Circuit need: Cutting a wire gives HIGH signal - 12V supply

Hi, I have a device that will trigger on a +3V to +45V signal, and send an SMS text message. The application is a boat & motor alarm/tracker. I would like to place a wire going from the boat to the outboard motor. If this wire is cut (they normally just cut off all wires/cables to save time), someone is stealing my motor.

My problem is to turn this "the wire has been cut"-signal into a +3V to +45V signal for the device. To complicate matters, I have +12V readily available, but no other voltages. I require minimal power consumption while the device is in standby.

A voltage divider + a logic NOT gate seems like a good starting point, but I'm concerned about getting a good signal when the input goes from HIGH (connected) to "float" (not connected, not even to ground).

Better ideas, please? :-)

Add a "sacrificial" wire to the existing cable assembly, cutting it would be like opening a switch.

Use 2 wires. both are tied together at your motor. The other ends are at the two resistors. Have your sensor at the resistor that is tied to 12 volts. At the moment the wires are intact, you'll have a low voltage. After cutting the cable, (or only 1 wire), the resistor will pull it up to 12 volts, and your alarm will be activated. You would have to find out what pulldown resistor is in the unit, so you can use a resistor small enough to override that, and large enough to not drain your battery too much.

What you asked is open loop/contact sensors for burglar alarm systems, it will stop neighbor kids but not much to do with pro. The pro will cut your Arduino wire before cut motor wire.

[quote author=Runaway Pancake link=topic=165839.msg1237238#msg1237238 date=1368277674] Add a "sacrificial" wire to the existing cable assembly, cutting it would be like opening a switch. [/quote]

This is a good idea, to extend it, Add one or more "sacrificial" wire to the existing cable assembly, pre-cutting it would be like whole system is defect. beat up cover of motor Looking and feeling, put burning and smoke sign on it , remove motor specification plate...

put security case to protect motor.

put warning sign to warn burglar.

Three important issues;- 1. burglar here is low(no) tech, against it should be low(no) tech. 2. those ADT's sign does not give any tech benefit, in matter fact show what system you use to help burglar breaking. but at least it tell burglar if you break me, you need 10+ more mins than the one next door has no sign. 3. "devalue" the system you try to protect. missing motor specification plate lead no re-sale value of motor.

MAS3: Use 2 wires. both are tied together at your motor. The other ends are at the two resistors. Have your sensor at the resistor that is tied to 12 volts. At the moment the wires are intact, you'll have a low voltage. After cutting the cable, (or only 1 wire), the resistor will pull it up to 12 volts, and your alarm will be activated. You would have to find out what pulldown resistor is in the unit, so you can use a resistor small enough to override that, and large enough to not drain your battery too much.

I'm sorry, I didn't quite get this, could you do a quick drawing?

All other replies in this thread are also much appreciated, and led me to understand that I haven't been sufficiently clear. 1) marking/blocking/tagging the motor won't help. It may be stolen in Norway, and will in that case be resold in a country our conventions of resale value do not apply. 2) I'm not planning on using any existing wires, the plan was to run a wire back/forth to the motor (2-lead cable connected at the end) and use the cutting of this pair as the signal 3) they will most probably simply snip off all wires & cables to the engine, lift it off and be off with it in 10 minutes. I'm not expecting them to ponder much or look hard for the transmitter. 4) actually, I won't be using an arduino for this as I found a device on ebay that combines GSM+GPS+Acellerometer+magnetic compass that will send sms's through a well-developed protocol, both easier & cheaper than I could cobble together an Ard+gsm+gps+accel+gyro+magnetometer+charger+battery

Please keep the ideas comimg :-)

Sure.

Have a look at this Frtizing! sketch.
Connect 12 volts to the red wire, and GND to the black wire.
Connect your sensor to the orange wire next to the resistor.

There will be about 1.2 volts with a fully charged battery at the sensor wire.
Unless orange and / or gray wires are cut.
Then there will be 0 volts at the gray wire and 12 volts at the orange and sensor wire.

