Wednesday, April 3, 2013

Simple Automatic Street Light System

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.

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.

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.

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.

 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.

Parts List

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, 200m

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