My students presented their final projects on a street corner this semester: sequencers and voltage controlled oscillators.
Read on for a sequencer project layout guide (and more fun video!)
To make crazy sound with this sequencer, combine it with a clock circuit and a VCO.
Clock ----> Sequencer ----> VCO ----> Amplifier
The "clock" send a continuous pulse to the sequencer, pushing it through each of its steps.
Previously the class had built 74c14-based oscillators, which resulted in a roomful of playable synth madness.
Nic Collins provides a great beginner's guide to this project, calling it The World's Simplest Circuit.
More advanced circuiteers might wade through HackADay's Logic Noise, which uses the interchangeable 40106 chip.
To build a 4017 sequencer, Collins suggests starting with the 74c14 (or 40106) chip for the clock.
Use a large value pot, (he suggests 1M, with a 2.2uF cap) and you'll get a very wide range of pulse frequencies.
A 555 timer can also work, and there's a bazillion circuit designs online for sequencers using a 555 clock with a 4017.
However... as a teacher, I've had mixed results with the 555. It's an important chip to know, but wow, is it sensitive! When I've used it in class, the failure rate on finished student projects can reach 50%. (One student told me his high school electronics class had similar problems with the 555.) It doesn't help that our classroom has carpet + radiators = static = dead chip. Hooking up the battery leads backwards can also fry it, and that's a very easy mistake for beginners.
This is a 4-step sequencer based on a 4017 decade counter. There's very few parts, but each step requires connecting three leads on a potentiometer, so it quickly becomes a rats nest on the breadboard.
So skip the breadboard, and go straight to soldering. We did point-to-point soldering on protoboard. Since my students are new to circuit design, I used the free circuit design software fritzing to create a layout for them. In fritzing's breadboard view, you can print out the image, actual size, and use it as. This is a very helpful tool in teaching.
(Not the most compact design: my goal was easy viewing in Fritzing's breadboard view.)
To use the actual-size print-out as a layout guide:
- Download and open the fritzing file here.
- Print it out from the breadboard view.
- Using a thumb tack, poke holes through the two pairs of x's.
- Line the holes up over the matching holes in the protoboard.
- Slide the ends of a piece of bare 22awg wire through one pair of x's, and twist the ends together.
- Repeat for the second pair of x's. Now the paper is anchored to the protoboard.
- Use the thumb tack to poke holes
for all the component leads.
- Presto! You've got a super easy layout guide.
- Just push the component leads through the matching holes, and solder them in place.
Fritzing doesn't show what the underside of the board looks like. So I added yellow lines to indicate what components are soldered together on the underside of the board.
Once you're done, test the circuit:
- Set the clock to a leisurely speed (an LED in parallel with the clock output makes it easy to check speed). I suggest 60 bpm, or slower, to give your multimeter time to measure.
- Plug the clock output into the sequencer.
- Turn all the sequencer pots completely down.
- Plug an audio cable into the output, and clip your multimeter to the cable to measure voltage - tip to red, sleeve to black.
- With all the pots turned down, you should get zero voltage. Turn the first pot up all the way. When the sequencer reaches that pot's step, the voltage level should shoot up.
- Turn the first pot back down and check each of the remaining pots the same way.
The output of the sequencer goes to a voltage controlled oscillator (VCO). The VCO produces various pitches, depending on the voltage level it receives from the sequencer.
Collins connects his sequencer to a 4046-based vco and a number of my students did that. You can hear the pure tones that result in the first video.
However... if you don't have time for a third chip...
A 74c14 synth can be turned into a "VCO".
- Add an audio jack in parallel with the battery of the 74c14 synth.
- Make sure the tip lead goes to the power bus and the sleeve lead goes to ground.
- Leave the power off (or disconnect the battery), and instead plug the VCO into the new jack.
(That was definitely not a problem for these guys.)