Stumped on trying to make a circuit

Thread Starter

shuck01

Joined Dec 22, 2018
2
I am trying to make something similar to what is called a pull pin or personal alarm, sometimes it is called a baggage alarm. I want it so when you pull or cut a wire it sounds a piezo buzzer, even if you put the wire back in place or reconnect the wire the buzzer doesn't stop. I only want the buzzer to stop when you disconnect the battery. I tried using a 555 timer to do this but haven't had any luck. Does anyone know a circuit or have a schematic for something like this so I can finish this?
 

dl324

Joined Mar 30, 2015
9,605
Welcome to AAC!

It would be helpful if you posted a schematic of your circuit so we know what we have to work with.
 

Thread Starter

shuck01

Joined Dec 22, 2018
2
Yep, just made a quick crude drawing, keep the laughing to a minimum please. I was trying to pull the wire going from the trigger(2) out of the reset(4) slot and have it activate. I guess I am doing this wrong.

 
Last edited:

Hymie

Joined Mar 30, 2018
741
The trigger (pin 2) is connected to +5V and the threshold (pin 6) is connected to 0V – so it will never work.

I suggest you revisit your proposed circuit.
 

djsfantasi

Joined Apr 11, 2010
5,956
Use a sub-mini audio or power jack, with a built-in cutoff switch. The switch is open with a plug inserted; it closes when the plug is removed. Use this to trigger an SCR as mentioned in post #5, and that will give you what you desire.
 

MrChips

Joined Oct 2, 2009
19,927
You have two issues that need to be dealt with separately.
1) an oscillator
2) a non-resettable switch

Firstly, work on getting the piezo oscillator working.

For part 2, you need something such as:

OR-gate latch
SET-only flip-flop
SCR circuit

Any such circuit should do the trick.
 
First, do yourself a favor and use a buzzer that doesn’t need an external osc.

Second, the simplest alarm is probably the SCR or a mechanical relay that uses one of the contacts to keep itself powered, the disadvantage is…you need to push a button to set when powered on. (relay)

But, if you really want to use a 555…here is another option…tested in LTSpice.

Pull_Alarm.JPG

EDIT:

Tested on breadboard as well.
 
Last edited:

the_arachnid

Joined Jan 3, 2019
14
I don't know if you want to make the circuit for fun or if you just want the product of it, but you can buy an alarm at the dollar store for fairly cheap. They have a reed switch on the main circuit and a magnet on a bar that you put on your window. You could have the pin have the magnet in it and then just have a small door with a spring close over the hole to prevent re-insertion. Just another idea.
 
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