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Alarm Dilema

KTW

Feb 22, 2015
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Feb 22, 2015
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I have built this alarm and it works well on the bench but with the alarm stationed in the house and the power, key and switch extended to the garage it goes into alarm after several hours of being switched on.
I have used another duplicate alarm and have tested the magnetic contacts and the only possible cause that I see is the extra length of wire, about 40 feet of 19 gauge to the garage from the house.
The alarm only activates when the scr receives current but if the contacts remain open where is the voltage coming from and why would it take several hours to activate?
The power supply is 12 volts, 2 amps.

alarm.jpg
 

Alec_t

Jul 7, 2015
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Could damp air be causing enough leakage across the switch to turn the SCR on? Which SCR are you using?
 

KTW

Feb 22, 2015
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The alarm was inside the house, no dampness.
The scr is a 106B1.
I performed a loop and ground on all the wires from the house to the garage, they are all clean and measure the same constant resistance.
I don't know if the added resistance of the wires from the house to garage are causing the problem but I have just about ruled everything else out.
I have moved the alarm to the garage today to test the system without the added resistance.
 

Alec_t

Jul 7, 2015
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Are the long wires configured as a twisted pair to reduce interference pickup effects?
 

KTW

Feb 22, 2015
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Hi Alec;
Yes 11 pair, 19 gauge,twisted pair, grease filled telephone cable.
 

Kiwi

Jan 28, 2013
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Haven't used many SCR's, but I was always told to connect a resistor close to the SCR across the gate and cathode to stop false triggering. From memory this was referred to as RGK?
This resistor would be across C2.
R1 and R2 may need to be lower resistance to compensate for this added resistor.
 

ChosunOne

Jun 20, 2010
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KTW, I can't tell from your diagram which switch is the key-operated switch and which is the "magnetic contacts" you mention. You refer to "the alarm", but I'm not sure if you're referring to the PCB or the sounder, or something else. Obviously not all the components in your schematic are on one PCB, so it represents a system of different components wired together.

The most common cause of false alarms from an opening (door or window) is spacing and imperfect alignment between the mag switch and the magnet paired with it. It may work fine when you first "arm" the system, but we don't live in a static world: All doors shift, warp, jostle, or otherwise move around a little bit, or a lot. Garage doors are notoriously tricky to cover because of all the "slop" in their movement---which is why there are special mag switches made for garage doors, to give the extra margin needed.

Rather than try to figure out where you placed the various components in your schematic, I'll just ask about your "magnetic contacts". Your schematic shows only one, so I'll assume you have one opening covered in the garage. WHICH opening (pedestrian door or overhead door) and what kind of magnetic switch are you using (standard alarm industry contact? Make and model?) and how is it installed (surface-mount? recessed? top of pedestrial door? latch side of ped door? Hinge side? Floor under OH door?)?

Your electronics may be working fine, and responding to the magnetic switch making and breaking randomly if the spacing and alignment is marginal.

BTW, we used to install "single zone" systems that did what you're doing, back about 50 years ago---it used a battery/power supply, two relays, a keyswitch, a sounder, wire, and however many magnetic switches were on the zone. And they were practically indestructible--there just wasn't that much that could go wrong with them.
 
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KTW

Feb 22, 2015
273
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273
Thanks for the response.
The alarm is an Elenco K-23, the magnetic man door contact is a Radio Shack.
Since I moved the alarm itself to the garage the problem has corrected itself.
I'm using the same alarm, door contacts, power supply and key switch.
Definitely something to do with the extra length of wire between the house and garage.
The garage isn't heated so that was the reason for placing the alarm in the house in the first place.

http://www.elenco.com/admin_data/pdffiles/K-23_REV-Q.pdf
 
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