I built an alarm circuit.

The plan

If a physical (door) contact (glued to the door) is interrupted (was disconnected by opening the door), an alarm should be signaled after 2 seconds. This is done via \$R_{Load}\$ which is a light bulb. When the contact is closed again, the alarm should remain switched on until I cancel it.

Edit: As I have seen, there is some misunderstanding here. It's up to me too. I want to clear the alarm with a separate button. Here you can see, for example, how a fire brigade beeper is alerted and you now have to press a certain button so that the red lamp stops shining. I mean that in principle.

How did I do that:

I used a 555 in flip flop circuit. I use an RC delay circuit to wait the 2 seconds. The contact must have remained open for 2 seconds. I did the circuit in PSpice for TI2020. Everything is currently working as it should. However, I had to cheat a bit because a lot of things just didn't compile in the simulation.

The opening and closing of the contact is simulated by \$U_3\$ and \$U_4\$.

The problems are:

1.) As soon as \$U_2\$ outputs the 6V voltage, the voltage should be taken from Trig input of the 555. To pull the voltage down, I unfortunately have to use a voltage-controlled switch because Pspice didn't let me connect “Trig” to “E” of a transistor. What can you do there?

2.) To cancel the alarm, “Threshold” must have more than 2/3 of the supply voltage. Believe me, I've tried everything. Time-controlled switch, pulse source back and forth until it finally worked. Strangely, \$V_4\$ must have a delay time of 0 seconds; everything else crashes (does that have real reasons? 🤨). In real, the transmission line should actually be connected to \$V_1\$ and a push button should be used.😕 What can you do there?

Unfortunately, I cannot offer you an .asc file.



  • \$\begingroup\$ Daniel, I had similar problems (not the same, perhaps.) We have a door that must generate an alarm, if opened (2 sec would be interesting but in my case having the alarm start immediately is just fine, too.) There are plenty of commercial products for this. But modern ones are largely wireless and therefore only alarm for a short period of time and then stop. That's not acceptable in my case. Older alarms (I have a bag of them) stay on. But they are glued to the door, so to speak, and we need the alarm to be wireless. I modified a commercial wireless system to get what I wanted. \$\endgroup\$
    – jonk
    Nov 4, 2021 at 20:44
  • \$\begingroup\$ For me, closing the door is 'acknowledgement.' I gather this is NOT the case for you? Could you clarify? \$\endgroup\$
    – jonk
    Nov 4, 2021 at 20:46
  • \$\begingroup\$ It appears you can't cancel the alarm until after you have closed the door because the output of the 555 will be high whenever TRIG input is low regardless of what THRESH is doing. Is that what you want? Also, shouldn't those two door switches be wired in series? \$\endgroup\$
    – user173271
    Nov 4, 2021 at 20:53
  • \$\begingroup\$ Are you married to this particular circuit? You could use an R-C delay to trigger a comparator and then have the comparator trigger an SCR. \$\endgroup\$
    – vir
    Nov 4, 2021 at 21:15
  • \$\begingroup\$ What do you mean interrupted? \$\endgroup\$
    – user263983
    Nov 4, 2021 at 21:15

2 Answers 2


With this circuit, when a door switch is opened, a pulse is sent to the NOR gate RS latch which latches on the alarm. The pulse has a fixed duration enabling the alarm to be cancelled even if the door is still open.

IC1a creates a 2 second delay. IC1b then creates the alarm pulse.

There are RC differentiators on both 555 TRIG inputs to create short trigger pulses into the 555s.

R7 & C7 create a several second power-on-reset to the RS latch to ensure that the circuit is only enabled several seconds after power-up. That is to say after any power-up pulses from the 555 circuitry have passed.

The circuit has a possible short-coming - It will not detect/respond to an open door at power-up.

Alarm Circuit

  • \$\begingroup\$ Hi James, I appreciate your effort. Unfortunately, I can't use ICs in my version. I will look at your proposal elsewhere, though, because it looks more professional than mine ;) \$\endgroup\$
    – Daniel
    Nov 5, 2021 at 12:37

Summary for future readers

I have now been able to solve yesterday's problems with the crashes of the simulations despite the limited number of components. Because I can't install ICs.

First of all, I simply used two time-controlled switches for the ability to switch off (“Thresh” must be > 2/3 of VCC for a short time). In the real world this is a push button.

In order to be able to pull the “Trig” pin to ground to trigger the alarm, I installed an NPN transistor. The simulation crashed yesterday; but I can't say that it wasn't me. I probably had mistakes in other lines. You just have to have everything right.

The switches U3 and U4 are the door contact in the real world.

I changed the operational amplifier, the other one was not allowed to be supplied with 6V.




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