Door Alarm Help Please

[MattDC]

Hello,

I am making an alarm for my workshop back door that will set off when the door is opened I have used a magnetic reed switch you would find on an alarm system as the switch I would like to add the ability to open the door and reset. I was planning to use a latching relay for this but I can get my head around how this will work with the reed switch being either open or closed not momentary. am i missing something really simple or is this a little more complicated?

any advise would be appreciated

 

[Arouse1973]

Hi Matt
I think the easiest option without having any other electronics is to have an external key switch which arms and disarms the alarm. Otherwise your going to need a timer of some kind to allow you to get out and shut the door. The key switch could just break the power to the relay.
Adam

 

[Colin Mitchell]

Do you want the alarm to go off after 10 seconds?

 

[KTW]

A normally open reed switch will activate when the door is opened and reset when it closes.

 

[Arouse1973]

A normally open reed switch will activate when the door is opened and reset when it closes.

Are you suggesting this as an option?
Adam

 

[AnalogKid]

I am making an alarm for my workshop back door that will set off when the door is opened I have used a magnetic reed switch you would find on an alarm system as the switch I would like to add the ability to open the door and reset. I was planning to use a latching relay for this but I can get my head around how this will work with the reed switch being either open or closed not momentary. am i missing something really simple or is this a little more complicated?

A little, but manageable. A complication in most alarm circuits is that when the switched is closed the alarm is to be off. This is backwards from normal controls, such as a doorbell or ceiling light, but not a big deal. What is your skill set/experience base? Are you looking for a solution of all relays and wires, or is building a small 2-transistor or 1-IC circuit an option?

ak

 

[KTW]

Are you suggesting this as an option?
Adam

Yes, stays on as long as the door is open.

 

[AnalogKid]

So a ff to disable the alarm sometimes, and the same action that normally disables the alarm clears the ff. This will fit in one ULN2003 (plus 5 resistors) with each stage acting as an open collector inverter. The whole circuit can run on anything from 5 to 48 Vdc, has input and output transient protection, and drives the beeper directly (up to 0.5 A).

Edit: 3 resistors.

ak

 

[Arouse1973]

Yes, stays on as long as the door is open.

What if the burglar shuts the door? I think he wants the alarm to stay on?
Adam

 

[AnalogKid]

Here is a first pass at a door alarm circuit. Each section of a ULN2004 is a darlington transistor with 1/2 amp collector current capability, base current limit and pull down resistors, input and output transient protection, and can run on anything from 5 to 18 Vdc. U1E and F form a flipflop to latch off the alarm. The reed switch is normally closed, and opens to sound the alarm. C? causes the circuit to power up in the alarm-off condition if no switches are connected. Not shown is a ground connection to U1 pin 8.

ak

 

[KTW]

What if the burglar shuts the door? I think he wants the alarm to stay on?
Adam

I thought he just wanted a signal to let him know someone has entered the room while he was say, working in the back.

 

[Arouse1973]

I thought he just wanted a signal to let him know someone has entered the room while he was say, working in the back.

Oh ok I thought it was for night time when he locked up.

 

[Arouse1973]

Here is a first pass at a door alarm circuit. Each section of a ULN2004 is a darlington transistor with 1/2 amp collector current capability, base current limit and pull down resistors, input and output transient protection, and can run on anything from 5 to 18 Vdc. U1E and F form a flipflop to latch off the alarm. The reed switch is normally closed, and opens to sound the alarm. C? causes the circuit to power up in the alarm-off condition if no switches are connected. Not shown is a ground connection to U1 pin 8.

ak
View attachment 22703

Nice one ak, does this latch the sounder on also? Or is the reset button just to allow the door to be opened and stay opened if he wants to. Then it arms back up when the door is shut. Do you need any switch de-bounce?
Thanks
Adam

 

[AnalogKid]

Nice one ak, does this latch the sounder on also? Or is the reset button just to allow the door to be opened and stay opened if he wants to. Then it arms back up when the door is shut. Do you need any switch de-bounce?

The request was for something to latch the beeper *off*. The FF does that. Only the door can turn on the beeper. There is no on-state latch, only off-state.
SW2 latches off the beeper. My guess is for those times when the door is open intentionally, such as ventilation in the summer.
Closing the door resets the FF so the beeper again comes and goes with door action.
No debouncing. Its a beeper, not and olympic trials lap timer.

ak

 

[AnalogKid]

Here is a version of the door alarm circuit using conventional logic gates. Note that there are three separate output stage options. U1D can sink over 20 mA, but is limited to 5 V beeper voltage. Because of its relatively high Rdson, Q1 is for low power beepers, but can handle higher output voltages. Q2 can sink over 100 mA at up to 40 V.

ak