Cell phone ringtone amplifier

[multiport]

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?

 

[audioguru]

When the microphone hears its own speaker output then acousical feedback howling will be continuous until it is turned off.

 

[multiport]

Yes I also tought so but I tried it with a simple LM386 amplifier connected directly to a microphone from an old landline telephone and it sounded really okay, no "beep sound" at all, only if I put the mic and the speaker really close to each other. Maybe the mic is so directional that it won't receive the sound from the speaker.

I could also use some kind of buzzer or a circuit that makes a sound, but how would I get it to ring only when someone calls the phone? The screen on the Nokia 1100 lights up for like 10 seconds also when the battery is empty or it receives a sms. Otherwise I could have it ringing all the time when the screen lights up.
I tried the circuit on this page but it didn't work reliably. Maybe the coil wasn't good enough. I also tought that it would ring also when the phone just sends some information to the GSM accesspoint tower, that would be really disturbing, but maybe a 3 seconds delay would help to that problem.


Do you have any ideas for a better solution than a mic with an amplifier?

And would a 4013 work as a light-triggered led, triggered directly from a transistor which the LDR is connected to?

 

[audioguru]

Yes I also tought so but I tried it with a simple LM386 amplifier connected directly to a microphone from an old landline telephone and it sounded really okay, no "beep sound" at all

An LM386 csan have a low gain of only 20 or a high gain of 200. yours probably had the low gain.

Maybe the mic is so directional that it won't receive the sound from the speaker.

The datasheets for most cheap electret mics show that they have a very wide pickup from all around, even from the rear.

Most people with hearing problems wear a hearing aid.

 

[multiport]

I'm using the gain 200 version, but the mic (from an 25 years old Siemens telephone) is maybe not so sensitive.

The person which would use the device has a hearing aid but wouldn't need to use it constantly if the phone would have a louder ringtone, or one with a lower pitch.