This is a simple circuit which will give you some visual indication of what your modulations are doing. There are 6 identical circuits on the board. The input op amp is wired as a comparator which outputs a high or low signal dependent on the input voltage and the reference voltage. The second op amp is a simple inverter to light up the opposing LEDs when appropriate. By default we recommend creating the reference voltage with a 100k and 1k resistor divider, resulting in a switch point of about 100 millivolts. You can of course change this voltage divider if you prefer the switch point to be higher. Use 100k and 10k for a switch point of about 1 volt or 100k and 27k for a switch point of about 2.5 volts. You can find this reference voltage divider close to the power header and it's 1nF capacitor. 

All of the SMD pads on the top board are for the LED current limiting resistors. We recommend 4k7 for standard brightness LEDs. 

Building the module is straightforward. Start with the lowest components such as resistors and capacitors, then ICs and diodes. Next solder the LEDs and I/O jacks. When you have placed all the components place the board connecting pin headers in their homes and place the two boards together while soldering the pin headers to ensure correct alignment between the boards.

Do note two small errors in the solder mask on the first run of boards. On the lower board one 100k resistor label 'R' has floated away from its home a bit - it should be obvious which one. And on the top board we placed the solder mask for the board connecting pin headers on the wrong side - the pin headers go on the bottom of the board not the top.

Also note that the jacks on the left and right side are connected directly to one another so do not try to mix signals in this module, as that could potentially damage your other modules. If using the left side as input, the right side is now output and vice versa.