Door Alarm

[oldrocker]

I'm putting together a door alarm and was wondering if there was another way other than magnetic reed switches for the alarm trip switch. The circuit I built needs the switch to be normally closed / held open (when the door is shut.). I was looking to use a normally closed momentary push button. Another thought was trying to make a DIY magnetic switch myself. Are there any ideas that anyone else has to make a passive door alarm switch?

 

[Sceadwian]

You could feed your circuit from an inverter. I'm not sure how you would go about making a momentary reed switch. The last switch I had was just like yours, it was disassemble though not sure if yours is. You could probably covert it to a NC if you bend the metal strip inside just right.

 

[oldrocker]

Well I'm not sure about using an inverter. Is that so I can go with a normally open switch instead of a normally closed?
I pulled a magnet from a tiny speaker. I was thinking that a thin strip of metal lying accross a two screws attached at one end shorting them together. When the magnet (on the door) gets close one end of the metal strip would lift off one of the screws breaking the circuit. I'm not sure if I can get that to work though.

 

[Sceadwian]

Your custom made NC switch should work if you can get the metal bent just right. An inverter is a very basic digital IC. If you input a high voltage it outputs low and vice versa, so it will invert the signal coming from your switch, making it look like a NC switch.
Depends on what voltages you're running at and what you're switching because inverters don't generally pass as much current as a switch can, if it's just a signal though it would work fine.

 

[Roff]

What sort of alarm are you wanting to activate when the door opens?

 

[oldrocker]

Thanks for the responses. Ok I do have some inverter IC's like the LM339 and a 4049 & 4069 hex IC. I'm not sure if these would work.
The switch makes or brakes 9 to 12 volts feeding a two tone siren circuit I made using two NE555 timer IC's.(UK siren) I can use another relay if I want the switch to be normally open but I don't think that makes the switch mechanizm design any easier to accomplish.

 

[oldrocker]

These are a couple of circuits I have used in my door alarm project.
The first is the siren circuit I used and the other is the delay to enter timer circuit, set for 30 seconds. I've added two more relays to the circuit. One is a latch for arming the siren circuit and the other seals around the door switch so the siren continues to sound even after the door is closed.
I used an old Radio Shack keypad to arm and disarm the alarm. Outside is a keyswitch that is momentary which arms and starts the enter delay circuit.

UK Siren circuit
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Delay to enter timer circuit
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[oldrocker]

Originally I stated that the circuit could handle from 9 to 12 volts. Well I was using a 9 volt power supply. When I tried a 12 volt supply it fried the 555 timer IC and the transister in the delay to enter circuit. I'm not sure why. All the componants were rated at 16 volts or higher. I fixed it and will stay with 9 volts. Pics of it will be posted soon.

 

[Roff]

Can you show us how those two circuits are interconnected, and where you would put the switch that you originally asked about?

 

[oldrocker]

I know this a crude drawing but this is the circuit I used. Keep in mind that the key switch (momentary) both arms the alarm and starts the delay timer circuit by grounding pin 2 of the 555 IC (delay to enter timer) this turns on R2 blocking the siren from sounding. Keep in mind that R2 is actually in the delay timer circuit in the schematic. After the door opens R3 seals on around the door switch.
The keypad starts a 3 second latch timer pulse to either arm or disarm the alarm (R1). The key switch is mounted outside the door and is used when leaving or entering the door it has a green LED above the key switch to show the delay timer is timing giving me time to enter and type the code in. The keypad is used to arm the alarm from the inside and to disarm the alarm with the key code.
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[oldrocker]

You can see the metel bracket that makes the door switch (opens switch) when the door is closed at the left of the first pic. I put the keypad and the speaker and both the delay to enter timer circuit and the siren circuit all in the same box. The key switch mounted between the storm door and the main door along with the green LED which is also in the delay to enter timer schematic.
And the door switch itself half a$$ed mounted to the door jam. It works great though.

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