This circuit uses light dependent resistors and a timer integrated circuit to automatically turn on a white LED night lamp when it gets dark and turn it off in the morning when light is detected. It also uses another light dependent resistor and transistor circuit to generate a morning alarm melody when light is detected. The circuit is powered by a 9V transformer with battery backup and provides both automatic night lighting and a morning alarm function.

1. C I I R U IU IIT EIAD E A S
C RC C T D S
AUTOMATIC NIGHT LAMP WITH SAN
I THE
O
MORNING ALARM
D. MOHAN KUMAR sistors (LDRs) for sensing darkness and Low-value capacitor C2 maintains the
light in the room. The resistance of LDR is monostable for continuous operation,
T
his circuit automatically turns on a very high in darkness, which reduces to eliminating the timer effect. By increasing
night lamp when bedroom light is minimum when LDR is fully illuminated. the value of C2, the ‘on’ time of the white
switched off. The lamp remains LDR1 detects darkness, while LDR2 de- LED can be adjusted to a predetermined
‘on’ until the light sensor senses daylight tects light in the morning. time.
in the morning. A super-bright white The circuit is designed around the LDR2 and associated components gen-
LED is used as the night lamp. It gives popular timer IC NE555 (IC2), which is erate the morning alarm at dawn. LDR2
bright and cool light in the room. When configured as a monostable. IC2 is acti- detects the ambient light in the room at
the sensor detects the daylight in the vated by a low pulse applied to its trigger sunrise and its resistance gradually falls
morning, a melodious morning alarm pin 2. Once triggered, output pin 3 of IC2 and transistor T1 starts conducting. When
sounds. goes high and remains in that position un- T1 conducts, melody-generator IC UM66
The circuit is powered from a til IC2 is triggered again at its pin 2. (IC3) gets supply voltage from the emitter
standard 0-9V transformer. Diodes D1 When LDR1 is illuminated with of T1 and it starts producing the melody.
through D4 rectify the AC voltage and the ambient light in the room, its resistance The musical tone generated by IC3 is
resulting DC voltage is smoothed by C1. remains low, which keeps trigger pin 2 amplified by single-transistor amplifier T2.
Regulator IC 7806 gives regulated 6V DC of IC2 at a positive potential. As a result, Resistor R7 limits the current to IC3 and
to the circuit. A battery backup is pro- output pin 3 of IC2 goes low and zener diode ZD limits the voltage to a
vided to power the circuit when mains the white LED remains off. As the safer level of 3.3 volts.
fails. When mains supply is available, the illumination of LDR1’s sensitive window The circuit can be easily
9V rechargeable battery charges via diode reduces, the resistance of the device in- assembled on a general-purpose PCB.
D5 and resistor R1 with a reasonably creases. Enclose it in a good-quality plastic case
constant current. In the event of mains In total darkness, the specified LDR with provisions for LDR and LED. Use a
failure, the battery automatically takes up has a resistance in excess of 280 kilo- reflective holder for white LED to
the load without any delay. Diode D5 ohms. When the resistance of LDR1 in- get a spotlight effect for reading. Place
prevents the battery from discharging back- creases, a short pulse is applied to trigger LDRs away from the white LED,
wards following the mains failure and pin 2 of IC2 via resistor R2 (150 kilo- preferably on the backside of the case, to
diode D6 provides current path from the ohms). This activates the monostable and avoid unnecessary illumination. The
battery. its output goes high, causing the white speaker should be small so as to make
The circuit utilises light-dependant re- LED to glow. the gadget compact.
ELECTRONICS FOR YOU DECEMBER 2003