I cant afford a function generator right now. But I constantly need one and have to keep driving to university lab and spend money on gas. Here is a solution.

1: Download this software:
http://heliso.tripod.com/download/generator/dsg.htm

2: Get some old computer speakers (everyone has some laying around)

3: Open up your speaker system and disconnect the signal wires from the speaker coils themselves.

Now plug the aux cable for the speaker system into your sound card

Now run the software and connect the speaker system signal wires, power and ground, into an oscilloscope.

Now you can use the speaker system's volume knob to adjust the amplitude and use the software to adjust the frequency and waveform type.

The major benefit of this setup is that the sound card is isolated from the output that is going to the oscilloscope. The speaker system's default amplifier is doing the isolation.

If you short circuit the signal wires, the worst that will happen is your crappy old speakers may get damaged, but your audio card is safe. I at least hope this is true. I really hope. When would this not be true? Anyone?

The output amplitude depends on your speaker system. I am getting 6.5V peak max output with sine wave before the signal begins to saturate and get distorted. My speaker system has a power supply of 10 volts. Makes sense.

This is not really meant to be used to drive anything major, but if you need a signal to test with an op amp or micro-controller, this is great. Once again it all depends on the kind of speakers you have.

Enjoy.

 

Very clever! Using audio signals to carry specific waveforms! Brilliant!

 

should be fine for low frequency sine but not so sure about other waveforms. how is the square wave or sawtooth?

 

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The major benefit of this setup is that the sound card is isolated from the output that is going to the oscilloscope. The speaker system's default amplifier is doing the isolation.
...

I have repaired and seen inside a lot of PC powered speakers. There is no proper "isolation" like optical isolation, and the amp ground is usually connetced to the PC ground, which is usually connected to the PC metal case and mains ground.

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If you short circuit the signal wires, the worst that will happen is your crappy old speakers may get damaged, but your audio card is safe. I at least hope this is true.
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Generally this is true and the amp will provide some decent level of protection for the sound card output.

Another benefit is the speaker amp provides some buffering so it makes a more powerful signal level than the PC sound card output itself.

If you want to optimise the protection then another good idea is to put a small fuse (100mA or 200mA) in line with the amp output.