There are many 555 based noise making circuits out there in the DIY noise and circuit bending community. Its popularity is partly due to the fact that it is a simple chip to use; it provides all the facilities for a square wave oscillator. But it is also due to the fact that there are countless 555 schematics out there churned over and over from book to book and now blog to blog who’s first incarnation begin probably in the era of ham radio or on the graph paper pages of a Forrest Mims book. It makes one wonder if the 555 timer is so used not because it’s the right tool for the job but more because it’s what’s in the prevalent circuit designs, leading to a kind of stagnant force of habit in the community. My main complaint about these 555 circuits is that they can only generate square waves. Saw tooth waves sound much meatier especially when run through a low pass filter. They are the perfect wave form for subtractive synthesis. Further a sawtooth waveform can easily be converted to a square wave or a triangle or even a sine wave. This is not the case for a square wave. At the heart of your typical electronic synthesizer you won’t find a 555. You will find instead a voltage controlled sawtooth oscillator. But these synth schematics are sophisticated and inaccessible to someone just getting into electronics so they don’t lend themselves to the kind of viral proliferation of say the Atari Punk Console. I decided to see how hard it would be to design a simple sawtooth relaxation oscillator using a common TL082 dual op-amp chip, which like the 555 can be found at RadioShack. The only other active components are a diode and an MPF102 JFET transistor, also available at RadioShack.
I used whatever components I had lying around and scavenged others off boards from my junk pile. Here is the final schematic:
The theory of operation is as follows: The first op-amp (left) charges the capacitor in its feedback loop. The output of the op amp is positive going and increases from 0v to 15v linearly, at a rate dependent on the negative current through the input resistor. But before it gets to 15v the second op amp, acting as a comparator, discharges it. The moment the output of the first op amp exceeds 7.5v (half the supply voltage) the second op amp switches its output from -15v to 15v. This fires the JFET which shorts out the capacitor, reseting the circuit to its original state. The diode prevents the input of the JFET from going above 0v and the resistor to ground pulls it to ground when it’s not being pulled to -15. The capacitor in the second op amp keeps the comparator from changing state before the main capacitor is fully discharged.
Here’s a video of it in operation. The meter is connected to the main capacitor, showing it charging up and then quickly discharging when it reaches the 7.5v threshold. I have a potentiometer hooked up between -15v and ground with the wiper connected to the input resistor – a variable voltage divider basically. This allows me to input any voltage between 0 and -15v to the circuit, which gives a corresponding frequency tone.
Here is an example of the circuit put to practical use as a light sensitive theremin. I replaced the 10k input resistor with a CDS cell (light sensitive resistor) allowing me to vary the frequency with the shadow of my hand
And another one of the light theremin at a lower frequency.
So this circuit was pretty easy to build. It is a little more complicated than a 555 oscillator, but it’s also little more interesting. A 555 is actually a relatively complex device but for the beginner it is usually treated like a black box. This circuit however has all the mechanisms exposed, making it perhaps a better education tool than the 555. The dual supply might be a little off-putting, requiring either two wall warts or two 9v batteries. Could this be modified to work with a single supply? One thing is for sure though, it sounds good, I dare say better than a 555 circuit. One more important point, with this circuit frequency modulation is possible. We could build a few of these circuits and be able to generate very rich FM sounds. We could also build a slew limiter for the input, or a random signal generator. The possibilities are virtually boundless with voltage control. To fully realize these options we would want to invert the input, and perhaps while we are at it scale the input exponentially. This unfortunately calls for another op amp.