A simple automatic 220V automatic street light circuit is explained in the following article which is built around a light activated circuit concept. The circuit can be used for automatically switching ON and switching OFF a roadway lamp or group of lamps in response to the varying ambient light levels.
The electrical unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in.
How it Works
The circuit can be used as an automatic day night operated light controller system or a simple light activated switch. Let’s try to understand the functioning of this useful circuit and how it is so simple to construct:
Referring to the circuit diagram we can see a very simple configuration consisting of just a couple transistors and a relay, which forms the basic control part of the circuit.
Of course we cannot forget about the LDR which is the prime sensing component of the circuit. The transistors are basically arranged such that they both complement each other oppositely, meaning when the left hand side transistor conducts, the right hand side transistor switches OFF and vice versa.
The left hand side transistor T1 is rigged as a voltage comparator using a resistive network. The resistor at the upper arm is the LDR and the lower arm resistor is the preset which is used to set the threshold values or levels. T2 is arranged as an inverter, and inverts the response received from T1.
How the LDR Works
Initially, assuming the light level is less, the LDR sustains a high resistance level across it, which does not allow enough current to reach the base of the transistor T1.
This allows the potential level at the collector to saturate T2 and consequently the relay remains activated in this condition.
When the light level increases and becomes sufficiently large on the LDR, its resistance level falls, this allows more current to pass through it which eventually reaches the base of T1.
How the Transistor Responds to LDR
The transistor T1 conducts, pulling its collector potential to ground. This inhibits the conduction of the transistor T2, switching OFF its collector load relay and the connected lamp.
Power Supply Details
The whole circuit can be built over a small piece of vero board and the entire assembly along with the power supply may be housed inside a sturdy little plastic box.
How the LDR is Positioned
The LDR must be placed outside the box, meaning its sensing surface should be exposed toward the ambient area from where the light level is required to be sensed.
Care should be taken that the light from the lamps does not in any way reach the LDR, which may result in false switching and oscillations.
R1, R2, R3 = 2K2,
VR1 = 10K preset,
C1 = 100uF/25V,
C2 = 10uF/25V,
D1 ---- D6 = 1N4007
T1, T2 = BC547,
Relay = 12 volt, 400 Ohm, SPDT,
LDR = any type with 10K to 47K resistance at ambient light.
Transformer = 0-12V, 200mA
Using opamp IC 741
The above explained automatic darkness activated street lamp circuit can be also made using an opamp, as shown below:
Here the IC 741 is designed as a comparator, wherein its non-inverting pin#3 is connected to a 10k preset or pot for creating a triggering reference at this pinout.
Pin#2 which is the inverting input of the IC is configured with a potential divider network made by a light dependent resistor or LDR and a 100K resistor.
The 10K preset is initially adjusted such that when the ambient light on the LDR reaches to the desired darkness threshold, the pin#6 goes high. This is done with some skill and patience by moving the preset slowly until pin#6 just goes high, which is identified by the switching ON of the connected relay and the illumination of the red LED.
This must be done by creating an artificial darkness threshold light level on the LDR inside a closed room and by using dim light for the purpose.
Once the preset is set, it may be sealed with some epoxy glue so that the adjustment remains fixed and unchanged.
After this the circuit may be enclosed inside a suitable box with a 12V adapter for powering the circuit, and the relay contacts wired with the desired road lamp.
Care must be taken to ensure that the lamp illumination never reaches the LDR, otherwise it may lead a continuous oscillations or flickering of the lamp as soon as it is triggered at twilight.