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What is this and why does it work like it does?

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JimmyG1976

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So 24 volts going to a voltage divider then into two polarized capacitors then into current limiting resistors for the LEDs.
The part that I can't wrap my head around is that you can put the 12 volts on either test point and the corresponding LED will light up one color and you can put the ground on either test point and it will light up the other color. The part that's really messing with me is that I can put 12 volts on one test point and ground on the other and they both will light up regardless of which test points the 12v and ground go. How does this work?

I breadboaded this tonight so I know it works. Nothing gets hot. The current draw on the LEDs was around 7ma if I remember right. I let it sit there and just go for about 10 minutes with no issues.

So what's it doing and does this type of circuit have a name? It seems like something is being both positive and negative at the same time.
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KlausST

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Hi,

the capacitors and the polarity make no sense to me. (They will see positive as well as negative voltage).

you can put the 12 volts on either test point and the corresponding LED will light up one color
I don´t think so.
Connecting 12V to TP1 and/or TP2 will cause no LED to light up.

You may use GND(0V) or 24V at TP1 / TP2.


The current draw on the LEDs was around 7ma if I remember right.
The LED current should vary between 1.7mA and 7.3mA depending on the 0V/24V connections at TP1 and TP2.

*****

The circuit may be useful to test 24V sensors like "inductive proximity sensors".
Some have open collector, open emitter, HIGH side / LOW side, or push pull outputs.

One color says HIGH side driven (let´s say red)
the other color says LOW side driven (let´s say green)
no color says OPEN

So a standard "Open collector to GND" will show green when active and OFF when inactive.

Klaus
 

JimmyG1976

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You read my mind Klaus, that is exactly what I intend to do with it. It works on a bread board with a dual mode NPN/PNP photoeye.

Banner makes one that is around $120. We have one at work and I took it apart and was extremely disappointed in what I found for $120.
A Hammond box, Generic 4 pole red/black, and a M12 female pigtail connector with this circuit inside.

schem1.PNG

I decided to use a single 9v battery and a boost converter hence the divider and capacitors.
They replicate the junction between BT2 and 3. Sure it's probably more complicated than it needs to be, but I like doing stuff like this.
I also surprised myself that I figured a way to do it. I'm pretty new to any form of DIY electronics.

I still don't understand how both LEDs can be on at the same time. It seems like there would be a short between the two capacitors if I were to hook up both channels of the sensor to TP1 and TP2.

Regardless my boss already bought the parts to make one for each technician at our plant..lol.
 
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