Audio to LED circuit.

[elektrobank]

I'm looking for a circuit that will do the following. It will plug into a headphone jack and will light up an LED when any audio signal is outputted. The brightness of the LED does not vary depending on the strength of the audio output or the volume of the device, it will always be on when there is any audio signal and off when there isn't. I will be sending it short pulses of audio, basically to make it blink at a certain intervals. The specs of the LED are: 1.2VDC 29mA. I'd like to try and use the smallest size battery as possible.

I've done a lot of circuit building, but not much design, so I would consider myself a bit of a newbie. Any help?

 

[audioguru]

An infrared LED is about 1.2V but you can't see it.
A red LED is about 2V.
A green LED is 2.2V for a dim old one or 3.5V for a very bright one.
A blue or white LED is also about 3.5V.
Which LED?

You need an amplifier, a rectifier then a transistor to turn on the LED.
The LED needs to have a current-limiting resistor.
A small battery will last for a couple of minutes before it is dead.

 

[elektrobank]

Thanks for specifying the part, can anyone provide a schematic? The LED isn't going to remain lit all the time, so I don't think it needs a very strong battery, and I'd like to keep the circuit compact, so I'd like to use as small a battery as possible. It's going to use an IR LED. Any help?
Thanks!

 

[audioguru]

You need to have an electret mic which draws 0.5mA all the time.
It needs to have a preamp which is made from a low voltage, low current opamp.
I don't know any good opamps like that because they have very poor audio performance.

An LM10 opamp operates with a supply as low as 1.1V. At 3V its idling current is 0.3mA.
It is noisy and has a frequency response to only about 3kHz.

The opamp drives a transistor that turns on the LED whenever audio is present.

A lithium coin cell would power the circuit and its life depends on the length of time the LED is lighted and the length of time the circuit is powered.

A CR2032 lithium coin cell would power the idling circuit for about 170 hours (one week). The current in the LED will quickly kill the little battery.