Adding a delay to a door alarm?
You have a circuit which uses a magnet and a reed relay switch to trigger an alarm. But the alarm is ON as soon as the magnet separates from the reed relay and OFF as soon as the magnet arrives close, again. You have this:







simulate this circuit &ndash; Schematic created using CircuitLab

You want this, where \$DELAY=30\:\textrm{s}\$:







simulate this circuit

It would be difficult (read as impossible) to find a reed relay that will introduce just exactly that kind of delay for you. That's not happening in this life.

So you need to replace the reed relay with a circuit that will emulate the reed relay's behavior to the existing circuit, but where it introduces a delay as long as you say you want it to be.

But you still need the fast-onset of the alarm edge and you still want it to respond to the magnet, too. So you need the reed relay. But you CANNOT have the reed relay directly attached where it is, because when the reed relay CLOSES it forms a short circuit and there is pretty much NO WAY you can delay the onset of that short circuit by imposing a parallel circuit that you attach.

(In the following, I'll use battery to mean either battery or power supply -- it doesn't matter which it is.)

Instead, you will have to insert a circuit that replaces the existing reed relay in its series position. The problem with this is that you need to first find out if the existing reed relay is floating within the circuit or else has one side connected either to (-) or to (+) of the battery (or power supply.)

I think, given what you already identified regarding the trigger pin going to a positive voltage, that one side of the reed relay is likely connected to the (-) battery terminal and that the other side of the reed relay connects to a pull-up resistor that, itself, has the other end tied to the (+) battery terminal. In short, the reed relay has one end grounded.

If so, then you will need to desolder one end of the reed relay (the end that is NOT connected to the (-) battery terminal) and tie that end instead into a circuit that you wire up. This circuit will have an output wire that will be soldered into the board where you just removed the reed relay end. The circuit will also need access to power, so the circuit will need access to the (+) and (-) terminal of your battery. Something like this:







simulate this circuit

You've mentioned \$5\:\textrm{V}\$ as the voltage. This suggests that there is a power supply and not a battery system here. But that's just details, really.

At this point, you now have a specification for what you want. You should have written all this up yourself, after testing your unit to validate the details written above (I think they are likely to be accurate, but they may not be.) Then you'd be able to ask for a circuit which would operate under these conditions to provide the necessary operation.



One thing you might do to verify that a BJT used in a circuit would actually be capable of replacing the reed relay, is to connect a forward-biased diode [this just means hooking the cathode to the (-) side of the reed relay and the anode to the (+) side] across the reed relay and then remove the magnet away from it. If it does NOT cause an alarm, then I think an external circuit will be workable. If it still gives an alarm when the magnet is removed, with the diode hooked up as mentioned, then there may be unforeseen complications I hadn't anticipated and more exploration would be required.



I'm stopping at this point. I think I've done enough of your work here. These are details you should have prepared and written up entirely on your own, so that others would know how to help you better and have a much better chance that their time isn't wasted on a solution that won't actually help you. To respect the time of others and to encourage meaningful help when you want it, you should take the time to work out as many details as possible and to carefully document those details as well as write up a very readable and thorough specification. The above provides a model you might follow in the future.



And you still need to verify my above assumptions about the reed relay wiring, the voltage details, and perform the test using the parallel connected diode.

Let me know when you get that testing done.