Ambient Light Sensor + Comparator


So I've been looking at options to create a way of controlling whether a flash will fire based on the amount of light in the surrounding environment.

Early note: Everything has to be surface mount and tiny

One of the companies I contacted about an ambient light sensor says an ALS would work to turn the circuit on\off only if I used a comparator.

I threw together a quick circuit, except with a phototransistor, and this is what I have, but the problem is I feel the amperage really sucks. I can't pull more than 14mA for a single LED (I need closer to 50mA), and about 8mA for two in parallel. So I must have something wrong, or maybe a comparator just can't source more than that?


Option 2 is using Transistors, which this circuit shows to source more, but still a bit under what I'd like.


So I'd like to know what you guys think. Do you think either of these will work with something like this? Is there a better way to do this? As always, thank you guys a ton!

Best Answer

Connect a transistor after the comparator to amplify the current.


simulate this circuit – Schematic created using CircuitLab

If you are using a push-pull comparator (which LM311 is not) then you may be able to remove R2 if the comparator is powered from V+ and its high output is very close to V+.

And yes, the current output of the LM311 is in the single-digit mA. This additionally means that you may need a Darlington transistor for Q1.