Is it possible to rig an LED to a headphone jack?

[skipper si]

This may sound like a stupid question and I'm probably setting myself up for a lot of abuse but is it at all possibly to rig an LED to a standard headphone jack so it lights up when you plug it into a socket?
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Si
www.81art.co.uk

[awright]

Yes, but it may not give you the results you want. There are several subtleties involved.

If you intend to have an LED plugged in at the same time you are listening you will probably introduce some distortion. Whether it is audible or not in the main channel depends on the manner in which the earphone socket is fed from the main amp and what resistance you have in series with the LED.

To get light output on both positive and negative signal peaks and to avoid biasing any capacitor that may be in series with the phones output (I don't know if they are there) and to avoid excessive reverse voltage on the LED you will probably want to put the LED inside a rectifier full bridge. You can avoid shorting the two signal channels together by providing separate half-bridges to each channel output and the completion half bridge to the ground connection (a total of six diodes). I also do not know if this is advisable to avoid damage to any headphone driver amp, but it seems like a good protective measure.

Different color LEDs have different forward voltage. Select LEDs with low forward voltage because you are going to be working with very low voltage signals - possibly too low for light action at comfortable listening levels. For the same reason, use Shottky diodes in the rectifier bridge. They have about half the forward foltage of ordinary diodes.

Provide an adjustable variable resistor in series with the LED inside the bridge to limit LED current and maximize load resistance seen by the headphone amp. A first guess would be a 500 ohm or 1K variable resistor in series with the LED inside the bridge. Probably a good idea to also provide 47 ohm reisitors in series with the output terminal of each channel to limit current somewhat if you reduce the pot to zero.

Then just try it out and see if you get the effect you want. I did a quick-and-dirty test with a red LED off the floor and ordinary silicon diodes and a 1K pot. On transient peaks of music I got bright LED flashes from the headphone output of my Sony receiver, but nothing at comfortable listening levels. You would get better results with Shottky diodes and an LED selected for low forward voltage, but it may still not satisfy you. But try it out. You can't do any damage if you provide the 47 ohm resistors in series with the headphone amp outputs.

If the setup recommended above does not give you the results you want you may have to resort to amplification to provide LED drive at lower signal levels.

Have fun.

awright