Note: I make reference to the Moog Werkstatt schematics throughout. Copyright prevents me reposting them here; they can be found on Moog’s website.
In its original form, the Werkstatt’s own keyboard generates the Gate signal to trigger the envelope, and there is no obvious ‘Gate Input’ on the header. The existing Gate Out can be (ab?)used as a Gate In, but it’s not ideal, because as with most of these header points, anything coming in here isn’t buffered from the internal signal.
Adding a proper Gate In to the Werkstatt is straightforward enough, though a little more involved than the CV input; my approach doesn’t require the cutting of any traces, the only hack-work being the hole in the enclosure for a jack socket. It does require the end of one wire to be soldered to rather small SMT (surface-mount) components though, so you’ll need a suitably fine tip for your iron and a steady hand.
How it Works
The Werkstatt’s keyboard scanner outputs a logic high at U19 pin 3 when it detects a key press. As well as stopping the scan and loading the current key value into a latch (which feeds the VCO CV), this signal is buffered to provide a Gate, and differentiated to provide a Trigger. The Key On signal is buffered inversely by the Schmitt trigger of U14-F before being flipped back positive by U14-D. In order to add our external gate without affecting any other part of the keyboard circuit, we only need to bring the input of U14-D low. In this way, we can use both the Werkstatt’s own keyboard and an external Gate without having to switch between control sources.
The solution is to use a simple NPN in saturation to take U14 pin 9 to ground when its base is taken high. In other words, a positive external Gate will take the gate inverter input low, just as does the keyboard gate detector. Because there is a diode in the way (D14), our added transistor is isolated from the keyboard scanner clock and data-bus, so there won’t be any accidental mis-readings of the keyboard CV.
Another advantage of this solution is that the Werkstatt’s own envelope retains its Gate/Trigger operating modes, as our external Gate also gets differentiated; we are activating the Werkstatt’s envelope, not over-riding it.
The modification takes just four components and a socket, and fits easily on the PCB. The hardest part is soldering the wire from the collector of the transistor to the appropriate point on the Werkstatt’s circuit – I chose to solder it across the connection between R89 and C64, as the two solder points make a convenient place to lay a thin wire and give it a firmer purchase.
I presume you’ll be doing both CV and Gate input mods; the socket ground can be wired to the CV In socket ground, which I wired to a solder tag around the nearby PCB mounting screw (see also the CV Input page).
33k 1/4W 1% MF resistor
100k 1/4W 1% MF resistor
1N4148 signal diode
BC549C NPN transistor
1/8” panel mount socket
These parts are what I had handy. Pretty much any NPN with reasonable gain can be used here, and the signal diode is a generic one.