The automatic lamp described below is a very simple and fully solid state device. Instead of an electromagnetic relay, TRIAC have been used to switch the lamp automatically. The Schmitt trigger stage using two transistors that provides flicker less ON and OFF operation of the lamp.
12 volts DC supply required for the three transistors and the associated light sensing circuit is derived directly from the 230 volts AC supply. The half wave rectification of the mains supply through a capacitor resistor & diode network that provides stable 12V DC supply to operate the transistor circuitry this small and light-weight power supply eliminates to use of bulky step down transformer.
The light sensing circuit consists of a voltage divider comprising of a 100K resistor, a 100K potentiometer and a LDR connected in series as shown in the diagram. When the light falls on the LDR its resistance drop to such a low value that it virtually provides No-Voltage Drop across it. As the sun goes down the intensity of the light falling on the LDR also reduces. This low light increases the resistance of the LDR and the voltage drop across it also increases. The voltage provided across the LDR is fed through a resistor-capacitor T network to provide the required time delay in second scale to the base-emitter circuit of the first transistor T1 of the Schmitt trigger stage. The main reason for providing the above mentioned delay to avoid the abrupt operation of the device due to temporary shadow or reflection of moving objects falling on the LDR. When this voltage drop reaches 4.4 V it brings the transistor into conduction. When the transistor comes onto conduction the voltage across the collector resistor increases and voltage drop across collector-emitter falls. The increase in the collector-resistor devices of the transistor TR3 in conduction and the fall in the voltage drop across the collector-emitter of the transistor TR1 brings the transistor TR2 out of saturation. The change in conducting state from TR2 to TR1 and difference in the ohmic values of their collector resistances causes a fall in their common emitter voltage. This drives transistor TR1 into complete non-conductive state. The conducting transistor TR3 drives sufficient current to the GATE of the TRIAC and the TRIAC switches ON the Lamp. The lamp once lighted it remains ON due to hysteresis provided by the Schmitt trigger action till the intensity of the light falls on the LDR increases sufficiently. When the voltage provided across the LDR falls below 3.9 volts. This feature avoids the flickering of the lamp during illumination.
On completion of the circuit on a vero-board or a PCB, the LDR can be mounted in such a manner that the illumination of the lamp should not be over the LDR and precautions has to be taken into consideration that only day light or indirect sun light should fall on the LDR. Now, adjust the sensitivity control for optimum result.