knob
with either small screws or an adhesive (see Fig. 3). Now attach this knob to
the tuning spindle.
At this stage please check that on rotating the folded
piece of aluminum mounted on the knob anti-clockwise, the volume control switch
gets activated, and that the reverse happens on turning the knob clockwise.
This completes the mechanical arrangement for the device.
The circuit shown in Fig. 4 is supposed to perform the
following three functions at the preset time:
1. It will switch on light (LI in the circuit).
2. It will either switch on or switch off an external
gadget attached to the device.
3. At constant intervals, it will repeatedly sound an
alarm.
Transformer XI is wired in series with the rotary
switch Sr of the volume control. At the preset time, when the
alarm rings, the switch Sr will activate and
switch on power to the transformer. A miniature bulb, L1, which is connected
across the secondary of the transformer, will immediately light up and provide
sufficient illumination in the room.
An external gadget can be attached to the 3-pin AC
mains socket P. The changeable contacts of the relay RL1 are connected to the
plug socket through SPDT switch S2, as shown in the circuit. The relay will
operate when power to the circuit is switched on via the rotary switch Sr and the mains supply, which hitherto was connected to
the N/C contacts of the relay, will get connected to the N/ O contacts. Thus,
by setting switch S2 to position 1 an external gadget can be switched on at the
preset time, whereas by setting switch S2 to position 2 the external gadget can
be switched off at the preset time (set in the alarm clock).
Only the commonly-available 555 timer ICs have been
used in the circuit. IC1 is connected in the astable mode having a mark-space
period of 7 minutes and 2 minutes respectively. The down going edge at the
output of this IC is used to trigger IC2 via capacitor C4 and resistors R5 and
R6. IC2 is wired as a monostable having a pulse width of 1 minute. IC3 is also
wired in the astable mode but has mark- space periods of 5 seconds and 2
seconds respectively.
The reset terminal, pin 4, of IC3 is connected to pin
3 of IC2. As such, IC3 will oscillate only during the period that the output of
IC2 is high, which is about 1 minute. The outputs of IC1 and IC3 are connected in the wired OR
logic, which is formed by diodes D2, D3 and resistor Rll. The output waveforms that are obtained at various
points in the circuit are shown in Fig. 5.
The output of the OR combination is fed to the base of
a npn transistor Tl, which saturates whenever the output of either IC1 or IC3
is high and discharges capacitor C9. As such IC4, which is wired in the
astable mode, is enabled only during periods that the outputs of IC1 and IC3
are concurrently low. The output of IC4 is amplified through
transistor T2 and fed to a speaker via current-limiting resistor R17.
Thus, the device will provide a beeping reminder for
one minute and a continuous sound for the next one minute followed by a silence
for seven minutes. This cycle will be repeated continuously, until you finally
wake up and switch off the circuit.
It will be observed that the initial silence period,
when the device is switched on, will be of about 10 minutes whereas
subsequently this will reduce to about 7 minutes. This occurs because initially
capacitor C2 charges from 0 volt to 2/3 Vcc whereas subsequently it charges
from 1/3 Vcc to 2/3 Vcc. The time periods for the two cases can be calculated
from the following equations:
Tinitial = 1.1 (Ra+Rb) C
Tsubsequent = 0.7 (Ra+Rb) C
The device can be reset by momentarily depressing a
press-to-close switch S3. The alarm will sound again after a gap of 10 minutes.
Switches S1 and S4 should be mounted on the cabinet in
such a way that they are not easily accessible. Switch S3 should be mounted at
a prominent position so that it can be easily operated.
To use the device, first set rotary switch Sr in the off position by turning the knob clockwise.
After winding the alarm key of the alarm clock, place it next to the device in
such a way that the alarm key gets coupled to the folded aluminum piece mounted
on the knob. Thus, at the preset time, when the alarm key starts unwinding, the
rotary switch will activate and the circuit shall start functioning.
A layout for the PCB is given in Fig 6 and 7 below.
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