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I have a burglar alarm remote key pad. It's a Honeywell Gen4, which is a good quality common unit in the UK. Honeywell are not beginners at electronics. It has a small SMD chip with about 20 pins that's probably an 8 bit micro controller. Next to it is a shiny 8MHz crystal. The are 9 traces like below scattered randomly about the PCB:-

folded PCB trace

This is approximately drawn to scale, and are about 7mm long. What is it for?

These two images show the PCB:- Track example 1 Track example 2

I don't think that this is a serpentine track for a high speed transmission line. The PCB runs at 8MHz. The folded tracks are all separate, not in pairs as differential lines might be. They are all exactly 5 folds. I'm not sure how they would reduce reflections from 1 - 50m long wires strewn randomly throughout a house, routed past mains cables. They are covered in solder mask and on the back of the PCB, so they're not cheap landing pads for the push buttons. Do not be fooled by the small switch in the middle of one of the images. That's a tamper switch on the back of the PCB. All the key switches are on the other side. The tracks are however adjacent to various connectors where the alarm circuit wires are connected. Comms is 9600 baud so unremarkable. The board runs at 12V.

The panel is fully wired. It is not wireless, so these are not antennae.

Before this gets marked as a duplicate, I don't think that this is for high speed operation as 8MHz isn't that high. An Arduino runs at twice this rate, and doesn't have these.

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    \$\begingroup\$ Is this a wired remote or a radio remote? \$\endgroup\$ – ThreePhaseEel Feb 13 '17 at 0:19
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    \$\begingroup\$ Could this be PCB length matching? Could this be a PCB trace antennae? (A photo of your board could help us answer your question.) \$\endgroup\$ – Nick Alexeev Feb 13 '17 at 0:29
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    \$\begingroup\$ @NickAlexeev I would only think that if the edges were curved. \$\endgroup\$ – Bradman175 Feb 13 '17 at 0:30
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    \$\begingroup\$ @ThreePhaseEel Sorry, it's fully wired. There is no wireless facility on this panel, and it communicates with a wire only main panel. It's a Honeywell Gen4, a very popular piece of kit in the UK. \$\endgroup\$ – Paul Uszak Feb 13 '17 at 0:34
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    \$\begingroup\$ Could it be a low-value PCB inductor? Please post photos. \$\endgroup\$ – DerStrom8 Feb 13 '17 at 1:14
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They are resistors! Low-value ones. They even look like their symbol :)

If you take another look you might note that the traces are thinner than any of the normal traces on the board.

Now some have suggested inductors or capacitors but those look differently, a coil-pattern gives inductance for instance.

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  • \$\begingroup\$ For what purpose? The connected wires (standard alarm cable) are about 130 Ohms/Km so there would be significant resistance along the length of a typical domestic installation. \$\endgroup\$ – Paul Uszak Feb 15 '17 at 12:40
  • \$\begingroup\$ Current sense perhaps? But resistors are used in all kinds of circuits. Literally all kinds. But without a schematic you have to be familiar with the circuit in question to give an accurate answer. \$\endgroup\$ – user1890202 Feb 15 '17 at 13:20
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    \$\begingroup\$ I'm leaning towards the cheap&crude current sense resistor hypothesis too. Some burglar alarms try to detect tampering by monitoring currents. \$\endgroup\$ – Nick Alexeev Nov 6 '17 at 9:01
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If the material is paper-based phenolic they may be part of a tamper-proofing strategy that would open up those traces if the PCB is physically cracked.

Even if it is not phenolic, you can find other security-related applications in patents such as US20110255253 where the traces are connected to a port or ADC converter as part of a tamper-proofing strategy.

enter image description here

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  • \$\begingroup\$ This had also crossed my mind. If there are little conductive rubber pads on the cover somewhere designed to mate up with these traces, it could be acting as a PCB switch (much like what you would find in a remote control for a television). If the cover is removed the connection is broken, setting off the alarm. \$\endgroup\$ – DerStrom8 Feb 13 '17 at 17:39
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    \$\begingroup\$ @DerStrom8 OP says they are covered with mask, but it could be capacitive - that would be clever. \$\endgroup\$ – Spehro Pefhany Feb 13 '17 at 17:58
  • \$\begingroup\$ Please add the details from the comments to your post. Without them it's not clear how that tamper-proofing is intended to work. Thanks. \$\endgroup\$ – try-catch-finally Feb 14 '17 at 22:11

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