Penguin Mint Function Generator
This little project arose from the need for a small, solid-state, relatively
disposable audio-frequency sine-wave generator.
Monolithic Function Generator
was a shoe-in choice, particularly since I had
a number of them lying around.
It took some searching for a suitable enclosure before the realization hit
that an Altoids
tin was the perfect size.
About two seconds later a Penguin
tin was substituted, and the project was underway.
(Be advised that those links lead to some thoroughly obnoxious site design.)
The circuit is copied from Figure 12 of the Exar
, "Circuit for Sine Wave Generation with Minimum Harmonic Distortion". A
voltage regulator was added to permit operation with a variety of 'wall wart'
transformers. The board layout was designed in
to fit on a small protoboard, Radio Shack part #
. I prefer to do a full board layout in advance for perfboard projects;
it usually speeds construction, and the final product is neater and more reliable.
If the project ends up being useful enough to copy, a PCB run is trivial.
For layouts destined for perfboard, it's a good idea to arrange component
pins on a .1" grid.
This circuit works well, and does seem useful enough to copy, so here's the
This layout does not include the power indicator (an LED and 1K resistor in
series on the regulated 12V side), and depicts DIP headers for connection to the
outboard components. I opted to solder the connections directly for greater
reliability, and also because there was no room for the headers--the box ended
up being a very tight fit.
A 1-meg pot for timing resistor R, combined with a value of .1uF for timing
capacitor C, resulted in a frequency range of 14.7Hz to 14.28KHz. A 25K pot
was found to allow maximum amplitude before clipping occurred. Peak to peak
output is 11V (non-adjustable) for the square-wave function; 7V for triangle;
and 4V for sine, flat across the spectrum with minor distortion of the square
wave above 10KHz.
Frequency adjustment is very non-linear in the upper range, making precise
tuning difficult. A log-taper pot may compensate for this to some degree.
I plan to add a multi-turn trimpot in series for the time being. Note that
scale linearity was not of particular concern since the XR2206 is not a
reliable frequency standard to begin with. The generator was intended for
use in conjunction with external measurement equipment, hence the lack of
any front-panel markings.
For power I used a generic "wall wart" style 12VDC transformer. An actual 12V
supply would not be sufficient for proper operation of the 7812 regulator, but
this transformer measured around 18V even under load. Cheap wall warts are
notorious for their poor regulation.
Contact: reboots at g-cipher.net
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