Automatic Light Controller Using 7806
Voltage regulator ICs (78xx series)
provide a steady output
voltage, as against a widely
fluctuating input supply, when the
common terminal is grounded. Any
voltage about zero volt (ground) connected
in the common terminal is added
to the output voltage. That means
the increase in the common terminal
voltage is reflected at the output. On
the other hand, if the common terminal
is disconnected from the ground, the
full input voltage is available at the
output.
This characteristic is utilised in the
present circuit. When the common
terminal is connected to the ground,
the regulator output is equivalent to
the rated voltage, and as soon as the
terminal is disconnected from the
ground, the output increases up to the
input voltage.
The common terminal is controlled
by a transistor, which works as a
switch on the terminal. For automatic
control of light, a light-dependent resistor
(LDR1) is connected to the base
of the transistor. In this way, the voltage
regulator is able to operate a light
bulb automatically as per the ambient
light.
To derive the power supply for
the circuit, the 50Hz, 230V AC mains
is stepped down by transformer X1
to deliver a secondary output of 12V,
250 mA. The secondary output of the
transformer is applied to a bridge rectifier
comprising diodes D1 through
D4, filtered by capacitor C1 and fed
to the input terminal of the regulator
(IC1).
The common terminal (pin 2) of IC1
is connected to the ground line of the
circuit through transistor BC557 (T1).
The transistor is biased by R2, R3, VR1
and LDR1. The grounding of IC1 is
controlled by transistor T1, while light
is sensed by LDR1. Using preset VR1,
you can adjust the light-sensing level
of transistor T1.
The output of IC1 is fed to the base
of transistor T2 (through resistor R4
and zener diode ZD1) and relay RL1.
LED1 connected across the positive
and ground supply lines acts as a
power-‘on’ indicator.
Normally, the resistance of LDR1
is low during daytime and high during
nighttime. During daytime, when
light falls on LDR1, pnp transistor T1
conducts. The common terminal of IC1
connects to the ground and IC1 outputs
6V. As a result, transistor T2 does not
conduct and the relay remains de-energised.
The light bulb remains ‘off’ as
the mains connection is not completed
through the relay contacts.
During nighttime, when no light
falls on LDR1, it offers a high resistance
at the base junction of transistor
T1. So the bias is greatly reduced and
T1 doesn’t conduct. Effectively, this
removes the common terminal of IC1
from ground and it directs the full
input DC to the output. Transistor T2
conducts and the relay energises to
light up the bulb as mains connection
completes through the relay contacts.
As LDR1 is in parallel to VR1+R3
combination, it effectively applies
only half of the total resistance of
the network formed by R3, VR1 and
LDR1 to the junction at T1 in total
darkness. In bright light, it greatly
reduces the total effective resistance
at the junction.
The circuit is
simple and can be
assembled on a small
g e n e r a l - p u r p o s e
PCB. Use a heat-sink
for IC1. Make sure
that LDR1 and the
light bulb are well
separated.
The circuit can be
used for streetlights,
tubelights or any
other home electrical
lighting system
that needs to be automated.
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