Backup Lamp Circuit

Description: This compact circuit is designed to be cost-effective, particularly useful during instances when the power supply is interrupted, such as while taking a shower. The backup lamp remains off as long as the CdS photocell is exposed to light from either the NE-2 neon bulb or ambient room light. In the event of a power failure at night, the lamp will activate. The 6-volt lamp should be positioned away from the photocell to ensure maximum brightness. The sensitivity can be adjusted by varying the 1-megohm resistor. Most N-channel power FETs, such as the VN-67, will function effectively if they operate well with a 6-volt supply. The NE-2 can be placed directly atop the photocell, ensuring that the leads do not contact other conductors. This design is both safe and portable, although it should be turned off at night when not in use. Alternatively, neon night lights can replace the NE-2, as they have a longer lifespan compared to 7-watt incandescent bulbs, minimizing battery drain. The standby lamp can be housed in an empty jar, with the photocell shielded from the lamp's light. Position the jar near the night light to deactivate the standby lamp in dark conditions. This configuration allows for easy transport, similar to an electronic candle, since the night light is not mechanically connected.

The circuit operates primarily on the principles of light sensing and automatic switching. The CdS photocell acts as a light-dependent resistor, changing its resistance based on the intensity of light it receives. When ambient light or light from the NE-2 bulb is present, the resistance of the photocell is low, preventing the activation of the lamp. However, when darkness falls, the resistance increases, allowing current to flow through the circuit and energizing the lamp.

For optimal performance, the 6-volt lamp must be positioned strategically to avoid directly illuminating the photocell, which could inadvertently keep the lamp off. The adjustable 1-megohm resistor allows for fine-tuning of the photocell's sensitivity, ensuring that the lamp responds appropriately to varying light conditions. The use of an N-channel power FET like the VN-67 is crucial for switching the lamp on and off, as it can handle the required current and voltage levels.

When assembling the circuit, care should be taken to ensure that the NE-2 bulb is securely mounted above the photocell while avoiding any short circuits with the leads. The choice of using a neon night light instead of an incandescent bulb is advantageous due to its durability and lower power consumption, which extends the operational life of the circuit.

The design's portability is enhanced by housing the circuit in an empty jar, which can be easily moved. Shielding the photocell from light emitted by the lamp ensures that the lamp only activates in complete darkness, providing a reliable backup lighting solution. This setup not only serves as a practical emergency light but also offers a unique, transportable lighting option, reminiscent of an electronic candle.This little circuit will pay for itself many times over the first time it is used, especially if you are in the shower. This back-up lamp stays off as long as light falls on the CdS photocell from either the NE-2 or from room ambient light.

So, if the power fails at night, the lamp will light. The 6 volt lamp should be directed away from the photocell to achieve full brightness and the 1 meg. resistor may be varied to adjust the sensitivity. Most N-channel power fets such as the VN-67 will work if they turn on well with 6 volts. The NE-2 may be placed directly on top of the cell but make sure the leads do not touch other conductors.

This design is safe and portable but it must be turned off at night when not in use. Alternately, neon night lights are available which could be used in place of the NE-2. These little round night lights won't burn out like the 7-watt incandescent bulbs which would result in unnecessary battery drain. The standby lamp may be built into an empty jar with the photocell shielded from the lamp. Place the jar close to the night light to turn off the standby lamp when the room is dark. This version may be freely carried away like an electronic candle since the nightlight is not mechanically connected.