Driving LED from sound card

[jeremys]

Hi - I haven't done any electronics for a decade or two, so sorry if I look stupid.

I working on a science experiment which requires a the brightness of an LED to be modulated with a waveform up to around 100-150 Hz, but down to low frequencies, maybe 1 Hz.

The frequency needs to be computer controlled. The easiest solution I could come up with would be to use a PC sound card headphone output to control the LED (my laptop only has headphone, not line-out).

To get the frequency response at low frequencies, I think my input waveform would have to be modulated on a carrier wave (maybe 10 kHz) in my computer program, and then demodulated externally. Maybe a simple envelope detector circuit (sorry link is missing, but it is a diode connected to capacitor and resistor in parallel) would demodulate the headphone output with 1uF capacitor and 1k resistor to (1 kHz time constant).

To drive the LED I suppose I need a transistor connected to the voltage output of this circuit, though I'm not sure on sensible component values.

Does this sound about right? I'm concerned that as the headphone output of a computer is supposed to drive a low impedance output, the simple demodulator might not work very well. It's not clear to me whether the sound card will act as a current or voltage source, and what the expected range in voltages should be. Unfortunately I don't have an oscilloscope so can't measure this.

 

[jeremys]

PS Here is the envelope detector circuit http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part9/page2.html

 

[Resqueline]

The modulation and envelope detector scheme sounds sound. Just put the LED in series with the resistor.
The problem as I see it is that the relatively low voltage one usually gets from the headphone output won't be quite enough to drive a LED directly. (What colour btw.?)

One way around that is using a small step-up transformer (or a step-down in reverse) before the envelope detector. This way you won't need a separately powered LED circuit. The 32 Ohms ouput impedance is a little low to drive a resistorless LED directly, but a transformer would change that.

Without a transformer I'd guess you'd get around a Volt from the envelope detector. Thus you could always use a voltage doubler rectifier circuit, getting 2 Volts, to make it work with a red LED. You'd need to control the volume carefully to adjust the brightness though.

Sound card output is a voltage source, with some resistance in series - thus limiting the available power and current.

 

[jeremys]

modulation

Hi - the modulation was just to be able to get low frequency waves through the sound card. I think the specs say they only output down to 20 Hz. I'll just give an LED a go and see whether it works before trying anything more complex. Thanks!

It was a white LED BTW. The experiment is to examine how sensitive the eye is to flickering.

 

[Resqueline]

White LED's require at least 3 Volts. You most certainly require a tripler rectifier or a transformer.