VC AVR LFO with variable symmetry. www.haraldswerk.de

Motivation

Voltage controlled AVR LFO with variable symmetry front view.

This is an old project dated back to December 2017. It was intended to learn some basics about the hard- and software of the ATMEGA series from AVR. It is kept simple. Just three analog inputs, one interrupt input and PWM output with filter are used. It is up to you what software you want to run on it. Here I made a simple voltage controlled VCO with variable symmetry. Speed, waveform and symmetry are voltage controlled. So you can change the triangle from ramp up to triangle to ramp down. Or make one halve of the sine very small. See screenshots below. This software was mainly written to test the hardware. To my surprise it worked sufficiently well for a LFO. So I leave it as is for the moment. No fancy accumulation with fixed point arithmetic and increment interpolation. Of course there is a lot room for improving the software. I know.

Specs and features

Voltage control for speed, waveform, symmetry
Bipolar and unipolar output
Square, triangle, sine, ramp up, ramp down waveform
20MHz crystal
19.5kHz PWM 10bit resolution
Runs on +/-15V and +/-12V
Power consumption around 30mA positive, 5mA negative rail

Implementation

Schematic

Voltage controlled AVR LFO with variable symmetry 01 — Voltage controlled AVR LFO with variable symmetry schematic 01

Voltage controlled AVR LFO with variable symmetry 02 — Voltage controlled AVR LFO with variable symmetry schematic 02

Description:

There is not that much to say here. You see a ATMega 328p running on a 20MHz crystal with three overvoltage protected analog inputs and a low pass filtered PWM output.