OPT-3 Instructions

With a little practice you should be able to duplicate your best performer (ratio of OFF resistance to ON resistance) with little effort.

First, build a small power source using a 9V battery, a clip, and a 1K resistor as shown below.

Next, glue the top of the LED to the center of the face of the LDR. You might want to make an alignment jig to hold them in line while the glue sets.
For adhesive, I like to use a drop of thick gel super glue. It works great and is clear so it doesn’t obstruct the light from the LED to the LDR.



Let’s test it before we encapsulate it. Connect the power source to the LED as shown, observing the polarity of the LED and the battery leads. The LED should light a bright white.
Next, connect your voltohmeter to the leads of the LDR. It is non polarized, so you can connect the leads either way. Set the meter on the 2K ohms scale.
With the LED energized, the ohmeter should read less than 1K ohms.
Disconnect the power source to the LED and the ohmeter should read an open. Because some room light is leaking through to the LDR, it probably won’t measure in the 10’s of megohms, but it will after you encapsulate it. If you want to see where it is ‘naked’ for OFF resistance, simply move to a higher ohms scale on your meter until you get some kind of reading besides an open (1 on the readout).

If your ON resistance is above 1K, make sure your 9V battery is up to snuff first. If it is, then you need to break the glue joint between the LED and the LDR and try again. If you used super glue, you can use acetone to dissolve it and re-apply. You’ll get the hang of it and will find the sweet spot after a couple of tries. I have had some go down to as low as 100 ohms with very little current going through the LED.

And that’s it. Now to encapsulate it.

Slide the tubing on the LED/LDR combo, centering it between the leads sticking out. The heat shrink tubing will shrink to about half its diameter when properly heated.
Use a heat gun or a hair dryer and apply even heat all around the tubing, holding the heat nozzle 6 to 8″ from the tubing.
Use any opaque adhesive to fill the ends of the tubing. The adhesive should have a high electrical resistance, since the OFF resistance of the LDR is over 20 Meg and we don’t want leakage through the adhesive to cause crosstalk in your circuit.
Preferably, use a thick adhesive so you don’t have to deal with it running while it is setting up.

And finally, don’t be tempted to cut the LED leads off even because you need to know which one is the longer one for the anode connection to the LED when you connect it in your circuit.