homemade vactrol

[1878]

Hello Everyone...

I've been building the UglyFace and run into a few problems along the road. It now seems everything is ok other than the (VTL5C3) Vactrol I used. I'm thinking of making my own to see if this solves the problems I've had. Has anyone got any hints or tips on what works best ?? I'm a newbie to all of this, so please don't think you're being too descriptive in your answers.

Thanks in advance.

[GibsonGM]

I'd think that vactrol would work well (?)   Have you tested it?  You sure you have the LED leads going in the right direction?  What are these "problems" you've had?

Smallbear has little Clairex LDRs for use in the Easyvibe...they work pretty well.  Couple that with a very bright red LED (even the RS type work), and you have your own vactrol.    You use a piece of heat shrink tubing...slide them both in there pointed at each other (touching) and heat the tubing til it encloses them.  Seal the ends with silicone or something dark.  That's all there is to it!  Be sure to mark cathode or anode of the LED, it's hard to tell after it's enclosed.

You can hook up the LED to a power source with resistor (1k, 2.2k, ok for testing).  Measure the LDR resistance with the LED off, should be high (1 to 5M...).  Fire up the LED, it should go low (100's of ohms to low kohms).  Do the dark test in a dark area - inside an old 35mm film canister works well. 

[Mark Hammer]

I find one of the biggest problems with homemade vactrols is that the conditions under which you would use it (essentially light-tight enclosure) are not the same ones under which you set it up and measure it.  I enclose the unit in heat shrink to block out the light (and keep it stable), but if I move the unit under the desk instead of on top, the measured resistance goes up (i.e., the heatshrink lets in light).  The unit obviously "works", but if you are aiming for some specific resistance target your bench-top radings can be different than your in-chassis functioning.

Another thing to consider is "squaring" the face of the LED to both reduce size and stabilize contact, as well as diffusing the light to spread it more broadly and evenly across the LDR.  That is, you take your average gum-drop shaped LED with a domed top and sand it so that the top gets a "brushcut" (i.e., flat).  This will shorten it and allow it to lay flush across the LDR.  You can consider laying a drop of cyanoacrylate (crazy glue) across the LED to adhere it to the LDR.

[petemoore]

  I rolled my own UF vactrol, grabbed an RS LDR and LEd , rolled them right up in typing paper.
 I put a hardcardboard stop block on the edge of a 2''x4'' piece of typing paper, then backed the LDR into the hotmelt glueline..this kept it at 90 degrees, then put a woodglue line along the edge of the 'EZ-wider' shaped typing paper sheet, and rolled it up, forming a stiff tube with an LDR in it.
 Then  make a plate sized to the ID of the LDr tube, and poked the LED leads through holes in the plate, hotmelt glued the plate to the back fo the LED, so that the leD mounting plate would 'piston' right in the LDR cylinder, trim plate as necessary.
 I guess the white paper walls help get more light on the LDr [no idea whether this is of any functional benefit], then I just 'pinch/glue' sealed the ends up, wrapped around it with electrical tape to block light, black paint would work.  

[AL]

I used an ink pen. Kinda tight fit and I haven't had the chance to test it yet but it's certainly blocking the light - and ink pens are everywhere (I think I just stepped on one) 

AL

[Mark Hammer]

Good ideas.  Let me be clear about something, though.  Heatshrink DOES block light...just not all of it.  An LDR that might read 180k on the benchtop with no LED illumination might read 300k when I stick it under the bench and away from the light.  That's the sort of difference I'm talking about.  The 300k is more representative of what will happen inside the chassis, and any fine tweaking (e.g., a series resistor limiting LED illumination) should be based on what will happen inside the chassis.

[petemoore]

  An opaque circuit box with no holes helps block light.

[GibsonGM]

One trick...punch 2 lead holes in a 35mm film canister...insert LDR and measure dark resistance.   Or punch 4 holes and hook the whole thing up in there, test light vs. dark.  But the little E.Vibe clairex's from smallbear have done the trick for me for several projects, so I'm going to go for them when I build my UglyFace next week.

[Iojan]

http://www.aleph.co.jp/~takeda/radio/phaser/indexE.html