[sdiy] Negative/Positive Triggers

[Theo]

Opamp as inverting comparator.
TL072 is a good one, or use a real comparator.

Trigger signal enters on the negative (-) input of the opamp.
Best use protection diodes to the power rails here.
(= diodes from power to the (-) input so that the diodes BLOCK)
Any voltage on the trigger input exceeding the supply voltage will flow 
to the power rails and not smoke your opamp.

Voltage divider on the (+) input of the opamp to set the threshold,
this avoids false triggers, 1.5V maybe a good one.
With 15V supply 100k/10k (with 12V, sorry I am lazy) 
100k from positive power to (+) input, 10k from (+) input to GND.

On the opamp output you put a diode facing _towards_ the opamp,
and a 1k resistor in series.
The other side of the resistor connects to your trigger selection switch.

Don't forget power for the opamp and decoupling caps. (100nF)
The unused second opamp in the TL072 should be either disabled:
(+) input to GND, (-) input to output.
Or you may use it to drive a LED showing trigger activity.

Success
Theo


From: Moho Disco <moho at mohodisco.com>



> How does one go about changing a negative-to-ground
> trigger to a positive-to-ground one?  I'd like to put in a
> switch on my Mono/poly that allows it to accept a positive
> trigger to the arpeggiator; currently it only accepts negative.
> Seems like it would be easy?
> 
> Lava
> 
>