# help with intruder/burglar alarm

#### channie

Joined Dec 22, 2013
2
I got a burglar alarm made from outside for my physics project. I showed it to my sir but he said it was an older version of the system(as in the circuit is open when the wire breaks instead of the usual ldr alarms i guess). It slightly confusing and i was wondering if a more in depth explanation of it could be givin? i have to show this in exam and even viva will be asked from it.
(the diagrams and all are given in the attachments)
thanks a lot

p.s. i know i shouldnt have got it done from outside

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#### Alec_t

Joined Sep 17, 2013
14,417
The circuit in the first pic has problems. That 'thin wire' is a near-short-circuit across the power supply, so the battery will die in next to no time. Also, the voltage spikes generated by the inductance of the speaker voice-coil will probably kill the BC546.

#### channie

Joined Dec 22, 2013
2
Ok my knowledge is very limited but I think I got it? If its no problem could you tell me how to correct it the circuit diagram?

#### Alec_t

Joined Sep 17, 2013
14,417
This is your project. Tell us how you would change the circuit to overcome the short-circuit and the voltage spikes. We don't do your homework for you; only give guidance.

#### Brownout

Joined Jan 10, 2012
2,390
Limit the current through the thin wire with a resistor. Hint: properly placed, the resistor would drop all the battery voltage, keeping the transistor base at 0v. I doubt voltage spikes from the speaker will be a problem, but here again you can use a resistor to absorbe some of the energy. This is a very primative circuit, so you should really be able to find a much better one with just a little effort.

Joined Jul 18, 2013
28,831
he said it was an older version of the system(as in the circuit is open when the wire breaks instead of the usual ldr alarms i guess).
The method often used now is a reed relay type switch that is held closed with a magnet to hold the circuit off, any movement of the magnet results in the reed relay opening and tripping the circuit.
Max.