PC Speaker to line out?

[Possessed Freak]

Old school here, at work we have a 286 laptop that we got working. For kicks, we are planning to bring this sucker to a tech seminar. I am also running a session during this and wanted to start with a gag of using this machine to run the "powerpoint" (ascii art). As such, we ordered a CGA to VGA converter card... but that leaves audio. Audio is of course the built in PC speaker. I want to adapt this to feed a line out. Since we will have an external box for the video adapter anyway, I could easily house the required electronics there too.

I tried some searches but turned up very little. Any advice on the matter is most appreciated.

Thanks.

 

[imagoon]

Most old school stuff like that is simply a square wave to a speaker. Most of the drive electronics already had impedance circuits built in so you could go speaker + - to an RCA input of some sort. Since the sound quality off a square wave PC speaker isn't going to be much of a concern, you likely could get away with cheap couple of hundred ohm resistor for safety reasons. You may not even need that.

 

[Possessed Freak]

I found this: http://www.epanorama.net/circuits/speaker_to_line.html

Would this work for the little 5v speaker connection?

 

[imagoon]

I found this: http://www.epanorama.net/circuits/speaker_to_line.html

Would this work for the little 5v speaker connection?

Yes. That would voltage divide the output. You may need to tweak the resistors because your source is sub .5 watt and may not even output enough voltage require the divider in the first place.

Please realize that the circuit at that link is designed for converting larger amplifier speaker outputs to low inputs.

The PC speaker won't put out that much to begin with.

EDIT:

Also since this is a laptop, have you verified what the output is? Most of those used smaller speakers or crystal speakers which are lower power and voltage to begin with.

 

[Possessed Freak]

Also since this is a laptop, have you verified what the output is? Most of those used smaller speakers or crystal speakers which are lower power and voltage to begin with.

No I have not verified the speaker yet. So I purchase a pack of resistors and just keep changing them to weaker ones until the line out levels are within spec?

 

[imagoon]

No I have not verified the speaker yet. So I purchase a pack of resistors and just keep changing them to weaker ones until the line out levels are within spec?

First you find out what the circuit input is. You can't do the math to build a proper divider bridge without that in the first place.

Once you have that you need to figure out what resistor ratio will give you a max of 2 volts at the expected max of the input. For example: if the expected input is 10 volts, you would need a 5:1 bridge so (based on the picture in the link) the top resistor would be 5k and the bottom 1k. Since this is low inputs the currents should be low so that is why you would use a 5k vs a 5 ohm and 1 ohm.

I = V/R

I = 10V / 5kOhm so I=0.002 Amps. This should be well with in the limits of the circuit. If you needed more current you can change both resistors to bump it up. For example only (you will never use this one)

I = 10V / 5Ohm so I = 2 amps.

Please note I am ignoring bridge current (the 1k does draw some etc) but that is more detail than is really needed here.

5V would be a 2.5:1 bridge
2V is 1:1 etc.