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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Bringing a pin LOW with a squarewave LFO?
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commathe



Joined: Jul 26, 2013
Posts: 83
Location: Beijing
Audio files: 5

PostPosted: Sun Feb 23, 2014 10:07 pm    Post subject: Bringing a pin LOW with a squarewave LFO? Reply with quote  Mark this post and the followings unread

Hey guys, I've been working on a guitar stompbox but this digital logic problem seems to suit here best.

I am using a chip (HT8950) that has two pins I want to hit with a square wave LFO to modulate the effect. There a problem I can't get around though: the pins have internal pull-ups and need to be brought LOW to trigger the effect. This has kind of thrown a wrench in the works for me as I don't know how to achieve this. I thought of using a JFET to sink the current from the pin to ground and using that to pull it low but I haven't been able to get it work.

Surely a few of you must have tricks up your sleeves?
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elektrouwe



Joined: May 27, 2012
Posts: 46
Location: Germany

PostPosted: Mon Feb 24, 2014 3:26 am    Post subject: Re: Bringing a pin LOW with a squarewave LFO? Reply with quote  Mark this post and the followings unread

commathe wrote:
... need to be brought LOW to trigger...

a Schottky-diode (Vforward<=0.3V) with cathode towards the LFO should do the job. GNDs of LFO andHT8950 connected. If you don't have one, any NPN with emitter grounded and collector pulling down the HT-input, and 10k from LFO to base should also work.
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commathe



Joined: Jul 26, 2013
Posts: 83
Location: Beijing
Audio files: 5

PostPosted: Tue Feb 25, 2014 12:03 am    Post subject: Reply with quote  Mark this post and the followings unread

I like the NPN idea! Thanks! It's funny because I was already using NPNs to drive LEDs as rate indicators but didn't think about using them to pull the pins low. Will definitely try it out!
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