How do I shine an LED when a PIR detects movement but keep it shining until a reset button is pressed?

Question

It's easy even for a beginner like myself to create a PCB where an LED lights up if a PIR chip detects movement, then goes off when movement is no longer detected. And if more movement is detected, the LED goes on again etc.

I'm struggling though with how to cheaply implement a PCB where an LED stays on continuously when the PIR first detects movement (regardless of how much time passes or whether the PIR detects additional movement), and only goes off when a user presses a button.

I've looked at latching relays but the cost per latched-relay is a lot for what I want to achieve given that my design has about 4 latched states I need to retain.

What component should I use instead to retain state?

(The design I need would still work if the ON state was retained continuously until the power was switched off... but I need the ON state to remain ON regardless of time or of any subsequent PIR movement detection. Not sure if that makes the requirements any easier...)

Answer 1

The best answer is to use a small microcontroller, since there are inevitably more things for it to do.

However, the caveman method to do just this and nothing else is to use a set/reset flip-flop:

Follow the logic thru and you will see that this circuit has memory. When IR goes low, OUT goes high. With RESET high (not asserted), this makes the output of IC1B go low, which keeps OUT high.

RESET only needs to be blipped low for a short time to clear OUT back to low, assuming IR is not asserted at that time.

You can get such flip-flops already all made for you. I show the inner workings to help you understand them. Of course this can still be done with half a single quad 2-input NAND chip.

Answer 2

Cheapest solution here would be a simple microcontroller that you code to work like you want it to. PIR sensor triggers the microcontroller, it triggers the led until you press a button on the microcontroller.