Remember to measure resistance to GND on your sensor wire at your device first, so you can be sure this 4700 Ohms will win from that resistance (if it’s there).

Thanks very, we're getting somewhere.. I tried R1=R2=4,7K and got 6V at the measuring point. With R1=47K I got ~1,2V at the measuring point with wires uncut and ~7V with them cut. However, the device has an internat 56K resistor, and wih the device attached I got ~2,5V at the measuring point. I've played with 22K and 33K as R1, but I am still struggling with getting consistent operation. If I attach the device straight to 12V, I get good, solid operation. Methinks I may have to add a transistor/relay to switch 12V based on the potential from the measuring point?

Of course when you take 4K7 and 4K7, you'll divide the voltage in half and you'll end up with 6 volts at that joint. But those are not the values i have drawn. I have 4K7 and 0K47 in that sketch. That's quite a difference. The voltages i mentioned were calculated with 14 volts (a fully charged battery is 13.8 volts). These values are correct as long as you have a ratio of 10:1 on the resistors, the voltage will have the same ratio (if you only measure the wire and the resistors with your unit disconnected). You measured 56 K internal resistance, is that to GND ? If you power up your device, do you measure any voltage at your sensor input ?

With that internal resistance (assumed it measures to GND), the combined resistance of 56K + 470R results in 466R, so that internal resistor wouldn't be significant.

Are you able to measure current (Amperes) ? In case you are, please measure the current on the input of your device when 12 volt is directly connected. Expect a low current. It is possible this input has a LED (of an optocoupler) to that pin and the resistors we are using will mess with that.

Strictly you do not need the smaller resistor, but i do feel better if you would use it (because in case of some mis wiring you'll have less of a disaster going on).

It's also possible to use a relay to get this working. But that relay will have a lower resistance and will do some more draining of your battery, as it will be always on unless the wire is cut. If you are using such a relay, you can switch 12 volts directly to your unit without anything (maybe a fuse) in between.

Hi, and thanks for your patience.
The input I am using on the device is actually meant to be connected directly to the ignition of a car - to sense that someone has turned the ignition on.
It is probably well protected on the inside - optocoupler / fuse etc. I measure the 56K to ground.

And: I think we may have this working now. I added a BC548 transistor, and fed the signal into it, thus:

The voltages listed are with the circuit closed, the voltages in (brackets) are with the “wire cut”.

This approach gives consistent operation of the device, with a current drain at rest at < 0,1mA

Do you see any flaws, if not we may mark this as [Solved] :slight_smile:

Well, that img-link sure worked well..

It's at: https://www.dropbox.com/s/6eqeag4pm46vojy/circuit.jpg

Sorry about the spamming here…
No, that only worked because the battery had drained sufficiently for the triggervoltage on base to drop below 1,5V.
With a fully charged battery, it became 1,5V. Time to break out the spreadsheet again… 56K and 3,3K, perhaps?

i think an op-amp would suit this question…

the opamp has 2 inputs , when the voltage is cut the opamp could be figured to spit out a high (or low) voltage…

sonnyyu: What you asked is open loop/contact sensors for burglar alarm systems

Here is my Google 2 cents base on that;-

Simple normally closed (NC) SCR security alarm circuit

Parts List

R1, R2 = 4K7, D1 = 1N4007, T1 = BC 547B, SCR1 = C106, B1 = DC 12V Buzzer.

Here, the transistor’s base is rendered inactive by tagging it with the ground potential and the tagged joint is attached to the particular gadget’s one of the body fitting screws.

If any attempt is made to steal or remove the unit from its position by disconnecting the wires, it will immediately trigger the transistor and the SCR, sounding the connected alarm.

Simple normally closed (NC) SCR security alarm circuit

Two important issues;-

  1. Once triggering, and reset (make closed again) is not supported, only way to reset is turn off power.
  2. No reinventing the wheel, especially no reinventing the square wheel.