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

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Hi all. Just found this forum/website while researching my predicament for the past few weeks...hope someone can help.

I'm switching alarm systems -- another wireless type that I'm trying to hack to use my existing wired sensors instead. I did this successfully with my old (current) system, by connecting the wired sensor wires to the wireless sensor's reed switch. So, instead of a magnet attached to my door activating the wireless sensor's reed switch on the door frame, I let my wired sensor on the door shunt the reed switch (mounted in my utility closet at the alarm box) which then triggers the alarm signal -- on an open condition. Effectively, I'm just using the wireless sensor to transmit the door action to the alarm base station.

All well & good with the current system, but the new system doesn't like the resistance (about 6 ohms) on my sensor wiring (multiple doors and windows wired in series) and apparently doesn't distinguish that from an open condition.

So, I inserted a Sainsmart 5v relay board into the mix, to handle the switching of the wireless sensor, with the relays triggered/energized via the wired sensor action. This works, but with two problems: I don't like having mechanical relays in this circuit, and I especially don't like having the relay coil energized all the time a door is closed -- or a window is open for that matter. But I have a few constraints. The wireless sensor is basically a black box to me -- I have no idea what the circuit looks like that contains the reed switch that I'm shunting, but it seems to trigger on the trailing edge, i.e., it only "fires" when it's been closed and then opened. For this reason, I'm not sure I could connect it directly to a MOSFET with its voltage and current flow.
Not to mention, these relays I'm using require a "LOW" (0, or ground) to trigger, and all I've got is a sensor circuit that is either closed or open -- and open doesn't translate into LOW very easily.

OK... I struggled with how to describe this problem with enough detail to make it understood, but without too much that you don't want to read it (TL,DR?), and you could say I've failed at both ends, but here's the bottom line:

When my series-wired sensors' circuit is closed: I need the wireless sensor's reed switched to be shunted (shorted);
When that wired circuit opens: I need to unshunt/unshort/open the reed switch circuit.
And...I'd prefer mechanical relays were not involved. (also prefer not to have to insert a microcontroller into the mix - has to be an easier way, right?)
And...the wireless sensor/reed switch circuit is a black box to me.

Any ideas/suggestions will be greatly appreciated. Have been head-banging on this for far too long.
 

Picbuster

Joined Dec 2, 2013
990
I do see your problem but it's a safety system and it should be extreme rigid.
trial and error seems to me not save
Advice: buy a proper one with zone alarm, Fire, Gas, Nox, Co2 plus alarm relay messages to an external guard. (like police, fire guard +++)

Picbuster
 

sghioto

Joined Dec 31, 2017
1,038
I don't see a simple solution other than using a relay. The circuit provided below is just a simple inverter to activate the relay when one of the sensors switches is open.
SG
EEE Alarm relay mod.png
 
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Thread Starter

jwrothwell

Joined Aug 9, 2018
23
I do see your problem but it's a safety system and it should be extreme rigid.
trial and error seems to me not save
Advice: buy a proper one with zone alarm, Fire, Gas, Nox, Co2 plus alarm relay messages to an external guard. (like police, fire guard +++)

Picbuster
Thank you for responding. I have bought a proper one that has all the various sensors, and it does send alerts to a central monitoring service, and once I've solved this one problem it will actually be a bit "safer" since it will no longer have the potential of a weak & possibly ineffective signal from the far-flung sensors (they will all be next to the base station in my revised setup). But thanks for the advice.
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
I don't see a simple solution other than using a relay. The circuit provided below is just a simple inverter to activate the relay when one of the sensors switches is open.
SG
View attachment 157871
Thank you so much -- much simpler than my ideas involving CMOS inverters and MOSFET drivers. Would love to get rid of that relay, tho, as I'm not sure about its coil being powered all night when a window is left open. Any idea how long it's safe to keep such a relay energized?
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
I'm having a hard time following, but does your alarm system need termination.

