Getting Practical

After building the square wave oscillator shown in the last post I found I couldn't get it to work. Turns out I have completely misunderstood some basic principles of schematics. I had assumed that points marked V+ went to the power supply (in my case a 9v wall wart type affair) and V- went to ground. Turns out that I was wrong, and there is such a thing as a negative voltage! After a bit of research I think I kind of understand it. Basically imagine you have two 12v batteries. if you wire them up in series you have a 24v supply, with 0v (what I would previously have used as ground) at the negative terminal of the first battery and +24v at the positive terminal of the second battery. But if you make ground the point between the two batteries, then instead of having +24V at one end and 0v at the other you suddenly have +12v and -12V.

This is still a problem for me as I only have this poxy 9v supply, and since my multimeter arrived in the post I've found out that it's actually outputting more like 15v!! Not to fear, I have a brilliant idea...

There's an old PC in my wardrobe that hasn't been in use for some time. Checking online I've found that ATX power supplies have cables to supply +12v, -12v, +5v and ground connections, so it could make the ideal poor mans hobby supply for my project!


Here is the pinout for the standard ATX connector:

My Supply had two sets of Molex connectors coming from it, so I cut one set off and used it to extend the other set to reach the top of my desk. I then ran a cable from pin 14 of the ATX connector as well. In order to make the supply switch on I had to put a jumper between pin 16 and 17. I used a piece of metal from inside a discarded Molex connector but any bit of wire you have lying around should do the trick.

Here I was glad I'd sprung for the nice breadboard with the screw terminals at the top, as I could connect all the cables there nicely, with colour coded screw on bits! the green terminal in the pic is connected to the audio in of my computer so that I can get a good listen to whatever oscillations I produce.

 

With this setup firmly entrenched on my desk I set about trying various circuits out, starting with the Square Wave Oscillator I mentioned last time. As I mentioned in my first posts, I'm typing all this retrospectively, So I don't have the circuit set up to take pictures anymore, but I built versions of it with the TL084 op amps and with the UTC4558s I found in the karaoke machine and both worked well. 

So! That's the square wave covered, now I just need Triangle, Sawtooth and Sine, and I'll have a VCO...ish. I wonder how you implement CV inputs?