Homemade line alarm

Hello All
I am a mechanic by trade and I don’t have much experience in electronics. I am trying to make an audible line alarm powered by 8- 1.5v aa batteries. I have a 6-28v alarm, key switch, 2n2222 transistor in a TO-18 package and an RY12W-K 12v signal relay. I plan on installing this in a double gang aluminum outlet box lined with spray-on rubber for insulation just in case. The “lines” are AV cable terminated by an AV connector soldered together (outside to inside) if you will. Sorry for the wording as I dont know the correct terminology. I will be using multiple AV cords and the general idea is that when one cord is seperated from the next, the alarm will sound. I am not sure how to wire in the transistor and the signal replay in with everything else. Any help would be greatly appreciated. Thanks I’m advance.

Should be possible, but you need to couple your sense signal to the cable without affecting the signal that's already there.

What is the signal that is already on the cable?
What are the connectors?
What are the equipment/devices that it runs between?

I will take some pictures in the morning.

The aluminum box is what I made and the plastic box is what I am trying to replicate. Upon further inspection of the plastic alarmist has what I think is a diode connected in the circuit as well as the transistor.
I actually got mine to work this evening without using the 8 pin sense relay and the transistor. I will upload pictures of what these two look like.
Why were these used in this type of alarm? Any advantages or disadvantages to using or not using the sense relay and transistor?

Looks like it’s a 100ohm resistor wired in the plastic alarm I am trying to replicate.

ChrisP58 said:

Should be possible, but you need to couple your sense signal to the cable without affecting the signal that's already there.

What is the signal that is already on the cable?
What are the connectors?
What are the equipment/devices that it runs between?

Click to expand...

Not sure of the original signal but I used the ground and got the alarm to work.
Connectors and lines are A/V cables.
The AV cable runs through my product that is setting on tables for the public to view. If someone tries to take the product and the AV is opened at any connection, the alarm will sound.

CP176 said:

The AV cable runs through my product that is setting on tables for the public to view. If someone tries to take the product and the AV is opened at any connection, the alarm will sound.

Click to expand...

I finally understand what it is.

My guess would be that the relay is needed to reverse the logic.
The relay is turned on when the cable is complete.
A break in the cable will cause the relay to turn off.
The NC (Normally closed) contacts will close and sound the alarm.
The diode would be across the relay coil to suppress any back EMF.

A transistor could be used instead of the relay to lower power consumption. In fact the diode wouldn't be needed if the transistor wasn't used.

Mike.

How would I wire this so when the the line (AV cord) is pulled part power is supplied to the coil causing the alarm to sound? Is this why a transistor and resistor was used in the other application?

This one time.... i made a trip alarm with a 9v battery, a backup alarm and duct tape. when my dog would open the garbage can it would close my tinfoil contacts and trigger the alarm and freak her right out.

You want the opposite of that where the trip happens when the switch opens... that is why for the transistor & resistor, the resistor just limits the power going to the base of the transistor, the transistor is kinda just a switch that is controlled by the base, the emitter is ground. and collector to alarm. Just be careful cos collector and emitter are polarity sensitive.

this basic circuit should work for you as a wireing diagram and tutorial:

The transistor also says 600ma, so if your buzzer Amps are less than that you dont need relay.

If higher power is needed then you dont need to use transistor, just relay since relays have NO, NC connections, and you only need resistor to limit power to the trigger coil.

as mentioned, using both is just a stragety for saving power consumption with relay.

At what points would the resistor be wired into on this circuit from your diagram you posted? Remember not very savvy on electronics when using resisters transistors and such.

Also I have to incorporate the ability cords into this to break ground to sound the alarm.

I'm assuming the "AV cables" do not pass any other signal & are just for the alarm?

My take on it would be that the transistor switches the relay and has a resistor from base to positive power, so the relay turns on and sounds the alarm (plus possibly one of its contacts shorting the transistor so it stays in the alarm state until it is reset).

The cable socket would just connect between base and 0V, so when the chain of cables is plugged in and the far end linked to a shorted-out socket on the alarm box, the transistor is held off.



Something like that, with another resistor from "input" to +12V (something in the 1K - 10K range probably) to switch the relay on, and the external cable socket connected across input and 0V to hold the transistor off unless the cable is disconnected.

Or it could have both ends of the cable chain used, with the path through all the centre pins ort whatever as the link that shorts the input.

Use a two pole relay with one normally open connected across the transistor collector & emitter, if you do not want it to reset just by reconnecting the cables. It would only reset then if power was switched off & back on again.

(The diagram shows 12V but anything from 5V to 12V or more is OK, as long as the relay voltage is correct and suits the battery voltage range from new to exhausted. That is 1.5V per cell new down to 1V when flat, for normal cells).

please use break cords as switch replacement