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The circuit shown here
is used to switch on a lamp when the tele- phone rings, if the ambient light
is insufficient. The circuit uses only two ICs and it can be implemented very
easily. A light dependent resistance (LDR), with about 5 kilo-ohms resistance
in the ambient light and greather than 100 kilo-ohms in darkness, is at the
heart of the circuit. The circuit is fully isolated from the phone lines and
it draws current only when the phone rings. The circuit provides automatic switching
on of a lamp during darkness when the phone is kept in a place such as the bedroom.
The lamp can be battery powered to provide light during power failure or load
shedding. This avoids delay in attending to a call. The light switches off automatically
after a programmable time period and it needs no attention at all. If required,
the lamp lighting period can be extended by simply pressing a pushbutton switch
(S1). The first part of the circuit functions as a ring detector. When telephone
is on-hook, around 48V DC is present across the TIP and RING terminals. The
diode in the opto-coupler is ‘off’ during this condition and it draws practically
no current from he telephone lines. The opto-coupler also isolates the circuit
from the telephone lines. Transistor in the opto-coupler is normally ‘off’ and
a voltage of +5V is present at the ring indicator line. When telephone rings,
an AC voltage of around 70-80V AC, which is present across the telephone lines,
is used to turn on the diode inside the opto-coupler (IC2) which in turn switches
on transistor inside the opto-coupler. The voltage at its collector passes through
0-volt level during ringing to trigger IC3 74LS123(A) monostable flip-flop.
The other opto-coupler (IC1) is used to detect the ambient light condition.
When there is sufficient light, LDR has a low resistance of about 5 kilo-ohms
and the transistor inside the opto-coupler is in ‘on’ state. When there is insufficient
light available, the resistance of LDR increases to a few mega-ohms and the
transistor switches to ‘off’ state. Thus the DC voltage present at the collector
of transistor inside the opto-coupler is normally 0V and it jumps to 5V when
there is no light or insufficient light. The 74LS123 retriggerable monostable
multivibrator is used to generate a programmable pulse-width. The first monostable
74LS123(A) generates a pulse from the trigger input available during ringing,
provided its pin 2 input (marked B) is logic high (i.e. during darkness). It
remains high for the programmed duration and switches back to 0V at the end
of the pulse period. This high-to-low transition (trailing edge) is used to
trigger the second monostable flip-flop 74LS123(B) in the same package. Output
of the second monostable is used to control a relay. The lamp being controlled
via the N/O contacts of the relay gets switched ‘on.’ The ‘on’ period can be
extended by simply pressing pushbutton switch S1. If nobody attends the phone,
the light turns off automatically after the specific time period equal to the
pulse-width of the second flip-flop. The light sensitivity of LDR can be changed
by changing resistance R2 connected at collector of the transistor in light
monitor circuit. Similarly, switch-on period of the lamp can be controlled by
changing capacitor C3’s value in the second 74123(B) monostable circuit |
