Have you replaced the mosfet?

This was the last one standing! The other one I accidentally powered in reverse into smokes.

I'll get some more I guess.

 

Do you have any other transistors on hand besides the mosfets?

 

Lemme see, i have these:
13003 / S8050 / TL431 / A684 / AZ431 / C945

 

Try this circuit:

 

Try this circuit:
View attachment 300345

SUCESS!

It worked! i fried one c945 through reverse current but luckily had another one.
I'm immensely greatful and wish you the best!

 

Just curious, what does the voltage read at the collector of Q1 when the relay is activated?

 

Just curious, what does the voltage read at the collector of Q1 when the relay is activated?

with the negative probe on ground, on idle it's 12.26v and on alarm it drops to 0.07v

 

with the negative probe on ground, on idle it's 12.26v and on alarm it drops to 0.07v

Perfect, meaning the transistor is in full conduction.

 

Just for kicks, try it without the optocoupler, but keeping the 100K to ground. That circuit should work as well.

 

Just for kicks, try it without the optocoupler, but keeping the 100K to ground. That circuit should work as well.

You mean like this?

 

You mean like this?

Forget I said anything.

 

Hmm...

OK!!

 

Best to leave it with the opto.

 

Oh yeah 100% i'm not touching it! If it works, don't fix it.

if i were to test without opto i'd build another one.

 

Best to leave it with the opto.

I agree, that’s why I told him ignore my suggestion.

Just for my own edification, why did we add the opto? Did something indicate isolation was needed?

 

From various attempts to get the circuit working it did indicate that was the issue but it could have been defective transistors. I should have asked for more voltage readings initially.

 

My take is that we could never get him to connect the alarm to circuit correctly.

 

I also now believe that the Transistors were defective but an opto makes sense in this case. The detector has an isolated voltage through two pins as the output. when the negative output is grounded it just takes 12v from power supply and activates the relay. when it's not grounded it's an open loop. my mistake was that I didn't paint the whole picture and the detector's pinout was out of the equation for quite some time.

 

Have you tried the circuit like this？
+12V
|
R1
|
+---|---+
| |
| R2
| |
| +----- Relay Coil
| |
+---B---| NPN Transistor (e.g., 2N3904)
| |
| |
| E
| |
| |
| |
GND GND

 

I just finished making another one for a second detector, then i got an idea. Couldn't we have just used the opto and the 4.7k resistor to achieve the same result? Pins 3 and 4 on the opto close the circuit when supplied with voltage on 1 and 2.