Sunday, December 18, 2011

VC Trigger Generator PCB

The VC Trigger Generator is a bit like an LFO, I suppose, but for generating triggers (these are the S-Triggers I was writing about earlier).  It has pads to allow for V-Triggers too; I've installed all of those parts, for completeness (they may help for any debugging, too!), but I'll be using the S-Triggers.

Yep, that's a LOT of transistors! 17 of them! (And only 2 are for the V-Trigger add-ons.)  Two SSM2210s have been replaced with discrete pairs. Note also the 2N4891 on the left (the little tin can)... hard to find, but I got 10 from Nikko in the UK. That should be plenty for what I may eventually build: they're only used here, the Sequencer, and the S&H.

There's a 2M resistor printed on the board; it should be 100k. It is well-documented on the schematic, etc., but just make sure you take note of it.

Another resistor is listed as "SELECT" - this is the resistor for the LED. I put a 1k resistor in there, which should be fine for the yellow LED I plan on using (Mouser 78-TLHY5401). There's not many LEDs in the Synthasystem: just here, the Peak Selector, and the Sequencer... this is the only one where you need to select a resistor value, though. In case you don't remember:

RLED = (Vcc - Vf) / If , where:
Vcc is the source voltage (here, per the schematic, +12V)
Vf is the forward voltage drop of the LED (look this up on the datasheet or Mouser page, 2.4V for mine)
If is the forward current of the LED (look this up on the datasheet or Mouser page, 10mA for mine)

So mine calculates out as (12 - 2.4) / .01 = 9.6 / .01 = 960R, and I rounded up to 1k.

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