Modding a wireless sensor alarm system to use wired sensors - need help triggering on open circuit

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
The MOSFET gate looks like a capacitor and has no significant current flow, so its drain-source current is completely separate from any current flowing in the gate control circuit.
The only thing in common between the two sides of the circuit is the common (ground) and there is no current flow in the common between the two sides.

If you are confused about current, just follow the current loop. Current can only flow in a loop so if there is no loop there is no current flow.
In this case there is a loop on the MOSFET output and a loop at the MOSFET input, but no loop between the input and output.
Success! ...with a little modification -- I had to add a 10k in series with the alarm sensor and Drain. Works perfectly.

I got sidetracked for a while when your circuit didn't work at first -- I put my ohm meter across Source & Drain and readings looked good -- .8 ohms when Gate was high (wired sensors closed) and very high resistance when Gate was low. But I also noticed that my wireless sensor fired whenever I was measuring this resistance (shades of quantum physics), but wouldn't fire if my meter wasn't connected. So I wasted a bunch of time trying various resistors across Drain & Source (none worked). It finally dawned on me to try inserting a resistor in series with the alarm sensor and voila!

Many thanks to crutschow and SG (would've gone with your earlier solution if I had had the parts on-hand)
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Well that makes little sense to me, but if it works, great. :cool:
I agree -- that wireless sensor circuit is a strange beast. It seems to be sensitive to the resistance delta between open and closed states as opposed to the actual values. I suppose it's got a comparator inside instead of just simple logic gates. Would love to see the schematic someday...
 

sghioto

Joined Dec 31, 2017
2,332
EDIT: Lets see here. The 6 ohms of resistance from the switches would not work but the relay circuit did.
Success! ...with a little modification -- I had to add a 10k in series with the alarm sensor and Drain. Works perfectly
That does seem strange. Maybe you don't need the mosfet, what would happen if you just install a 10K resistor in series with the switches and the alarm sensor ?
SG.
 
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Thread Starter

jwrothwell

Joined Aug 9, 2018
23
EDIT: Lets see here. The 6 ohms of resistance from the switches would not work but the relay circuit did.

That does seem strange. Maybe you don't need the mosfet, what would happen if you just install a 10K resistor in series with the switches and the alarm sensor ?
SG.
OMG. (And I never say that!) That works! Even tho that's what I was essentially doing when I added the resistor in series with the MOSFET, it's the exact opposite of what I would have expected.

What started this whole project was, after I soldered a couple wires across the reed switch in the wireless sensor and was able to trigger the sensor by shorting the wires together and then removing the short (of course), I then connected those wires to the cable coming from my wired switches and found that opening a door didn't cause the wireless sensor to fire. That's when I took a resistance reading of my wired switches line & found that it was about 6 ohms when closed -- both the doors and and the circuit. So, the only difference seemed to be, when I directly shorted the reed switch I had zero ohms; if I "shorted" it via the wired switches, 6 ohms. So the last thing I expected to work was to add another 10,000 ohms.

What the hell is going on here? Any ideas? How can it work at 0, not at 6, but again at 10k? And yes, I've just repeated the various scenarios -- triggers consistently with a direct jumper across the reed & with 10k across it -- but not with 6ohms. And I've checked -- there's no voltage on the line, DC nor AC. Could there be some kind of capacitance effect? It just makes no sense. (as crutschow famously once said ;-)

And I just spent the day yesterday dismantling my prototype board and soldering everything into a PCB...
 
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Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Just for the purposes of a complete record, for any future forensic circuit analysts...zone 2 wouldn't work with just the added series resistor -- had to revert to MOSFET circuit for that zone's 8 ohm resting state.
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Did you try different values of resistors?
SG
Yes, and since I'm flying blind, I went up & down the scale. Nothing worked on this zone except the MOSFET switcher. So I've resigned myself to the fact that I'll need to use that for each zone. A fringe benefit of the switcher/driver circuit is it lets me add an LED on each zone for visual status of all the doors and windows, by zone. But I'll still lie awake for a more nights trying to figure out what the heck is going on with these wireless sensors and their very strange behavior.
 

marcf

Joined Dec 29, 2014
260
Yes, and since I'm flying blind, I went up & down the scale. Nothing worked on this zone except the MOSFET switcher. So I've resigned myself to the fact that I'll need to use that for each zone. A fringe benefit of the switcher/driver circuit is it lets me add an LED on each zone for visual status of all the doors and windows, by zone. But I'll still lie awake for a more nights trying to figure out what the heck is going on with these wireless sensors and their very strange behavior.
Despite all of the strange observations, this sounds to me like a normally closed Class B intrusion alarm with a normal window of operation, Either the protected loop circuit heading to ground or heading to battery voltage would cause an alarm. Only one value for the EOLR (end of line resistor would be valid. I have a 'concept' diagram if you are interested.
 

MisterBill2

Joined Jan 23, 2018
8,440
What I think I read is that you abandoned the wireless system and went to a wired system. I was going to suggest that if your sensor connection wires were all usable to abandon the wireless system and return to the wired system. That would be more reliable and much harder to defeat, if it is installed correctly. So I think that you made a good choice.
Wireless systems can be intentionally defeated with a more powerful RF noise source covering their communications frequency band . And that defeat method leaves no trace when it is switched off. Sorry but I can't reveal the source of this information.
 

marcf

Joined Dec 29, 2014
260
This is NOT an actual working circuit of a Class B alarm system, but it reflects how one is wired, and what is going on in the control box. Other details would include the activation of indicating appliances (horns, strobes, etc).
 

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