Specifications

‘bUSY’. The last two displays are not used in the
word ‘bUSY’. Rails ‘A’ and ‘B’ are active low for the
common-anode displays used here. Segments that are
to be always ‘off’ are left disconnected and are shown
as hollow lines. Those segments which are either lit
during ‘toiLEt’ display (pulled ‘low’ via bus ‘A’) or
during ‘bUSY’ display (pulled ‘low‘ via bus ‘B’) are
shown shaded in the figure.
Connect a +5V supply rail to the common anode
pin of all the displays. To test the circuit at this stage,
temporarily ground
either rail ‘A’ or rail
‘B’ (but not both),
and check whether
the display shows
‘toiLET’ and ‘busy’. Use a 9V DC adapter
as the power supply source and stabilise it
through a 7805 regulator.
Normally, switch S1 is open and transistor T1
is forward biased. T1 conducts and thus rail ‘A’ goes
to near 0V, to display the word ‘toiLEt’. If switch S1
is closed, T1 switches ‘off’ and turns ‘on’ transistor
T2 to take point ‘B’ to near ground potential, and thus
the display changes over to indicate the word ‘bUSY’.
Thus, when toilet door is open, the magnetically-
operated (or micro-switch operated) reed switch is
open and the display indicates ‘toiLEt’. Now to make
the message changeover to ‘bUSY’, when someone
goes inside and locks the door, the switch needs to
be closed on closure of the toilet door. One may also
use other methods to achieve the same results.
D.K. KAUSHIK
FEATHER-TOUCH
SWITCHES FOR MAINS
A
n ordinary AC switchboard con-
tains separate switches for switch
ing ‘on’/’off’ electric bulbs,
tubelights, fans, etc. A very simple, inter-
esting circuit presented here describes a
feather-touch switchboard which may be
used for switching ‘on’/‘off’ four or even
more devices. The membrane or micro-
switches (push-to-on type) may be used
with this circuit, which look very elegant.
By momentary depression of a switch, the
electrical appliance will be ‘on’/‘off’, inde-
pendently.
To understand the principle and de-
sign of the circuit, let us consider an ex-
isting switchboard consisting of four
switches. One live wire, one neutral wire,
and four wires for four switches are con-
nected to the switchboard, as shown in
the illustration below the circuit diagram.
The switches are removed and the above-
mentioned wires (live, neutral, L1, L2,
L3, and L4) are connected to the circuit,
as shown in the main diagram.
The circuit comprises four commonly
available ICs and four micro-relays, in ad-
dition to four micro-switches/membrane
switches (push-to-on type) and a few other
passive components. IC 7805 is a 5-volt
regulator used for supplying 5V to IC2
and IC3 (7476 ICs). These ICs are dual J-
K flip-flops. The four J-K flip-flops being
used in toggle mode toggle with each clock
pulse. The clock pulses are generated by
the push-to-on switches S1 through S4
when these are momentarily depressed.
When a switch is momentarily depressed,
its corresponding output changes its ex-
isting state (i.e. changes from ‘high’ to
S.C. DWIVEDI
170