is led driver ok?

[docb]

I want to add a photoresistor to this common LED driver circuit.
Is this circuit change ok?

Any issue with the photocell having too much current through it?

thanks





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[(*steve*)]

hahaha, yeah, heaps.

You would be very lucky to find a photocell with a range of resistance suited to this (i.e. minimum resistance in the tenths of ohms)

 

[docb]

hahaha, yeah, heaps.

You would be very lucky to find a photocell with a range of resistance suited to this (i.e. minimum resistance in the tenths of ohms)

I think I need about 75ma, so just a 1/10th watt resistor, at 17ohms.

Which part if this plan is a problem? Is it that photocells all are way out of this range?
An alternate way to do this?

 

[(*steve*)]

well, you show a 3W LED, if you assume it's 1A (ish) at full power then the resistance you need to drop 1.25V is 1.25 ohms (Sorry, for some reason I was thinking it was 3A across 0.6V)

In any case, your typical photo-resistor has a resistance of several hundreds to a few thousand ohms in bright light, raising to several hundred thousand ohms in darkness.

I'm pretty sure your circuit wouldn't be too effective. It would either have the LED too dim in daylight, or too bright in darkness.

However if you have a particular photo resistor in mind, and know both its resistance, and the LED current you require in bright light and darkness, it is possible to calculate if it is possible to achieve them. I remain sceptical.

Without such measurements it is simply impossible to say. Even if it is possible to get 2 points right, there is no guarantee that the intermediate values would be acceptable (although this is probably less of an issue).

 

[docb]

Any suggestions for a photo cell that has as low a resistance as possible when exposed to light?

The ones I saw all stayed rather high, but closer to 50ohms would be nice.

 

[Zander]

Any suggestions for a photo cell that has as low a resistance as possible when exposed to light?

The ones I saw all stayed rather high, but closer to 50ohms would be nice.

Try

http://www.mouser.com/search/Default.aspx


Figure out it's lowest resistance as it stands, and then put a resistor in series with it that gives you your desired resistance.

If you want to get a lower resistance, I think you have to put a resistor parallel to the photo cell.

Are you making a light meter? I have a book that has nice light meter circuit in it.

 

[docb]

If you want to get a lower resistance, I think you have to put a resistor parallel to the photo cell.

If the psr has a resistance of say 1k minimum. and I need to see 100ish, added a resistor in parallel will not give me much range at all.

100+psr at 1k = 90.909
100+psr at 5k = 98.039
100+psr at 50k = 99.8

See why this won't work?