Alarm sensor expansion

[Qbits]

I have an IR sensor and wish to extend this to 2 IR sensors using the same 4 wires. If I install a new sensor with its own wiring it means running the cables all around the building which is almost impossible as the installation is complete and hidden.

There is a +12n and ground input (same for all sensors) plus an alarm line and a common. All alarms connect to the common. This just leaves the signal line.
I assume that this is held high for 'normal' or pulled low for 'alarm' just in case the wires are cut.

If I connect a second sensor then both sensors will send a high signal. If one sensor detects an intrusion it will go low. Now the 2 inputs are competing so how or what simple circuit can I use if either input line goes low the output will also go low.

Access to parts is limited here in the Pacific and so I have resistors, capacitors, op amps, comparators and a few transistors to build it from.
Any ideas as a direction to study would be invaluable.
Qbits

 

[Lightium]

Just connect the sensors in parellel.

 

[Tony Stewart]

Normally it is a transistor switch that can be wired OR.

 

[rjenkinsgb]

Measure the voltage between the alarm and common wires, with and without the sensor triggered.

From that, it should be possible to suggest a suitable connection scheme.

Are they generic / commercial four terminal PIR alarm sensors?

 

[Qbits]

Thanks for the inputs. It may be possible to wire in parallel as suggested by Lightium but without a circuit diagram from the sensor I dont know what sending a back voltage down the signal will do?
rejenkingsb suggested testing the volts and when off it has 0.58v on the signal line which jumps to 3.3v with an alarm condition. Maybe a logic gate will not be required but some form of buffering might be useful.

It is a Paradox DG85 4 wire commercial sensor

 

[rjenkinsgb]

OK, so it has a normally-closed output.

You should be able to connect another one's alarm relay (normally closed) output contacts in series?
(And link power straight across).

 

[Qbits]

You should be able to connect another one's alarm relay (normally closed) output contacts in series?
(And link power straight across).

I cannot see a way to connect them in series and if I could I fail to see how it would ever alarm.

In parallel would be easy and should work fine if the impedance does not affect them when coupled together.

 

[Lightium]

What configuration are your code switches in?

 

[Diver300]

I cannot see a way to connect them in series and if I could I fail to see how it would ever alarm.

In parallel would be easy and should work fine if the impedance does not affect them when coupled together.

Put the red and black in parallel on both sensors.

Put the alarm outputs of the two sensors in series. Alarms like that usually alarm when contacts are opened, so if two contacts are in series, either can open the circuit and raise the alarm.

 

[Qbits]

What configuration are your code switches in?

Its cyclone season here so when or if it stops raining I can open the sensor and have a look.
The installer has basically just left us in the lurch to try and fix it ourselves.

Diver - thanks for the wiring diag. Now I see how to connect it but because the signal goes high to 3.3v when an alarm condition is registered I am lost how it could be a relay. Unless the 3.3v is generated on the control board and is grounded by the relay, when the relay opens then it lets the signal line on the board go to 3.3v and hence sound the alarm. That may make sense as a tamper proof design so if the line is cut it effectively opens the circuit.

 

[Qbits]

The attached file shows 2 PIR sensors wired to main PCB. There are spaces 3 to 8 for expansion and all those inputs have a 2k2 resistor to ground which I assume is to hold the 3.3v signal generated by the board to ground. This presumably stops these slots from causing an alarm. If that makes sense then a series connection would seem appropriate.

 

[rjenkinsgb]

The sensor alarm and tamper contacts are closed when everything is OK.

That means the alarm panel zone input normally reads just the 2k2 terminating resistor, which means the connections are neither shorted or open circuit.

The panel internally has a pull-up resistor on each zone input and reads the voltage at the terminal.
It can calculate the loop resistance from that, so it can differentiate a short circuit, open circuit or correctly terminated loop.

You can add as many more normally closed contacts in series with the alarm loop as you like, and any one being triggered (opening) will cause an alarm, as the overall loop has been broken.


The simplest way would be using four core cable.

Connect red & black to both the original and added PIR, then move the terminating resistor to same position in the new sensor. Use the terminals that was in for the two "loop" wires to the new sensor and connect that as the original one was.

The terminating or end-of-line resistor should always be in the farthest most device on each input, if possible.

This does not show the resistor, but demonstrates the concept:

These are some of the valid possibilities for connecting sensors to alarm zones such as your:

 

[rjenkinsgb]

For a bit more info on how these type of alarm zone inputs work, this is a grab of some of the "live" zone status page on my system -
The "loop" reading is the zone input voltage as a 0-255 value for 0V to full supply.

The ones showing around 145 - 150 are reading the EOL resistor at the end of each loop, while the 250+ ones have the loops open.

(It's an HAI Omnipro, which can work with multiple types of sensors, communicating devices & inputs, so things like the garage door status are used in logic sequences, they are not alarms themselves).

 

[Qbits]

Thanks for the details and diagrams. The way I understand it is, there is a relay inside the sensor that connects the sensor line to ground via a 2k2 resistor. This being NC will pull down the sensor line therefore no alarm. When the relay opens the sensor line is free to float or rise to 3.3v therefore triggering an alarm condition.

I have modified the layout given in the manual to reflect how I think it should be wired.
The 2k2 has been removed and then connected to the second sensor. If either relay opens then it should alarm.

I hope this is what you suggested.

Many thanks for all the help.
Qbits

 

[rjenkinsgb]

Pretty much, though the blue wire from the very right should connect to the other tamper terminal on the left unit, rather than having two blue external connections.

(The tamper switches are also normally closed, in series with the zone loop, from what you have posted earlier).

 

[Qbits]

OK I will connect the blue line and the power to the other unit thus avoiding the running of the extra wire.

Thanks