Telephone rings in the dead of the night and you grope in darkness for the light switch or may be for the cordless handset. Here is ingenious circuit which makes a light bulb glow whenever the telephone rings.

This is a good application where mains are triggered by triac by the action of an opto-coupler. The circuit is shown in Schematic 31.

Opto-coupler is device where the signal is coupled from input to the out put optically. It offers full isolation for both sides. Usually a LED is placed on one side internally and a photo transistor on the other, all housed in an opaque package similar to an IC. Unlike a transformer, opto coupler also allows DC linking. MCT2E opto coupler consists of an infrared LED (Pins 1, 2) driving a silicon phototransistor (Pins 4, 5, 6) in a 6 pin DIL package. Pinout details are given in Figure .

IR LED across pins 1 and 2 in MC2E opto-coupler is connected to the telephone line with a 47 k resistor. When the telephone rings, LED lights up internally. It is sensed by the phototransistor across pins 4,5 and 6. This drives the gate and the triac fires making the bulb glow.

You are warned that the circuit operates mains. There are only few components to be mounted. Check the pin-out of MCT2E and fix it properly. See that no arcing develops across the tracks or components. Use of optocoupler here, effectively isolates it from mains

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