What is it?

The schematic of the device is based oh the 555 timer in monovibrator configuration. The time is set to around half of minute. A preamplifier done by NPN small signal transistor is put in the front of the timer. The inductance with value ~ 100uH detects higher activity of the phone when ringing and triggers the timer. It seems that two physical principles works with this device:

• list text hereWhen the phone is called it communicates with the main station or the GSM cell switching his transmitter on higher power and these signals are detected by the coil, converted to AC current and amplified by the preamplifier.
• list text hereWhen called the phone speaker starts to produce sound. The coil of the microspeaker creates electromagnetic waves which are caught by the coil and converted to electrical signal

Why did you make it?

My mother lives a great distance from me. She is old and can hardly hear. When I call her at the phone, she rarely hears it. To improve things, I decided to design a device that gives light signals when the phone rings. I usually call her in the evening when she's watching TV. This device located next to the TV would attract attention and the probability that she will find that the phone is ringing is higher.

What makes it special?

Very simple construction using cheap and widely spread parts. The sensing coil is embedded directly in the PCB.