I needed a Function Generator for testing. Usually I just whip one up on the breadboard. I have had a number of these instruments over the years but they mostly gathered dust as I didn't use them that much. I had these great little Exar 2206 function generator IC's hanging around and fresh off of my experiments with pulse width modulation tests. So I figured "what the hey" why not build a portable one that II can use more often. As can be seen from the photo on the left the frequency range is limited from 125Hz to 20KHz. It will generate triangular, sine waves and square waves. The sine wave will have a max peak of 3 Volts and the yellow area will generate a 3 Volt peak for a triangular waveform. The square wave is independent and is a good waveform at 12 Volts peak. I choose to run the function generator at +12 Volts DC.

And like I have said before if you build a nice circuit into a proper box you will use it more often and it doesn't end up in the junk box so easy. As always consult the PDF for great details as maybe you will want to try this on your own.

I built a nice box for this project from PVC plastic (scrap from electronic surplus). The front panel was drawn out with AutoCad, then printed on glossy photo paper, and finally glued to a flat piece of plastic. As you can see this is a great way to drill the holes for the rheostats, switches and terminal blocks.You can get them in just the right spot and centered. Even with all the planning I did, I forgot how I was going to fasten the top panel to the box, thus the junky looking "L" brackets I glued on. But "what the hey", it is mine and I can do it any way I want to. It was also good to do the front panel mask up in AutoCad , because when I prototyped the circuit up on the breadboard I was able to get accurate marks for the amplitude and frequency knobs. It was especially hard in this case because I didn't have any linear pots at 50K only log pots. Thus you can see the gaps in the markings for the frequency and amplitude pots on the front panel.
I use AutoCad to do the layout for the 0.1" spacing of my perfboard.When you are getting ready to solder your components on the board you tend to spend some time moving them around trying to get a workable placement. It only takes a few minutes with AutoCad to so most of that work. Wires wires everywhere makes it look messy, but a with a good solder job, and building it sturdy, it is as good as a commercially produced jobbie. Again it is my box, and I don't intend to make a million of them to sell or anything. I love those little terminal blocks on 0.1" spacing. They are one of the few components I have bought recently. They are real cheap but oh so useful. Pretty simple circuit when you see it laid out like that, with the Exar 2206 DIP IC, 4 resistors, a trimpot and 2 small capacitors.

The circuit schematic on the left is fairly simple. the key component is the timing capacitor C2 a 0.1uF ceramic capacitor. You can consult the data sheet for the Exar 2206 . All frequencies generated are keyed to this timing capacitor. The frequency is set from pin 7, with the R15 330R resistor and the external R14 rheostat. As mentioned I didn't have a 50K linear rheostat and was stuck with using a 50K Logarithmic pot. This caused me much consternation.The square wave is generated from pin 11 via the pullup resistor R13. this is generated all the time at the frequency set. The amplitude control is via pin 3. First R11 and rheostat R12 divide the supply voltage in half. I placed a rheostat R12 into the divider just so I could fine tune the DC level. The output sine or triangular waveform rides on this DC level so you can get your + and - swing. Again the square wave is unaffected by any amplitude adjustments. the sine or triangular is produced at pin 2 depending on the switch setting between pin 13 and pin 14 and resistor R16.

I have still not checked out all the uses of this great little Exar 2206 IC. One other function that might be useful is the FSK generation feature. By hooking another timing resistor to pin 8 (not shown) and toggling the FSK input pin 9, you can produce the second tone for FSK generation. Pretty cool little IC eh?

With this circuit I didn't follow some of my commandments for good circuit design. I don't have a fuse for protection in case I put the main power in reverse, which I have been known to do. I should have at least put a diode in for reverse protection. I also didn't put in any Leds for indication that at least I have power to the unit. All in all it is fairly fail proof and easy to use, and I think Exar is still producing them so they must be a fairly useful and popular IC.

I tryed out the Exar 2206 as a Pulse wdth modulator and the circuit on the left worked great.

Ued as a speed control for a DC motor it accelerated the DC Motor very smoothly.

I Think I will get a few more of these chips ---that is if they make them anymore!

Here is the PDF for the circuit only.

Revised 2013 by Larry Gentleman