In this series, we build a simple LFO module for Eurorack. In the previous step, we designed a dual LED indicator circuit. This will show us the polarity and relative position of the triangle wave. Let’s put it on the breadboard to see if it actually works.
First, let’s simulate the waveform of the LFO circuit. We need a 10V peak to peak triangle(-ish) signal. Here I use my cheapo Chinese Feeltech Function Signal Generator (FY3200S). The frequency is set to 1Hz.
So let’s clip the output of the function generator to a patch cable…
… and plug it in jack socket J1 of the Moffenzeef Eurorack Power Breakout.
I mistakingly patched it to J2 😱
Place 100k terminating resistor from J1 to ground. This is to provide a current limited path to ground in case of shorts or other nastiness.
Power to the op-amp
So now we can connect the ground rails on the breadboard.
Hook up the -12V to pin 4 of the op-amp IC.
Then, connect the 12V rail to pin 7 of the op-amp.
Build the indicator LED circuit
The 10Vpp triangle wave from the function generator goes from J1 to pin 3. This is the non-inverting (+) input of the op-amp.
So now we take the output of the op-amp (pin 6) to the dual LED. Be sure to connect it to the longest lead for the correct polarity. This will give us green light when the LFO swings to a positive voltage and red when negative.
Create a feedback loop from the shortest terminal of the dual LED to the inverting input of the op-amp (pin 2).
Finally, we put the 470 Ohm current limiting resistor from the short lead of the dual LED to ground.
Here’s where I discovered that the triangle wave was patched into the wrong jack. Switched it to J1 et voila… blinkenlights.
That what easy. ☺️
The indicator LED in action
Power it up. Here’s a short clip of the dual LED light in action. On the scope you see the op-amp jumping and skipping the forward voltage range of the LED(s). This provides for a smooth fading action.
Next time we build the final part of the LFO module: the output stage.