How to Use a Photocell to Trigger a Brighter Lamp When My Office Phone LED Lights Up

The discussion centers on designing a circuit to detect when an office phone's LED indicator lights up and subsequently activate a much brighter lamp for better visibility. The original idea of directly connecting a photocell (LDR) in series with a lamp and battery is insufficient because the photocell's resistance change caused by the phone LED is too small to power a lamp directly. Multiple contributors recommend using a transistor amplifier stage, such as a single NPN transistor (e.g., PN2222A or BC548), to amplify the photocell signal and drive the lamp. The photocell used is a cadmium sulphide LDR with typical resistance values around 1kΩ in light and 10kΩ in dark, which is not low enough to power a lamp directly. A transistor circuit with appropriate base and collector resistors (e.g., 47kΩ and 470Ω) and a variable resistor (trimpot) for tuning sensitivity is advised. The transistor acts as a current amplifier, allowing a small current from the photocell to control a larger current to the lamp or LED. For initial testing and troubleshooting, it is recommended to use a low-current LED (e.g., a 10mm SuperBright LED rated for 20mA) instead of a filament lamp, as filament lamps require higher current and more complex driving circuitry. The transistor pinout and orientation are important, with the emitter, base, and collector identified for the PN2222A transistor. The photocell should be placed in a light-tight enclosure over the phone LED to prevent ambient light interference. The final working prototype used a 10k trimpot to adjust sensitivity, successfully turning an LED on and off in response to the phone LED. Scaling up to a brighter lamp would require consideration of the lamp's voltage and current specifications and possibly a more robust transistor or additional amplification stages. The use of Darlington transistors or MOSFETs was suggested for higher sensitivity or current handling. The discussion also touched on alternative light detection methods, such as using LEDs as light sensors or op-amp comparator circuits, but the transistor amplifier approach was the primary solution. Components were sourced from RadioShack and SparkFun, and solderless breadboards were used for prototyping. The final device was housed in a custom enclosure with the photocell mounted in a black suction cup pocket and the LED indicator placed with a parabolic reflector for visibility.
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