Welcome to Robot Dialogs. This is a place where I will record my progress, mistakes, errors, failures, lessons learned and hopefully occasional success in my designs and implementations of robots and other technical endeavors. I will try to make it as educational, informative and entertaining as I can...

Wednesday, January 18, 2023

Eurorack Voltage Detector

 This is a Eurorack Voltage Detector module design. The idea for this comes from combining two previous modules I designed; the Level Display module and the recent Comparator module. This module uses op-amps in the comparator configuration to compare a signal against several reference voltages and display the output of the comparison on an LED. 

The voltage levels compared against are approximately: ±9.8V, ±4.9V, ±3.2V, ±0.9V

The module has two independent sets of comparators so two different signals can be plugged in at the same time. 

The main use for this in my rig is when interfacing modules from various manufacturer's the acceptable input voltages aren't always the same and it's annoying to have to look that up for each module. I usually remember to add the output voltage range in the engraving on my panels so there's no guess work, others don't usually do that. This module will let me quickly see what voltage a signal is at and attenuate/amplify it as needed before plugging it into something else. The Level Display module, one of the first I made for myself, is better for AC (audio) signals and designed for showing the level in more common decibels like in most audio amplifiers. It also used an Arduino Pro, this module however can be used for AC and DC signals and only uses op-amps.

I started the design with some simulations and a prototype on the breadboard (only a few voltages on the breadboard version though, not the entire circuit).

After that I had two PCBs made and designed the front panel.

Overall the design has worked as planned. There may be additional uses for the front panel and the 2.7k LED resistors use a lot of current but that's easily tweaked with different values.

Module Width: 3 HP
+12V Current: 45mA LEDs off, 155mA LEDs on w/ 2.7k resistors
-12V Current: 45mA LEDs off, 153mA LEDs on w/ 2.7k resistors

Design Files: voltage_detector

Eurorack Comparator Module

 A Eurorack Comparator module in 6HP.

Like the Analog Switch this module comes from my journey to create a noise gate with modules. Most of my attempts to make a monolithic noise gate have required some form of comparator to determine when the signal goes above (or below) the noise floor, so making it an independent module is likely to be useful.

The core of this module is a dual comparator LM393. The input signal is rectified by the op-amps that buffer the input so both inputs on the comparator are operating in the positive range but one of them is the negative half of the input and the other is the positive half. This creates a window from -LEVEL to +LEVEL where the comparator outputs GND and if the input goes above/below ±LEVEL the output is 10V (or 5V if desired).

I made several simulations of the circuit to improve idle noise issues and allow adequate adjustability of the desired level to detect. The latest simulation is here. There's a simple RC filter on the signal input with a19.4kHz cutoff frequency. This helps reject any non-audio signals some of which I found would trigger the comparator. A more robust filter could probably be created with the two op-amps used to invert/buffer the input, but I didn't feel like going down that rabbit hole.

The LEVEL the comparator checks against is voltage controllable or can be set with the knob. There are a few trim pots on the PCB to allow the output to be trimmed to the desired range(s). I like signals to be 0-10V, but 0-5V should also be possible. The logic outputs are both 0:+10V and make good inputs to the Analog Switch module or as gates to some other module.

The module started as usual on the breadboard.

Once satisfied with the design I had a PCB made and designed a front panel for the module.
Issues on v1 PCB:
  • Negative of the LM393 should be to GND like on the breadboard... cut the trace to -12V and short C8 to GND with a 0 Ohm.
Calibrating the output takes a few simple steps:
  1. Adjust the LEVEL knob to mid-range and provide an input signal that can trigger the comparator.
  2. Adjust the output voltage of the INV. OUT output to the desired 0-10V level with trim pot A (R_ADJ100).
  3. Adjust the output voltage of the OUT output to the desired 0-10V level with trim pot B (R_ADJ101).
Module Width: 6 HP
+12V Current: 12mA
-12V Current: 3mA

Design Files: vc_comparator

Eurorack Analog Switch Module

This design comes from a long term project/idea to make a noise gate for Eurorack. I've attempted several designs for a noise gate over the last year but haven't been happy with the result (yet). In the latest attempt I used an analog switch and a comparator and had some success. I decided that those two parts of the noise gate would be fine as individual modules so that's where this came from.

It's really a very simple module, there's a buffered input and ouput and a gate input to turn the switch on or off. When it's on the audio or whatever input signal is passed through to the output.

The schematic calls out the DG418LDJ (normally open) analog switch. The DG417 should work as well for a normally closed version.

The gate circuit is using a few generic NPN transistors to turn whatever the gate input is into a sharp 5V logic signal for the analog switch. There's a simulation here.

Of course the design started on the bread board.

Then I designed and ordered a few PCBs and a nice front panel for it.


Issues on v1 PCB:
  • So far the only issue is that any input over 7.8V is clipped. This is probably something to do with the internal rail limits in the DG418LDJ. To fix that v2 could attenuate the input and then use more gain on the output to bring it back to a known level. That might introduce noise but making a circuit to allow that option is pretty easy. I likely won't bother with this anytime soon.

Module Width: 4HP
+12V Current: 18mA
-12V Current: 5mA

Design Files: analog_switch