This intelligent switch circuit enables an emergency light system to automatically switch on during a power outage. It detects a power failure using a transformer, rectifier, and capacitor, which causes a relay to energize. A photo transistor and Darlington pair detect darkness and complete the battery supply path, energizing another relay to turn on the lights only when both power is out and it is dark. The operation is summarized in a table showing that the switch turns on only when mains power is absent and it is dark, otherwise it remains off.

1. INTELLIGENT SWITCH
T
his intelligent switch circuit ena- remains de-en-
bles automatic, switching on of ergised. How-
an emergency light system during ever, during
darkness in the event of mains failure. darkness, pho-
The mains power failure condition to transistor
is detected by the section consisting is cut-off and
of mains step-down transformer X1 therefore tran-
followed by bridge rectifier comprising sistor T2 re-
diodes D1 through D4 and smoothing ceives forward
capacitor C1. If the mains is available base bias via
then it causes energisation of relay resistor R1
RL1 which has two sets of changeover (connected to
contacts. positive rail),
The light/darkness condition is de- as resistor R2
tected by the circuit comprising photo- is no more
transistor FPT100/2N5777 followed by grounded (via
Darlington pair comprising transistors photo-transis-
T2 and T3. However, this section will tor T1). As a
function only when mains supply is result, relay
not available (i.e. when relay RL1 is in RL2 gets en-
de-energised state) since battery supply ergised.
(negative lead) path gets completed via Thus it
lower N/C contact of relay RL1. would be ob-
During daylight, photo transistor served that
conducts and places transistor T2 base when mains is
near ground potential. Thus Darlington absent (relay RL1 de-energised) and it output path is complete. In any other
pair remains cut-off and relay RL2 is dark (relay RL2 energised), the switch condition switch output path would get
broken. The switch output terminals can
Period Conditions Switch status
be used (in series with supply) to control
a lighting system directly or indirectly
During daylight
[
(when mains is present
(when mains is absent) ] intelligent switch is ‘off ’.
through another contactor/heavy-duty
relay depending upon the load.
During night darkness
[ (when mains is absent) ]
(when mains is present) intelligent switch is ‘off ’.
intelligent switch is ‘on’.
The working of the intelligent switch
is summarised in the table.
200 ELECTRONICS PROJECTS Vol. 19