Cool Mods for the MOTM-800

Here is one of my MOTM-800 envelope generators, modified to add an output LED. Its intensity is a function of the control voltage present at the output. It's very handy for visually seeing the envelope, and for providing a sense of signal flow in a complex patch. Besides, blinky LEDs are cool! See the description below on circuit mounting details, and the very simple schematic.

Also, it would be cool to have a manual gate pushbutton for each envelope generator. There is just space below the LED, between the SUSTAIN and RELEASE knobs for this mod. I haven't tried this yet, mainly because I need to track down a nice miniature pushbutton switch first. The LED mod provides a place to mount this circuit as well. I've included a simple schematic below for the pushbutton which should work.

How to do it:

First, the disclaimer: As with any electronics modification project, you can screw things up if you are not careful. I've tested the LED mod and it works great. I haven't tested the switch mod yet. Proceed at your own risk. If you start adding lots of LEDs to your system, make sure your power supply can handle the extra load.

First, you need to mount a small daughter board. I went to Radio Slack, er Shack, bought their protoboard, and sawed it into strips. I got enough to mod 4 800s. Using 2 6-32 threaded aluminum standoffs (5/16" long works well), replace the KEPs nuts holding the pcb bracket, adding a small flat washer and lock washer between the pcb bracket and the standoff. Then, mount the board on top with short #6-32 machine screws. I mounted the board copper side up, components underneath, so that I could bring the LED leads all the way through the daughter board when mounted, and still solder on top where I could reach. Power for the opamp is brought from the back side of the main board power connector. Be careful you don't short anything.

It's an extremely simple circuit consisting of 1 dual opamp , 1 LED, and 1 resistor. This circuit also could be used most anywhere else you needed a visual indicator of the signal level. For bipolar signals, you'd maybe want to use a bicolor LED as in the MOTM-320. Referring to the schemo, the first half of the opamp buffers the signal, the second provides a current driver to power the LED. The 470 ohm resistor sets the max current to a little less than 10 mA for a +5 volt input. You can connect the input to the OUT + jack to get an output indicator that varies in intensity with the envelope, or to the GATE jack to get a simple gate indicator. Note that if you are using it as an output indicator, and you have nothing plugged into the gate or trigger jack, and the sustain level is set above zero, the LED will glow. This is a good reminder that the 800 does in fact send its DC sustain level to the output when not connected to a gate or trigger.

Dual opamp - almost anything will do, as long as it can sink 10mA. I used a crummy old 1458 I had laying around, and it was plenty fast enough.

LED - I chose green since MOTM-320s use green to display the amplitude of the signal when in a positive range. Use the Lumex part to match the MOTM standard (GREEN:Digikey # = 67-1156-ND, Lumex number = SSI-LXH387GD). If you can't deal with DigiKey, maybe Paul Schreiber will sell you some.

To add a manual gate switch, try adding this.

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