27-07-2012, 01:48 PM
Staircase Light With Auto Switch-Off
Staircase Light.pdf (Size: 297.83 KB / Downloads: 40)
We are all familiar with the
electrical wiring arrangement
that connects an electrical
bulb with two switches: one at
the bottom of a staircase and the other
at the top. Wiring is done such that
either of the two switches can be used
to switch the bulb on or off. In such a
wiring arrangement, while climbing
up the staircase which is in dark, the
switch located at the bottom of the
staircase is used to switch on the light.
After you have climbed the staircase,
you use the switch located there to
switch off the light.
The circuit presented here is an
electronic-cum-electrical arrangement
to get a similar facility as provided
by the hard-wired electrical system,
but you need to operate the switch
only once. Whereas in the hard-wired
arrangement if you forget to switch
off the light once you have traversed
the staircase, light would remain ‘on,’
wasting energy.
In this circuit also, we have two
micro-switches—one located at the top
and the other located at the bottom of
the staircase—that can be pushed and
released easily during climb-up from
the bottom of the staircase or climbdown
from the top of the staircase. With
every push and release of either of the
two switches, bulb L1 lights up for a
preset time period of, say, 40 seconds,
which is considered adequate for climbing
up or going down the staircase. The
bulb goes off automatically after the set
40 seconds. You can change this ‘on’
time by changing the values of resistor
R7 and/or capacitor C4 depending
upon your requirement.
Switches S1 and S2 are the two
micro-switches, which provide low inputs
to the respective de-bouncing circuits.
Each de-bouncing circuit is built
around two NAND gates connected
back to back. The de-bouncing circuits
ensure a clean, bounce-free pulse at the
output every time the micro-switch is
pressed and released. The outputs from
the two de-bouncing circuits are ORed
using diodes D1 and D2 (1N4001). So
every time you press and release either
7
of the micro-switches, you get a positive-
going pulse at the junction of the
cathodes of diodes D1 and D2.
These pulses are used to trigger the
monostable circuit built around timer
IC2. On the trailing edge of the pulse,
the output of the monostable goes
high for a time period of 40 seconds.
This drives relay-driver transistor
2N2222 (T1) wired as a switch. Relay
RL1 gets energised and closes N/O
contacts of the relay, wired in series
with the mains and the bulb (L1). Bulb
L1 switches off when the relay gets
de-energised after 40-second pulse period.
Free-wheeling diode D4 (1N4001)
protects transistor T1 against transients
during relay switch-off operation.
The circuit operates off a 9V battery,
which gets connected to the circuit
through ‘on’/‘off’ switch S3. You
can also use regulated 9V power supply.
Assemble the circuit on a generalpurpose
PCB and house in a small box.
Connect micro-switches S1 and S2 near
top and bottom of the staircase through
flexible wires and bulb in the middle of
the staircase.