Simple LFO 1.5

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.



Simple LFO Front Panel
Simple LFO Front Panel

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.

3D view

PCB component side

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.

Potentiometers:

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 plain 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

Detail of Diodes D2 and D3: the white ring is Up.

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

Wiring of the various controls. (V1.5)

Documentation

5 thoughts on “Simple LFO 1.5

  1. Capacitor R2 ceramic 1uF is nowhere to be found at the moment, is there any other alternative? electrolitic use would be easy to solve

  2. Hello! I am currently breadboarding your lfo but the minimun frequency i get is about far from your mentioned 25mHz (would be about 400 seconds period time), i get about 7.5. i am using 1uf capacitor. Any idea what might be wrong?

    Thanks for your schematics!!!

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