The Simple LFO is a… simple LFO module for your Eurorack system.
Two waveforms available: Triangle and Square wave. Adjustable amplitude and Low/High range switch.
It’s easy to build, easy to understand and maintain. Only easy-to-find through-hole components.

Description

The Simple LFO module is a Low Frequency Square and Triangle Oscillator, designed to be integrated into an analog synthesizer. It is compatible with the Eurorack format: 10-pin power supply connector and voltages (+12V, 0V and -12V). The frequency, ranging from 2.5 mHz up to 250 Hz (audio domain), is adjusted with the potentiometer R1 (Rate). There is no Voltage Control (CV) input.

Powered at -12 / +12V, the maximum output amplitude swings between -6.3 / +6.3V peak-to- peak for the triangle waveform and -5V / +5V for the square waveform. The output signal
can be attenuated with the potentiometer R9 (Amplitude).

The waveform is selected between Triangle and Square with the switch S2 (Signal). If required, both waveforms can be used, separately, but in this case, only one waveform can be attenuated via R9. Also, there’s a phase shift of 90° between the two signals: the square wave is “low” when the triangle waveform is “going up”, and the square wave is “high” when the triangle wave is “going down”.

The oscillator is based around two operational amplifiers. The first op amp (U1A, R1-R3, C1, C2) is an integrator circuit. The second part (U1B, R5-R8) is a Schmitt trigger comparator.

The output is buffered, insuring a stable frequency, independent of the output load. It can be directly linked to the CV input of any voltage controlled analog module. U1C is the output buffer. U1D drives the LED.

This circuit has been inspired, among others, by:

• ASM-1 LFO
• MFOS Sound Lab Mini-Synth LFO

Revisions

V1.5:

• Simplification: R4 removed
• Output Normalization: Square waveform reduced by half

V1.4:

• Corrected an inversion in the LED circuitry, which prevented the LED to be lit as expected.

Simple LFO is available on Tindie!

Frequency Ranges

Range	Lowest Frequency	Highest Frequency
LO	2.5 mHz (0.0025 Hz)	2.4 Hz
HI	250 mHz (0.25 Hz)	245 Hz

Power requirements

+12V	10mA
-12V	10mA

About the BOM

Many component values are not critical in the Simple LFO. It’s a robust circuit which can tolerate many experiments and modifications of the values.

The 2 input capacitors C5 and C6 can have any value between 1µF and 100µF. A common value is 10µF. One important thing is the minimal operating voltage of C5 and C6: it mustn’t go below 25V.

TL084 or TL074?

The TL08x and TL07x are interchangeable. They are pin-to-pin compatible, the voltage levels are identical.
The TL07x family is a lower noise version of the TL08x family.
However, the signal levels involved in synth modules is probably somehow immune to noise problems.
So feel free to use whichever version you want.

Potentiometers:

• https://synthcube.com/cart/synth-diy/parts/potentiometers/alpha-9mm-round-shaft-pc-mount-pots
• https://www.thonk.co.uk/shop/alpha-9mm-pots-right-angle/
• http://smallbear-electronics.mybigcommerce.com/alpha-single-gang-9mm-pc-mount/
• https://www.musikding.de/Alpha-Pots-9mm

Assembly procedure

This is a step-by-step assembly procedure.

Click on the links below to view in full page:

• Simple LFO V1.5b Assembly procedure (Top view)
• Simple LFO V1.5b Assembly procedure (3D view)

Last minute details

About LED D1

The LED D1 is a dual LED (bi-colored). It is lit in one color for positive excursion of the selected waveform and lit in other color when the signal goes negative.

To address either one LED or the other, the polarity has to be reversed. For example, if pin #1 is positive and pin #2 is negative, the first LED is lit. If you reverse the voltage (pin #1 negative and pin #2 positive), the second LED is lit.

You can use a regular LED instead, and place it in any polarity you want, but it will be lit on only one excursion (either positive or negative) of the selected waveform.

About the diodes D2 and D3

D2 and D3 are Protection Diodes: they protect against reversed voltages on the power connector. You can override the diodes with shortcuts (jumper wires or clipped legs from components).

They are to be mounted vertically, over the circle printed on the PCB, with their Cathode pin pointing Up. The Cathode side of the diode is the one with the white ring.
In other words, the Cathode must be connected to the square pad while the Anode is connected to the round pad.

The 1N4007 can be replaced by any 1N4000 diode (1N4004 etc) or by equivalent Schottky diodes 1N5817, 1N5819 etc. Schottky diodes have lower forward voltage.

Wiring

Videos

Documentation

• Schematics (lfo-1.5b-20210205.pdf)
• Interactive BOM (lfo-1.5b-ibom-2020-10-01)
• LFO 1.5b BOM & doc (LFO-1.5b-BOM-doc-20210228.pdf)

External links

• Reddit / SynthDIY Forum – “NOOB Question about the David Haillant LFO”
• exsertus.com – Modular Synthesizer Project Part 6 – LFO
• lookmumnocomputer.discourse.group – Adding sync to an LFO
• Circuit Lab – Haillant LFO with sync
• SYNTHÉTISEUR MODULAIRE DIY – SIMPLE LFO

The Simple LFO has been inspired, among others, by:

• The ASM-1 Synthesizer by Gene Stopp and Magnus Danielson
• Music From Outer Space Sound Lab Mini-Synth by Ray Wilson

You can find the Simple LFO on Tindie!