LDR VCA type thing

mike page · Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk

Trying to get this little fella working but no joy. The left hand side could be any oscillator but I thought I'd include the one im using in case the resistance of the input is a factor.
(the left hand side works fine putting out a nice triangle 8v p-p, And the led is flashing and is coupled in the dark with the LDR)

Just connecting the oscillator to the lm324 seems to kill the signal.

I thought I felt confident about op amps as buffers but it seems im missing something!

Any thoughts?

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Grumble · Posted: Thu Feb 16, 2017 5:10 am Post subject:

First of all: start measuring the resistance of the LDR when the LED is off, and when the LED is fully lit.
Read some more HERE

mike page · Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk

Grumble · Posted: Thu Feb 16, 2017 8:15 am Post subject:

If you have measured these resistances "in vivo" then is should work..

PHOBoS · Posted: Thu Feb 16, 2017 8:22 am Post subject:

Do you happen to have a TL074 or a similar J-FET opamp laying around to
test with ? I think it should work with a LM324 but it being a single supply
opamp might mean there is some internal biasing that messes things up.

mike page · Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk

I checked my wiring and had a silly short. It works! And sounds good. Thanks for your replies all the same.

mike page · Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk

out of interest... if i rearrange the LDR voltage divider as in the diagram I would get the opposite response. Right?

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mike page · Joined: Sep 26, 2016 Posts: 134 Location: norwich, uk

oh and apologies if this is in the wrong bit of this forum! (I guess its not strictly lunetta chat)

PHOBoS · Posted: Thu Feb 16, 2017 11:32 am Post subject:

good to hear it works and yes that circuit should have the opposite responce.
There is something to say about it not being linear but generally speaking
more light results in a lower level.

As for it not being strictly lunetta, those kind of simple circuits do go very well
with lunettas so its fine Very Happy

StasV · Joined: Jul 21, 2017 Posts: 1 Location: Slovenia, Ljubljana

Hey Mike

I was reading through the forum, got fed up with voyeuring and decided to finally create an account and start giving the ideas back to the community Smile

As to designing "a proper" LDR/LED optical VCA combo - you could use a NPN inverter like http://dlb.sa.edu.au/rehsmoodle/file.php/282/kpsec.freeuk.com/trinvert.gif to invert the VC that goes to the LED. If I'm not mistaken the upper part of the attached schematic (by Thesquarewaveparade) enables the same thing - the LED will constantly emit light and fade out when a signal is applied
Hope this helps! Smile

Ciao!

VtoR.jpg

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voltage to resistance schematic

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