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 |
