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Anyone know where I could get some LDRs or optocouplers wuth a max resistance of about 5 or 10k, or if they're even made?

I have never come across a ldr with such a low off-resistance/high on-resistance... you could try adding resistors in series/parallel but I don't think that you will get the desired results out of that... you could try photo-transistors tho

no sir, you can't. well, maybe you can. 5k is really pushing the limit of whats possible with $$$ parts. its better to look at the schematic and get creative with modding the circuit to take a 100k or 1M dark resistance. 1M dark are the cheapest most common kind of LDR. at $7 a photocouple, I'm not buying. you might be lucky enough to get he good shit @ $5 per unit.

put a parallel 10k resistor, depending on the ldr it will still go from about 100ohm to 10k.

Thanks for the input everyone.

I figured there weren't any around, I've run across very few hints that there are some that low...obviously not common enough.

I'll try a few alternatives. Reworking the schem is probably the most logical...

if your 1M LDR goes to 50k in light, do the math. with a 10k resistor in parallel you get 9.9K to 8.33K

that is not much of a range at all. the response is also not even close to linear. the exponential curve will make it function like an on/off switch even if you could scale your range with an opamp. do you want to post the schematic?

Maybe you could set up the LED to stay bright enough to keep it up that high?

is there a voltage range you're trying to use - that is, for the control in question...
that info might make things easier.

eatyourguitar wrote:if your 1M LDR goes to 50k in light, do the math. with a 10k resistor in parallel you get 9.9K to 8.33K

that is not much of a range at all. the response is also not even close to linear. the exponential curve will make it function like an on/off switch even if you could scale your range with an opamp. do you want to post the schematic?

Depends, the "ON" resistance often doesn't mean the lowest an LDR will go. The "ON" resistance manufacturers give just gives you a rough idea at some test conditions( xyz lumens which corresponds to often only a dimly lit LED), but with an ultrabright LED (or daylight) you can go further than the test conditions. The ones tayda sells are supposedly "ON" 10k, but I got 100ohms out of them..

mysteriousj wrote:Depends, the "ON" resistance often doesn't mean the lowest an LDR will go. The "ON" resistance manufacturers give just gives you a rough idea at some test conditions( xyz lumens which corresponds to often only a dimly lit LED), but with an ultrabright LED (or daylight) you can go further than the test conditions. The ones tayda sells are supposedly "ON" 10k, but I got 100ohms out of them..

+1.

also the specs usually mean it does that in a certain time period.

i also had no problem getting the tayda ldr in the 1k-10k range with a standard led.

True but you cant get 10k off as far as i know.