I need to build a device that would help a hearing impaired person to hear when the cell phone rings. Old phones use quite high-frequency ringtones ("non-polyphonic"/mp3) and the person doesn't want to switch to a newer phone.
So I am thinking of a device that has a LDR resistor that detects when the phones display backlight lights, and the signal drives a transistor that gives power to a microphone amplifier. The phone would have to be upside-down so that the room lighting can't get to the LDR. A microphone picks up the ringtone sound from the phones internal speaker and amplifies it to a bigger speaker. I think of using a simple LM386 amplifier and a small speaker. When the backlight shuts off the amplifier shuts down again, so that it doesn't amplify all sounds in the house.
The phone has to be on a plate that has a switch that senses if the phone is there. When the phone is lifted away it should disconnect the speaker or shutdown the entire circuit.
I have tested this circuit and it works quite fine, but I would like to hear if someone has better ideas? First I was thinking of something that detects the microwave signal but I'm not sure it is reliable enough...
Another thing: I would like to have a led on the device which would light up/blink after someone has called/sms:ed the phone, and it should keep lighting or blinking until the phone is lifted from the plate. I'm thinking of using a 4013 flip-flop, can this be used? It would be triggered directly when the LDR receives light from the display, and resetted when the phone is lifted from the plate. Would this work?