1
\$\begingroup\$

I have a wireless security door sensor, which has a reed switch, and I'm trying to replace the reed switch with my existing wired security door sensors (thus not having to mount the wireless door sensor on my door when I already have one there).

I soldered two short wires to where the reed switch connects to the circuit board, and when I touch the two wires, it completes the circuit as expected. However, if I connect these wires to my wired sensor via 30' wire, then when I first close the circuit, the wireless sensor registers it as closed, but when I break the circuit, it continues to register the sensor as closed.

I confirmed with a multimeter that the circuit opens/closes when I trigger the door sensor. I also just used a 20' wire without a wired sensor on the end, confirming it fails in that case. When I use an 8' wire, everything works as it should.

I'm using 22/2 unshielded wire, so I'm wondering if this, combined with the distance is my issue.

Can anyone with more electronics experience explain why this would be failing? And, are there any solutions with my existing wiring?

\$\endgroup\$
  • \$\begingroup\$ did you confirm that the wire is not shorting out? \$\endgroup\$ – jsotola Jul 24 '18 at 0:15
  • \$\begingroup\$ I assume the multimeter test (resistance of infinite when disconnected and 0 when connected) was one way of confirming, but I tried a separate 20' wire with the sensors, which had the same issue. I assume the multimeter test would not be stable if shorting out, but I could be wrong. The 20' wire also did not show any cuts or kinks, so I think it's fairly sound. \$\endgroup\$ – Dolan Antenucci Jul 24 '18 at 2:30
  • 1
    \$\begingroup\$ There must be AC noise on the wire. Try 10K or 1K after release across wires and see if it measures <1 Ohm and 10k.or 1K, then try 1nF cap. \$\endgroup\$ – Sunnyskyguy EE75 Jul 24 '18 at 5:45
  • \$\begingroup\$ @TonyEErocketscientist -- at 20k and 2k, registers as 0.0 Ohm, and at 200, registers 0.8 Ohm. This is on freestanding 20' wire (not connected to anything but multimeter and itself at one end). \$\endgroup\$ – Dolan Antenucci Jul 24 '18 at 17:59
1
\$\begingroup\$

The sensor input is working with ac voltage.

Wires that are not working have a higher capacity that triggers the alarm. The weird not releasing issue is caused by the sensor input hysteresis.

No easy fix, just to change the wire.

Edit:

Actually there is but it works only if not battery powered , use the sensor to drive a reed relay placed near the alarm input or a mosfet transistor but this depends on the input.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the reply. Regarding the wire, what type of wire would work, if any for this setup? Note that the new wireless sensor runs off battery (3v lithium). I don't think the old wired sensor has a battery, but could be wrong. \$\endgroup\$ – Dolan Antenucci Jul 24 '18 at 18:36
  • \$\begingroup\$ @DolanAntenucci Then my answer is not entirely true but the issue is the same. To save energy battery powered sensors are pulling up the input for a very short time from time to time. If the switch is not closed the input voltage should look like a short pulse. Due the wire capacity the pulse is flattened and the switch might be read as closed. Same solution, shorter wires or with lower capacity. Try to use a capacimeter. \$\endgroup\$ – Dorian Jul 25 '18 at 8:07
  • \$\begingroup\$ I ended up deciding to move the sensors so that a shorter wire would work. Thanks for the help \$\endgroup\$ – Dolan Antenucci Jul 25 '18 at 20:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.