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Help with daylight to dark circuit energizing buzzer
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Qmavam:
I'm trying to help someone make this circuit, while also trying to get them to understand. (seems to be the harder part.)
He had a non working circuit, asking on another group and got so many conflicting answers, many saying the circuit wont work. (good chance it didn't)
So I took him offline and just doing emails.
The first thing. I got him to built a 555 oneshot. It took a while, but now we can pull pin 2 to ground and the buzzer buzzes 5 seconds. All is good.
Next I had him set up, 9v to LDR to resistor to ground, the center point is 5.6v with light and 1v when dark. Then I had him connect this to Pin 2 of the 555. Now he says it is 5.6v with light and 4.3v in the dark. (I don't really get that) He is now adjusting the resistor to pull pin 2 down below 1/3Vc.
Where my knowledge is running out is, I know Pin 2 needs to be a pulse otherwise Pin 3 stays high, (as long as Pin 2 is low). That means I need to differentiate the voltage into Pin 2. But I know a slow light to dark transition will fail with a normal differentiation. So I expect I'm going to need a circuit between the ldr and Pin 2. This fellow has a 741 and only PNP transistors. ¯\_(ツ)_/¯
Do you have an idea to turn a negative going slow voltage transition into a fast negative pulse?
Thanks all.
moffy:
It would be more helpful if you drew the circuit rather than use words to describe it. Even if its only on a piece of paper and you upload the photo.
TerminalJack505:
You can use an LM741 op amp as a comparator and do something like the attached. Your friend will need a couple of diodes and some resistors and caps in addition to the LM741. The resistor values will likely need some adjustment.
Connect the output of this to the 555's trigger pin.
TerminalJack505:
I re-read your post and notice that you are using 9V and not 12V like I used in my example so here's an improved version of the circuit. I put a small amount of pre-bias across the output cap so that the voltage can be pulled down lower than it was originally. I don't think I show the LDR being hooked up the same as you are doing but I'm sure you can figure it out.
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