# How to sense the pull on a metal wire

I'm building a small interactive exhibition display for a museum. The showcased artifacts are protected against theft with a metal security recoiler like this:

If a visitor picks up one of the artifacts, I want to show educational information on a screen. Since all artifacts are secured with a security recoiler, I want to use the movement of the security wire as a trigger.

Ideally I don't want to modify the existing security recoilers.

I have a mechanical solution which is not super robust, that's why I'm looking for a pure electronic solution (Hall-Effekt, etc).

Since these anti-theft wires are open-ended I'm not sure if I could use something like hall-effect sensor etc.

Any ideas are welcome. Maybe there is a better type of sensor.

• this is not a very clear description ... someone pulls on the metal wire – jsotola Jan 7 '18 at 20:35
• I agree with @jsotola – you don't want to measure "pull" (==strain) but "movement", as far as I can tell. – Marcus Müller Jan 7 '18 at 20:38
• why do we have to force information out of you? please provide full description, including physical sizes of the components of your device. – jsotola Jan 7 '18 at 20:44
• Sounds like the wire is wound onto a spool (presumably via a return spring). Sense rotation of that spool instead. – Brian Drummond Jan 7 '18 at 20:55
• It would be so much easier if you could modify the recoiler. Or use a string pot in the first place. – mkeith Jan 7 '18 at 21:03

## 4 Answers

You could probably sense motion of the cable with an optical mouse chip. That particular one may no longer be available.

• the chip quoted is for an optical mouse? How can that work? – Chris Jan 7 '18 at 21:08
• @Chris Point it at the cable. The mouse chip takes low resolution photos and compares successive images with an on-chip DSP to determine motion. – Spehro Pefhany Jan 7 '18 at 21:34
• I like that idea. – Marcus Müller Jan 7 '18 at 21:56
• This is not a bad idea, I would be concerned about contamination though... Kids can be messy little... er..um... patrons – Trevor_G Jan 8 '18 at 1:47
• @Trevor Yes, I think the real answer is probably simpler but it may violate the requirements of no modifications. – Spehro Pefhany Jan 8 '18 at 2:03

You could use a hall effect sensor to detect when a metal attachment or magnet on the wire is "home".

There are lots of different kinds, and prices, one of which I am sure you can make fit your needs with a little help from the model-shop.

Perhaps add a second one of those nice cylindrical cable clamps that goes into a pipe at the recoiler to line it up with the hall sensor.

The nice thing about these is it does not matter if Timmy drops his Ice-Cream on it.

BTW: If you have one of those recoilers you can open up, you may find there is some detail on the bobbin the wire coils up on that you can detect moving from outside the case with a hall effect sensor from underneath. It's a "Hail Mary", but worth a look before you go fabricating something.

• Or, just use a reed switch and permanent magnet, let a soft iron bead on the string be the pole piece that connects 'em. – Whit3rd Jan 8 '18 at 3:15
• @Whit3rd yup, though making it kid proof might be a little more challenging. – Trevor_G Jan 8 '18 at 3:18
• @Trevor thanks for pointing to the parts! Very helpful. Will the hall-effect sensor work just by moving the metal wire over it? – Chris Jan 8 '18 at 10:44
• @Chris I'd say no, it would probably be too finicky and unreliable with just a wire. But a little wire clamp is cheap. You would have to play with it to get the best results. The nice thing about them is, with the right one, they are just a switch and quite rugged. – Trevor_G Jan 8 '18 at 11:19

Another idea:- Maybe you could feed the cable through a 3D printed box with pulleys and a hairpin turn so tugging on the cable actuates the switch.

• Or just mount the recoiler on something with a switch... – Chris Stratton Jan 8 '18 at 2:10
• @ChrisStratton better - just needs to be strong enough to not compromise security- should be easy. – Spehro Pefhany Jan 8 '18 at 2:11
• The OP didn't mention it but this stuff also needs to be almost bullet proof. You tie it down.. some kid is going to try and yank it out of the wall.. Eventually one will probably succeed too :( – Trevor_G Jan 8 '18 at 2:12
• @Trevor There were some WWII displays down by the CNE grounds in Toronto, IIRC kids damaged a tank and (more sadly) a Lancaster bomber. – Spehro Pefhany Jan 8 '18 at 4:23
• @SpehroPefhany from experience I can tell, that the challenge is to make it robut enough to withstand real world usage. You only need to pull strongly and your 3D printed can breaks into pieces. – Chris Jan 8 '18 at 10:43

Mount the spool item on an elastic pillar, with a tilt sensor inside. Any tug on the string will be sensed. Or, if the string is long enough, feed it through a curved path, with an inside curve on the path connected to a pressure switch. Tension on the string will press the switch.

• wow this is interessting. You mean by using something like an accelerometer? – Chris Jan 8 '18 at 10:43
• @Chris - not so much an accelerometer, as a ball in a bowl that makes or breaks electrical contact when it's level. Pendulum and mercury switches are variants. Every car alarm has something of this sort. – Whit3rd Jan 9 '18 at 0:48