I think that depends on the particular diode. I've seen black diodes with white marked ends and translucent diodes with black marked ends. I believe the marked ends is always the cathode, but not necessarily always the same color.

 

That's correct. Thought you were using a plain black diode. However, I always check them wiht a meter anyway.

 

Shouldn't the white marked end be connected to the positive rail? That's the cathode, right?

The 1N4148 diode looks like this:

The cathode is marked with a black stripe (not a white) and I've connected that one to the positive rail. By the way, I have solved that problem. By mistake, I put in a Zener diode that looks identical and it pretty much shorted my circuit.

 

The 1N4148 diode looks like this:

The cathode is marked with a black stripe (not a white) and I've connected that one to the positive rail. By the way, I have solved that problem. By mistake, I put in a Zener diode that looks identical and it pretty much shorted my circuit.

Congrats on getting it fixed! Thanks for the update - it's always nice to hear how mysteries like this work out in the end.

 

Hello again everyone!

I'm back with yet another circuit for the same problem, still trying to minimize current consumption. I have omitted the 555 from the circuit and threw in a cmos AND gate. One of the inputs is connected to the output of the PIR sensor, the other one is set to go high with a delay provided by the capacitor and the resistor. I have only used CMOS IC gates so far but I want to try this "home-made" gate. Problem is, I have very little experience with MOSFETs as I usually use simple BJTs. With BJTs you have to add current limiting resistors, but I'm not sure about mosfets. Do I need to add any resistors (pull up/down, current limiting) in this circuit or it will work without frying anything?

 

You normally don't need a resistor in series with a MOSFET gate since the input of a MOSFET looks like a high impedance capacitor.

Note that diode D14 may prevent Q22's output from going all the way to 12V, thus Q26 may not fully turn off. If so you can add a pull-up resistor (say 10kΩ) from 12V to Q22's drain.

 

What is D15?

 

What is D15?

I think it's an SCR since the OP is looking for a latch function.

 

What is D15?

Crutschow is right. It is an SCR.

 

Crutschow is right. It is an SCR.

The confusion is because that's not a standard symbol for an SCR, which is this: