DIY Function Generator

By Paul Marossy
Last updated 06/16/03


I just finished building this project. Although there are kits you can buy, such as the one offers (which produces a pseudo-sinewave), I thought I'd build my own instead just for the challenge of it.
circuit is simple. It uses a single IC chip, the ICL8038 function generator chip which produces simultaneous sine, square and sawtooth wave forms. There are relatively few parts in the circuit - just two resistors, one transistor, five trimpots and the function generator chip. The ICL8038 can operate on a unipolar power supply between 10 and 36 volts DC, and it can also be used with a bi-polar power supply.
I built it per the schematic and it works, except I had to change the 1M resistor between the power supply and the square wave output to a 2M trimpot to get the waveform to function properly and be at the right amplitude in comparison with the sine and sawtooth waveforms. Check out the pictures below.

This is the top of the circuit board. I used some non-coppered perfboard I had lying about to build the circuit on. Whenever I use perfboard, I like to mark up my perfboard with some fine point Sharpie markers and get all the connections worked out before I actually construct the circuit. I find it easier to do it this way.
This is the back side. It's a little more challenging using this type of perfboard over the copper padded type.
Here is a view of the front. The enclosure comes from a defunct 4-way data switch box. I gutted it and created some graphics for the faceplate. It measures 7.5"x2.25"x5" deep.

For the frequency range switch, I used a recycled rotary switch from an old parallel port A/B switch box. To make it work with this circuit, I had to disassemble it and rearrange the insides a little bit, but now it does exactly what I want it to. (I know, I could have just bought a new rotary switch, but I had this switch lying around...) Since I am using a single female BNC jack and a single 1/4" jack wired in parallel, I decided to use three SPST switches to switch between the different waveforms. One of the switches will be a on-center off-on type. I figure the middle position would make a nice "kill switch" which will prevent any waveforms from reaching the output jacks. I like the idea, because if I don't want any output, I can just flip that switch and leave the unit powered up. Of course, one could just use another rotary switch with a SPST switch that could act as a kill switch as well. I used the SPST switches mainly because I had a bunch of them lying around waiting for a new home...
This homebrew function generator isn't as fancy or accurate as the ones that are on the market, but for a do-it-yourselfer hobbyist type, it's adequate. I have found that the sine wave isn't totally accurate when I switch between frequency ranges, but I have incorporated a pot which corrects any waveform offsets, so it still quite useable and pretty accurate. Not too bad for a $20 project.


My Guitar Effects Prototyping Board