In this series, we design and build an LFO module for our Eurorack modular synth. No module is really complete without blinkenlights. In this part, we design the LFO indicator light with a red/green dual LED, choose the current limiting resistor and smooth out the fading with an op-amp.
Adding an indicator light to the LFO module is not that hard. I picked a dual LED which lights up green or red based on the polarity of the voltage. These are basically two diodes in one package.
One thing to do is to calculate the resistor value to limit the current flowing through the LED. Another thing is to come up with a way to gradually light the LED up and back down again.
Choosing the LED current limitting resistor
As every diode, an LED has a forward voltage. These differ from color to color. In our case, it is 1.7V for the green color and 2.0 for the red color. It also needs 20 mA of forward current. The source voltage of the LFO is at max 10Vpp.
The calculation of the current limiting resistor is as follows.
I rounded off to the nearest E12 resistor value which is 470 Ohm.
Design the indicator LED circuit
We use an op-amp in buffer configuration to separate the LFO triangle signal from the indicator light circuit. The dual LED goes at the op-amp’s output followed by the current limiting resistor to ground.
But there’s an interesting catch. 😋
If we place the LED after the feedback loop, the light will be off whenever the op-amp’s output goes lower than the forward voltage. So that’s when the LFO is roughly between 2V and -2V.
However, if we place it inside the feedback loop, the op-amp will try and keep the feedback voltage as close to the input voltage. This results in a much smoother fading of the light.
The effect is illustrated in the circuits below.
The blue scope line shows the voltage drop across the LED when it is placed in the feedback loop. The red line shows how the op-amp reacts to that by quickly ‘skipping’ the forward voltage of the LED. The yellow line displays the normal voltage drop when the LED is placed after the feedback loop.
The effect is maybe not that apparent from the simulator visual, but the difference is quite remarkable. See the next post in the series for a video demonstration.
Next time we’ll build the indicator LED circuit on the breadboard and see it in action.