The attached circuit is for a flash trigger, when a flash of light is seen by D1, pin 7 of U1 drops low. I would like to get a high output from this rather than low. Can I just swap the inputs to U1B to achieve this or would that upset the previous stage? If not, is there a simple but efficient way, e.g. adding a transistor, that I could get a high output from this? It is designed to be quite sensitive and I don't want to loose the sensitivity.

Thanks

 

I'd just reverse the inputs on U1B but if you'd rather not tweak the existing opamp circuit, just add a P-type MOSFET (or a PNP transistor) to the positive supply to the LED. (The LED appears to be drawn backwards.) A low voltage on the gate (or base) will turn on the transistor and light the LED.

 

Thanks @wayneh well spotted on the LED. I re-drew the circuit from an original that had Vcc at the bottom so kept getting confused. I'll see how it goes with reversing the inputs otherwise I'll use the P-type MOSFET.

 

I should add that fully turning off a PNP transistor requires U1B to hit the upper power rail voltage. I believe your chosen opamp can but you should check the data sheet before you try it. A P-type MOSFET might be fully off with its gate at some voltage below the rail. All these details are addressed in data sheets.

 

Thanks again @wayneh I'll check but I think you are right that that op amp is rail to rail

 

1. How many volts of the battery?
2. Does the direction of led(D99) is right?
3. Why you added the big 3.3K(R6) resistor, does the current enough for led?

 

3. Why you added the big 3.3K(R6) resistor, does the current enough for led?

I didn't even look at that before. Yes, R6 should be removed. It might be OK for powering everything but the LED, but it serves no purpose I can see.

 

It's a circuit I copied from an old-ish (~2001) magazine. Input voltage is 3.6V from a lithium cell, I had wondered about R6 it seemed a bit pointless to me. Ignore D99, the original circuit had an opto coupler at that point and I just replaced it with an LED to compete the netlist.

Apologies for the confusion and thanks again for looking