LDR with really low minimum resistance for a VACTROL vibrato

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

so i'm building a vactrol for a vibrato in a bent Casio SK 5 with a LTC 1799 alternative crystal clock. I have a little 555 circuit that is creating a breathing LED to be my pitch vibrato.

Thing is I want my LDR to oscillte between 0 Ohms and say 1K Ohms. Is this possible? is anything like this possible?

What sort of LDRs are out there

AlanP · Posted: Fri Jul 24, 2015 12:14 pm Post subject:

Have you tried a resistor in parallel with the vactrol's resistance side?

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

yeah i have. from my understanding, putting say a 500 Ohm resistor in parallel with a LDR that has a range of 5K-500K will result in the resistance varying around 500 Ohms, but never getting close to zero.

It'll change the range, but not lower it.

PHOBoS · Posted: Sat Jul 25, 2015 2:06 pm Post subject:

no, you're not going to get it lower, they usually have a range somewhere between 500 ohms and >10M.
Well you probably could put a bunch of them in parallel all controlled by either the same LED or maybe
a couple LED's, But that still wouldn't get you a very low resistance. Maybe using a FET instead could work.

Or drop the LTC1799 altogether and use a 74HCT4046 instead, which has a build in VCO so you can contol
it directly with a voltage. You could still use the vactrol setup or connect it (with some adjusting) instead of the LED.

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

Thanks Phobos.

Any tips/links on the 74HCT4046?

I found this http://www.electronics-tutorials.com/devices/74hc4046.htm

PHOBoS · Posted: Tue Jul 28, 2015 12:59 pm Post subject:

hmm let's see, you don't need the whole comparator phase locked loop stuff, just the VCO section which is very simple.
just a small capacitor (between pins 6,7) and a resistor from pin 11 to GND which sets the range, you could also use a pot here.
I think in your case the best solution would be to use a small mixer circuit on the CV input (pin 9). With a potentiometer
to set the pitch and an input for your tremolo (with a pot. used as an attenuator). This will let you control the pitch
and tremolo depth independently. You could take a look at this circuit: http://electro-music.com/forum/post-372062.html#372062
I think the 74HC4046 would actually more useful than the 74HCT4046, but a standard CD4046 will be too slow for your needs.

Another thing I was thinking about is that you could use the LTC1799 but with a smaller capacitor, this would give you
the same speed at a larger resistance so then the Vactrol might work. I don't know how you're using it though.

Bogus Noise · Joined: Jun 03, 2009 Posts: 65 Location: Sheffield

The H11F1 is also a really decent alternative to a vactrol. It doesn't have any of the slew that vactrols have so you can pitch your modulation osc up really high and get some wicked clangy noises.

I'd also be tempted to use a TL071/TL072 for the LFO, as 555 waveforms lose their shape when they're pitched up too high.

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

sorry to revive this, but some more discoveries!

the LTC 1799 datasheet describes it as having a VCO setting - https://cds.linear.com/docs/en/datasheet/1799fc.pdf with a oscillating voltage between 0-1.13V

now that is a tiny voltage range, but i was thinking a circuit designed to create a pulsing LED with a few resistors might do the trick?

thoughts?