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 Forum index » DIY Hardware and Software » Circuit Bending
Wiring a this Comparator for single supply ?
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thepaddiscuile



Joined: Jul 11, 2007
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Location: ireland

PostPosted: Thu Aug 22, 2019 7:02 am    Post subject: Wiring a this Comparator for single supply ?
Subject description: tl072 comparator
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Hi Guys ;]

Trying to get this to work on a single supply of 12v
I think it originally works on +-12v

I send a gate or pulse in and the cap and resistor reduce it to a spike..
The comparator has a threshold of around 1v set by the two resistors 100k and 10k..
I should get a crisp 1ms pulse out the other side..
Needless to say it doesn't appear to work on single supply of 12v..

Anyone have any suggestions on how I can get it to operate on a single supply instead ?

Cheers

Patrick


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thepaddiscuile



Joined: Jul 11, 2007
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PostPosted: Thu Aug 22, 2019 7:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry I should mention that the chip used is a TL072 !!
I'm also using this..
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wackelpeter



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PostPosted: Thu Aug 22, 2019 10:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Not having checked your schematic at all but a TL072 is naturally suitable for dual supplies. If you want to build an comparator on a single supply use an LM358 instead. Otherwise the TLC07X series are OPamps designed for single supply operation.

https://www.ti.com/lit/ds/symlink/tlc072-q1.pdf

As it seems you would want something like the CGS gate to trigger converter, then you should look up for the CGS24 schematics.

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ixtern



Joined: Jun 25, 2018
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PostPosted: Fri Aug 23, 2019 12:35 am    Post subject: Reply with quote  Mark this post and the followings unread

For single supply operation I suggest using old, cheap but still good LM311.
Below is part of my VCO using LM311 with single supply +12V. You may adapt it for your needs.
Notice that on your circuit input there is a differentiating RC resulting in converting rectangle like positive pulses into positive and negative saw-like spikes. You may need to play with R and C input values to get proper output pulse


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thepaddiscuile



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PostPosted: Fri Aug 23, 2019 4:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the replies.. Yes I realize I can do it with a single supply chip but !!

To be honest I just didn't want to go down the road of buying more chips for a simple circuit.. I have 1 unused op-amp on a tl072 that I'm using for a drum circuit..
But If it can't be done then it can't be done !!
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thepaddiscuile



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PostPosted: Fri Aug 23, 2019 4:21 am    Post subject: Reply with quote  Mark this post and the followings unread

ixtern wrote:


Notice that on your circuit input there is a differentiating RC resulting in converting rectangle like positive pulses into positive and negative saw-like spikes. You may need to play with R and C input values to get proper output pulse


Yes this doesn't matter too much, as I'm really just after a clean pulse in the 1-6ms range, this is then going to a pulse shaper circuit that lets me stretch or shorten the pulse to my tastes..
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PHOBoS



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PostPosted: Fri Aug 23, 2019 4:53 am    Post subject: Reply with quote  Mark this post and the followings unread

although it might not be ideal a TL072 should work on a single supply. So I wonder, what is your input signal, does it go high AND low ?
otherwise you'd need a resistor to GND on the input (before the capacitor). Similarly are you checking the signal before or after the diode ?
If after you might again need to add a resistor to GND. Also what happens if you wire a potentiometer between +12V and GND and connect
that directly to pin 3 of the opamp, does it trigger the comparator when you turn the pot ?

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thepaddiscuile



Joined: Jul 11, 2007
Posts: 28
Location: ireland

PostPosted: Fri Aug 23, 2019 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

These are things I haven't tested... I haven't tried adding resistors to ground, Wonder what values they would need to be ?

And your right.. Im feeding it with a saw tooth from a dixie II modular oscillator which now that I think of it, would have half its cycle in the negative voltage range..The pwm from the dixie isn't short enough to give a pulse to use, its more of a narrow square.. As far as my ears tell, the saw gives a nice pulsey whack when the cycle starts.

How would one arrange a diode on the input to remove the negative swings ?
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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Fri Aug 23, 2019 6:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Triggering with a saw .. you mght have to change it from an upgoing saw to a downgoing one .. your circuit as is will will trigger on positive signals .. so the "fast bit" of the saw needs be upgoing.

The other thing is .. with a downgoing edge on the input you'll get a negative going spike on the input of the opamp which is outside its supply voltage range .. which may lock it up .. this can be solved with a diode fron the + input to ground to short out the negative pulses (its "arrow" pointing towards the input).

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also .. could someone please turn down the thermostat a bit.
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