LDR's (Light Dependent Resistors) their uses.

[Murphyv310]

Hi.
I have a couple of LDR's of unknown type they are 13mm in diameter and are blue with the exception of the "lens".
I had a little play about with one today with a 3volt supply the LDR and a 100k load fed to the scope. Prior to that the spread was 50 ohms on my Fluke in bright sunlight to 15k in the shed with no light on but still some creeping through the window.
On the scope I could see it responded to an infra red remote and you could easily see the pulse train which was actually quite square.
Next trick was to unscrew the lens off my Pye Lynx and put the LDR in the end and shroud it from external light, I then put on one of the 405 line sets and focus it to the screen, the set was displaying a test card, on the scope you could lock it to both line and frame but on line you could clearly see that ghostly image you get when frame rate bleeds into line, the test card could be seen although with little contrast, moving my hand around the screen and you could see it removing parts of the test card and the waveform changing.

Now this begs the question of the LDR's speed, will it resolve up to a few Mhz or are they not that fast. It is one component I have had little to do with. Might be a suitable component for a slow scan system.

[Mark Hennessy]

They're normally pretty slow - perhaps only good for 10s of Hz, depending on the model. Certainly not quick enough to resolve the 38kHz of the modulated IR signals from a remote, but perhaps they might show you the "envelope".

Easiest way to test is with a nice bright LED mounted tightly against it. In the past, I've used a black 35mm film canister to keep ambient light away from it. Feed it with a square wave, and use a 2 channel 'scope to compare the input to the output. All good fun

[Refugee]

In one of my boxes of parts I have the "bubble" off the top of a street lamp. It contains a big LDR in series with a resistor that is fixed to a thermostat that switches the lamp.

[Nick]

These things contain cadmium sulphide which is highly toxic - many cadmium compounds are carcinogenic.

They are slow. ISTR the most common ones were ORP12s - about 3K in light to over 1M in the dark.

Still in use and available today - the classic Jim Williams ultra-low-distortion Wien Bridge oscillator uses one to stabilise the loop as did HP's first product, the 200A oscillator which was based on Hewlett's PhD thesis.

Farnell have a load of different ones - the data sheets lack in detail but imply settling times in the seconds rather than microseconds range.

[Murphyv310]

Thanks guys for the info. I'll pop off a photo of these ones and one of the trace off a remote.

[Murphyv310]

                   
Hi.
Here are a couple of pictures of the LDR I am using and a small board I made up this morning.
You'll see from the trace (the best I could do) the train of pulses from the remote.
Also a trace of the ghost like image off the test card from the Ekco. I'll be honest far from easy with only two hands but moving the lens closer to the set the circle gets bigger and moving your hand in between the screen and lens disturbs the trace, a bit hit and miss TBH but I'd like to try this on a lower line rate and view it properly.
I was thinking of modifying one of those little repro Baird scanners, one for a camera and another for display. OK Ok I've too much time on my hands I know.