AnaLogiCool

171

03:22:26

Comparator to Bistable Latch

First, undo the reset switch located at the top of the RS component. Then over at the comparator, notice the 100K resistor at the non inverting input- that's my photoresistor. I have the comparator set up to give an output signal which acts as SET, when enough light falls upon the LDR thereby reducing its resistance to approximately 17K ohm. 
So reduce the 100K down past 18K and you'll see what I mean. 
Here's my problem: I've got this circuit in a breadboard right now, but it isn't working. I separated the comparator from the RS and tested them both individually- they work great. But when I join them, no joy. 
As you can see the circuit operates splendidly here in simulation- so why doesn't the real one work? I am perplexed, and would very much appreciate your thoughts...

published 11 years ago

Sine_eyed

11 years ago

Keep in mind the circuit will want to return to its prior state (a higher resistance at the LDR) in order to be properly RESET. So adjust the original resistor back above 18K and then double tap the RESET switch at the top of the RS Latch..

faceblast

11 years ago

comparators in the wild cannot source current, they can only sink it, their output is open collector. Different from an opamp. Easy fix though, just forward bias the Transistor on, then use the Comparator to turn it off

ferlop

10 years ago

How about reducing the 10k resistor to the base of the transistor to maybe 3k or 5k. that increases the base current a little, kind of sounds like the breadboard set up is not interfacing or coupling properly.

ferlop

10 years ago

What type of comparator is it? faceblast got it.

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