A super simple automatic light activated switch circuit is explained here. The circuit can be used for automatically switching ON and switching OFF a street lamp or group of lamps in response to the varying ambient light levels.
The unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in.
How the Circuit Works
The circuit can be used as an automatic street light controller system or a simple light activated switch. Let’s try to understand the functioning of this useful yet simple automatic street light circuit and how it is so simple to construct:
Referring to the proposed automatic street light circuit figure 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 power supply is a standard transformer, bridge, capacitor network, which supplies a clean DC to the circuit for executing the proposed actions.
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.