Alarm circuit should be improved

Question

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?

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Unfortunately, I cannot offer you an .asc file.

Answer 1

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.

Answer 2

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.