https://www.structuredhomewiring.com/SecuritySystem/TamperProofWiring/
Thank you for your reply. I'm studying the info at this link and one statement jumped out at me right away: "Note that End Of Line Resistors (EOLRs) can only be used if the alarm panel has been designed to use them." And of course, since I've removed my alarm panel in favor of my bridge to the wireless sensors of the new alarm system, your question would more aptly be, "Does your circuit need End Of Line resistors?". And to that, I'd have to say, I don't know. I suppose it would depend on how the circuit eventually ends up being wired. Do you have an opinion on this, as to how it would work? Thanks again.
 

sghioto

Joined Dec 31, 2017
1,038
Any idea how long it's safe to keep such a relay energized?
They are usually designed for continuous use. You mentioned the wireless module would not operate when you connected the series switches across the reed switch because of the 6 ohms of resistance, seems that the relay option would work the best. Possible will work with a analog switch, will look into this.
SG
 
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Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Here's a small chip that would probably work if you can handle surface mount components.


https://datasheets.maximintegrated.com/en/ds/MAX4624-MAX4625.pdf
SG
View attachment 157889
Yes, that looks great as a direct replacement for one of my relays, but I'm wondering...do you think this would be functionally equivalent for my purposes (since I really don't need SPDT):

https://www.mouser.com/datasheet/2/427/dg611e-1155094.pdf

And could I wire it the same as you indicated for the MAX4624? (Repeating 4 times for 4 reed switches) They cost about 10 times less per switch, plus would allow me to use just 2 IC's for my 8 zones. Thanks.
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Here's a small chip that would probably work if you can handle surface mount components.


https://datasheets.maximintegrated.com/en/ds/MAX4624-MAX4625.pdf
SG
View attachment 157889
Sorry...Forget that DG611E -- I just noticed that it has an On resistance of 160 ohms. So, same questions for:

https://www.mouser.com/datasheet/2/256/MAX4661-MAX4663-258235.pdf

A bit pricier, but still a third less per switch than the 4624, and I'm thinking that the 2.5 ohm On resistance will be ok.
 

crutschow

Joined Mar 14, 2008
24,354
Here's a simple circuit using one N-MOSFET that should do what you want.
When all the sensors are closed, 5V is applied to the MOSFET gate and it is turned on giving a very low resistance from the Alarm terminal to ground.
When a sensor is opened than the MOSFET gate voltage is pulled to ground by R1 and the MOSFET turns off, giving a very high resistance to ground at the Alarm terminal.

M1 can be just about any logic-level type N-MOSFET (Vgs maximum threshold of 2V or less) with the desired ON resistance (well below an ohm is very common).

upload_2018-8-10_23-48-14.png
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Here's a simple circuit using one N-MOSFET that should do what you want.
When all the sensors are closed, 5V is applied to the MOSFET gate and it is turned on giving a very low resistance from the Alarm terminal to ground.
When a sensor is opened than the MOSFET gate voltage is pulled to ground by R1 and the MOSFET turns off, giving a very high resistance to ground at the Alarm terminal.

M1 can be just about any logic-level type N-MOSFET (Vgs maximum threshold of 2V or less) with the desired ON resistance (well below an ohm is very common).

View attachment 157896
Thanks! I like this, both for its simplicity and the fact that I already have a bunch of N-MOSFETs lying around (IRLZ14PBF -- that should work, right?) I was hesitant before about putting the MOSFET across the reed switch of my black box sensor, but after studying these various datasheets for the past several hours I'm feeling more confident that it won't fry anything. Gotta get a few z's now but I'm going to try this first thing in the morning. Worst case, if something fries I'll just throw some bacon on it and make the most of it. ;-)
 

Thread Starter

jwrothwell

Joined Aug 9, 2018
23
Specs look good with an on resistance of .2 ohms and logic level drive (5 volts). Your not going to fry anything it's just whether the wireless module will work with that setup.
SG
My concern all along with the MOSFET approach is that I'm introducing its own current flow across the switch terminals of a totally different circuit, with possibly an opposite direction of flow than that other circuit. I can't quite wrap my head around what the effects of that would be.
 

crutschow

Joined Mar 14, 2008
24,354
My concern all along with the MOSFET approach is that I'm introducing its own current flow across the switch terminals of a totally different circuit, with possibly an opposite direction of flow than that other circuit.
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.
 

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.
Gotcha- thanks again!
 
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