Resistance Light and Dark Values

I forgot to state that it is Cadmium Sulfide (CdS) Photocell.

Features:

Resistance (Dark): 1M? Resistance (Light): 12k? Lead length: 1.44"

Dark= no light Light = >= 10 or 100lux

Light dependent resistors change resistance with light. 1,000,000 ohms when dark 12,000 ohms when light shines on it. (but those are just nominal values)

I am trying to get my laser alarm working.

No luck so far.

I am wondering if the cadmium photo cell may be the problem?

Maybe I need an LDR that responds to light quicker.

Andy

They're not slow. Do you have a meter to measure the resistance while you illuminate it with your laser? It may not be interested in that wavelength of light.

Yeah, they're slow. Not seconds, but dramatically slower and less well-behaved than photodiodes or even phototransistors.

Using a CdS or CdSe cell is very rarely the right answer. If you modulate the light source and detect the AC at the other end with a photodiode, transimpedance amp, and synchronous detector, you can easily make an 'electric eye' that works fine in direct sunlight.

Cheers

Phil Hobbs

range.

You really can't beat a photodiode, though.

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rating

Does anyone know of an alarm circuit that they have made and works?

I have tried 2 different circuits and both have failed.

Andy

Please put the resistance meter on your LDR and give use the resistance Dark and then with laser light on it. You will need to change the scale between the two measurements. Mikek

Yes, it can. A photodiode takes time to turn on or off, about the same a LDR takes. LDR are reversible, standard photodiodes are only one way.

A fast photodiode will respond in picoseconds. Even slow ones are in the tens-of-nanosecond range if used with the right transimpedance amp.

They're also highly linear and stable, neither of which is true of CdS cells.

Cheers

Phil Hobbs

For an alarm, do you care to 10ms for instance ?

perating

In the dark, .83 on the 2M scale.

With laser, .3 on 2k scale. From a distance of 6 feet.

So, 830,000 and 300 ohms.

Andy

Of course not, but you gave Bad Info about photodiodes, who are sort of friends of mine. ;)

Cheers

Phil Hobbs

A bad friend which act only in one way !

Photo diodes are much much faster than CDS- LDR's. Even faster than photo transistors. To get the most speed out of diodes and photo transistors it depends on current and biasing. Over-drive the piss out of them and they slow down a bit.

But for his application, with an un modulated light source, a CDS photocell is fast enough, especially with the 555 circuit he was building where it is directly coupled to the 555. It should be able to meander all over the place in it's own sweet time and still trigger the alarm.

My gripe with CDS cells is their susceptibility to moisture. I live in a humid climate and these things will go bad just sitting in a parts drawer unless they are sealed in glass,

No problem, stick two back to back.

When do you actually need bidirectional conduction? Audio or linear applications maybe, that's about it, and he's just looking for on-off control here.

A LDR doesn't need power to operate !