electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
Live streaming at radio.electro-music.com

  host / artist show at your time
  Twyndyllyngs Live tonight Chez Mosc
Please visit the chat
 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Voltage controlled Resistance
Post new topic   Reply to topic Moderators: mosc
Page 1 of 1 [8 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
synaesthesia



Joined: May 27, 2014
Posts: 279
Location: Germany
Audio files: 85

PostPosted: Sat Mar 30, 2019 4:40 am    Post subject: Voltage controlled Resistance
Subject description: simple circuit with DIY vactrol
Reply with quote  Mark this post and the followings unread

Very often I wished that I could control a potentiometer in a Lunetta circuit with a voltage controlled resistor. Of course, you immediately think about vactrols. Commercially available vactrols are nice but expensive. A simple do-it-yourself vactrol is easy to make, but the non-linearities of the LED and the LDR make it really hard to control.
This is a circuit that I am using to tame those non-itineraries and create a well-behaving voltage controlled resistance. The DIY vactrol consists of a bright flat-head LED and two GL5528 LDRs in a heat shrink tube package. I chose an amber LED simply because I happened to have those available. The reason for using a flathead type LED is that it nicely fits next to two GL55xx LDRs in a 5mm shrink tube when you bend away the two wires right at the body. Two layers of black shrink tube are required to seal the package properly, so I wrapped the result in another piece of 6mm shrink tube. I tried several LDRS like GL5516, GL5528, GL5539, GL5549 and found the GL5516 and GL5528 types work better, because I do not need the high resistance when dark that the other types provide.
Resistor R1 forms a simple voltage divider with LDR1 and is used to feed back the current voltage from the divider to the opamp, which then regulates the voltage at the LED. You can adjust the range of the controllable resistance by changing the value of R1 and the Vdd voltage that you apply. I am using 5 Volts Vdd here. The 1M resistor and the small capacity at the opamp dampen the regulation a bit. The second LDR2 is illuminated by the same LED and thus has approximately the same resistance as LDR1. This LDR can be used to replace a resistor or potentiometer in your circuit.
The nice feature of this circuit is that it can replace a resistor or potentiometer with two floating terminals, for example in a filter, and make it voltage controllable. The response of the resistance to the control voltage is a bit delayed, because the LDRs respond rather slow, but more than fast enough to use this circuit with a voltage coming from an LFO.
My control voltages are typically in the range of 0 to 3.5 Volts, because I used LM358s at 5 Volts frequently, so that is the range that I measured. The image below shows the circuit and the resulting resistance at LDR2 for control voltages in the range of 0 to 3.5 Volts. Quite neat, isn't it?


Voltage controlled Resistance circuit.JPG
 Description:
 Filesize:  43.09 KB
 Viewed:  80 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

Voltage controlled Resistance circuit.JPG


Back to top
View user's profile Send private message
PHOBoS



Joined: Jan 14, 2010
Posts: 4454
Location: Moon Base
Audio files: 625

PostPosted: Sat Mar 30, 2019 8:39 am    Post subject: Reply with quote  Mark this post and the followings unread

excellent! Very Happy
nice idea with the feedback.
With flathead do you mean a rectangular/square LED or a round one with a flat top ?

_________________
"My perf, it's full of holes!"
http://phobos.000space.com/
SoundCloud BandCamp MixCloud Stickney Synthyards Captain Collider
Back to top
View user's profile Send private message Visit poster's website AIM Address Yahoo Messenger MSN Messenger
synaesthesia



Joined: May 27, 2014
Posts: 279
Location: Germany
Audio files: 85

PostPosted: Sat Mar 30, 2019 8:46 am    Post subject: Reply with quote  Mark this post and the followings unread

I think the correct name is straw hat. Rounded top, but for wide angles.
Back to top
View user's profile Send private message
Grumble



Joined: Nov 23, 2015
Posts: 995
Location: Netherlands
Audio files: 27

PostPosted: Sat Mar 30, 2019 9:09 am    Post subject: Reply with quote  Mark this post and the followings unread

https://www.edn.com/design/analog/4368893/Use-a-photoelectric-FET-optocoupler-as-a-linear-voltage-controlled-potentiometer
_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
synaesthesia



Joined: May 27, 2014
Posts: 279
Location: Germany
Audio files: 85

PostPosted: Sun Mar 31, 2019 4:48 am    Post subject: Reply with quote  Mark this post and the followings unread

The response is still not linear, but that can be fixed. Using the second opamp in the LM358 package, I am creating a constant current source for the LDR1 sensor. This gives me a linear response between CV=0.25 and CV=3.5 Volts. The minimum resistance of the LDR at CV=0V is about 2800 Ohms in my case. The desired maximum resistance at CV=3.5V can be controlled by R1.


Voltage controlled Resistance circuit.JPG
 Description:
 Filesize:  54.68 KB
 Viewed:  71 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

Voltage controlled Resistance circuit.JPG


Back to top
View user's profile Send private message
Grumble



Joined: Nov 23, 2015
Posts: 995
Location: Netherlands
Audio files: 27

PostPosted: Sun Mar 31, 2019 1:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.
This way you only need one opamp and it is linear (for as much as the two ldr’s are the same and at equal distance from the light)

_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
piedwagtail



Joined: Apr 15, 2006
Posts: 286
Location: shoreditch
Audio files: 3

PostPosted: Sun Mar 31, 2019 2:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interesting thinking!
I'm not saying this is of any use but I did play with the n-channel depletion fet LND150 as a LED driver.

R


LND150_characteristics.jpg
 Description:
 Filesize:  21.02 KB
 Viewed:  1598 Time(s)

LND150_characteristics.jpg


Back to top
View user's profile Send private message Visit poster's website
Grumble



Joined: Nov 23, 2015
Posts: 995
Location: Netherlands
Audio files: 27

PostPosted: Sun Mar 31, 2019 10:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

An improvement could be to mount each LDR to its own LED and place the LED's in series.
_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic Moderators: mosc
Page 1 of 1 [8 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
e-m mkii

Please support our site. If you click through and buy from
our affiliate partners, we earn a small commission.


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use