One of those modules you should want in your arsenal when starting a "DIY modular adventure" is the Low Frequency Oscillator: even the simplest LFO, when used with taste, can give your patches that movement and "life" they need.

In this article I will show you the simple module PCB I designed.

There we go!

Features and Description

This is a simple LFO circuit with control over waveshape and frequency I found while lurking the net. The original circuit design is by user nicolas3141 at electro-music.com forum and was posted in 2007. HERE the link to the original post.

The project is probably the one with few components between those I have realized at today for my modular synthesizer, but nonetheless is very effective in doing it's job.

LFO functions are limited to the frequency control and waveform shape. Main shapes are square and triangle. "Shape" on the square wave acts on the duty cycle, but it's the triangle wave that deserves more attention: it can be shaped continuously from backward saw to triangle to forward saw.

Both waves can be used at the same time.

The opamp I used is a TL074 quad opamp, while Nicolas used an LM324 in his project. These are pin-to-pin compatible so you can use the easiest to source or cheaper in your area.

Nicolas reports a different peak-to-peak voltage value depending on the LEDs color, and I can confirm: use blue LEDs to have +/- 3V voltage range, or white for +/-2.5V.

The 1uF capacitor (C1) must be non-polarized. Changing its value will change the frequency range of the oscillator (bigger -> lower frequencies, smaller -> bigger frequencies).

The circuit works on +/-12V dual voltage, but should also work on +/-15V.

I placed a voltage invertion protection diode between the +12V and -12V lines.

Tests

Attached pictures show some picture of LFO's characteristics by changing frequency and waveshape.

Triangular wave can be shaped from a falling saw to rising saw with continuity. At lowest frequencies (< 150 mHz) the signal goes flat, so probably it could be a nice idea to add a resistor to modify a little the low frequency behaviour.

Square wave changes it's duty cicle by turning the shape potentiometer.

I have used white LEDs, so the voltage amplitude is close to +/-2.6V.

As you can see, both waveshapes are not DC biases at any frequency.

I could measure frequencies up to 9.5 Hz and down to 150 mHz with both waveshapes, which is more than adeguate in my opinion.

I have made some testing by changing potentiometers values (I was running short with 100K ohm pots and I had 10K ohm potentiometers by hand, soo...). Reducing shape potentiometer makes shaping less effective, especially at lower frequencies. Changing the frequency potentiometer had less evident effects: the range was the same, but this was also affecting the shape behaviour. I also noticed a strong effect on DC biasing on the square wave, so I cannot suggest to use potentiometers other than 100K ohm.

PCB and BOM

All components values are silkscreened on the PCB to make assembly easier.

The board is intended to be mounted perpendicular to the front panel. If your case is not deep enought, you can lock the board on the case bottom and use wires to gain electrical connection with elements (potentiometers, buttons, jacks and so on) on the front panel.

List of components:

Capacitors

- 1x 1uF electrolitic

- 2x 100n electrolitic

Resistors and Potentiometers

- 2x 100K ohm single turn, mono, linear potentiometer

- 3x 1K ohm resistor

- 1x 100K ohm resistor

- 3x 10K ohm resistor

- 1x 47K ohm resistor

Diodes and LEDs

- 2x 1N4148

- 1x 1N4004

- 2x LED (blue)

Amplifier

1x TL074 Op-Amp

Others

1x (5x2) IDC connector