Here is a White LED Lamp that turns on when the telephone rings. The cool white light helps to locate the phone in darkness and also helps to note the message or telephone number. The lamp remains on for three minutes then turns off.
The telephone lamp circuit is powered by a 9 volt PP3 battery and connected to the telephone lines. Capacitors C1,C2 and resistors R1 and R2 converts AC signals to DC to trigger the NPN transistor T1.When the phone is on hook, there will be around 48 volts in the lines which reduces to 12 volts when the phone is Off hook. Capacitor C1 and C2 reduces the line voltage to low value DC and diode D1 rectify the AC signals.
The lamp circuit is built around the timer IC555. It is designed as a monostable timer to turn on the White LEDs for three minutes. Resistor R6 and capacitor C4 determines the time delay. When the telephone rings, transistor T1 triggers and its collector turns low. This triggers the monostable and its output becomes high to switch on the White LEDs.