Tap Tempo

[Heinz]

The idea of a tap tempo LFO for tremolo has always intrigued me. Internet research only revealed some microcontroller-based designs, which are very elegant 1-chip solutions but require the ability to program microcontrollers. I used a more basic approach using TTL logic chips, since they are readily available, dirt cheap and don't need programming. This circuit uses 5 ICs and should cost less than 1€ altogether.

It can produce an LFO signal from 0.25Hz to approx. 60Hz (if you can tap fast enough) and has a resolution of about 15ms. The first tap starts the time measurement, the second tap stores the time. If the time between first and second tap exceeds the maximum capacity (about 4s) it will automatically reset without changing the output signal. An optional indicator LED can be added to display the measuring state.

The circuit still isn't quite finished. It needs an analog wave shaping circuit that transforms the square wave into a more triangular or sine shaped signal. And finally, it needs a PCB. I'll keep posting updates when something new emerges.

Note: I have built and tested this circuit

[Shrapnel]

Have you thought of a cap to smooth the transitions?

[Heinz]

Yes, I have tried an RC low-pass filter. It works to some extent but I'm not too happy with it.

The frequency range of the LFO is pretty wide. If I choose a low cap value the signal gets very smooth at low frequencies but is almost completely filtered out at higher frequencies. If the cap is small it works well for high frequencies but the low frequencies remain nearly square. A simple solution could be a value somewhere in between that rounds the signal at low frequencies just enough to sound good and filters just a bit at higher frequencies. But the basic effect remains: square at low and weak at high frequencies.

A more complex solution would involve an active component that varies the filter according to the LFO frequency. I'd like that much better if it is not overly complicated.

[PRR]

> wave shaping circuit that transforms the square wave into a more triangular or sine shaped signal.

Why?

EDIT: oh, wait. You mean the output, not the tap-pad signal.

The output square-edge can trigger a narrow band-pass filter, say F=200Hz Q=7, to give a consistent "thud" at any rate. Like a drummer: the arm-rate is unrelated to the drum-pitch.

[Heinz]

Here's the final schematic with a sine wave shaper. It's digitally controlled RC filter that varies capacitance according to the cycle period. A quad of diodes polishes the resulting triangular signal into a pretty nice sine wave.

I'll start designing a PCB and post it when it's finished.