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open circuit triggers relay

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=106721
Printed Date: May 15, 2025 at 2:30 PM


Topic: open circuit triggers relay

Posted By: orbitald
Subject: open circuit triggers relay
Date Posted: August 12, 2008 at 3:00 AM

Hi again,

In this application I am using four PIR sensor to monitor an area. When any one of the four sensors sense someone I want to trigger a relay that will creat a contact closure that will close contacts on the "next song" button of an MP3 player. This system is powered by an standard 12V automotive battery.
 
I am using four Honeywell Aurora PIR Sensors and thought it best to hook them up in series so that if any of the sensors goes off it triggers a relay:
 
I plan on using this Constant to Momentary Output (CMO) relay circuit to turn the sensor triggered relay into a pulse that the MP3 player will understand (if the button is held down it wont go to the next song - it wants a momentary pulse)
 
I tried hooking up two of the PIR sensors in series with 12V on one end the other to pin 86 of a relay and then GND to pin 85 and then using the 87b output to trigger the CMO relay for a pulse output. This works OK for two sensors but there is a 3V drop across each sensor leaving only about 7V on the relay side. I'm guessing this wont work for four PIR sensors.
 
Is there a relay circuit that triggers on an open circuit without running 12V though the series of PIR sensors? I know many of you would probably use transistors and such but my knowledge is somewhat limited and if I can get away with just using relays or off the shelf components that would be easier for me.

I'l try and answer any questions more quickly - I'm not used to such a wonderful forum...
 
Thanks again,

David



Replies:

Posted By: orbitald
Date Posted: August 12, 2008 at 3:17 AM

Oh I guess I should say that when the PIR sensors are trigger their contact goes "Open"

And I would like to minimize battery usage as this is run on solar.

Thanks again,

David




Posted By: i am an idiot
Date Posted: August 12, 2008 at 6:12 AM
Do the sensors have a common and a normally open set of contacts?



Posted By: KPierson
Date Posted: August 12, 2008 at 6:54 AM

Did you try it with all four sensors wired in series?  Despite the "voltage drop" I bet it would still trigger the automotive relay.

However, if you want low current a standard autmotive relay isn't what you want.  If you want off the shelf you should look in to the PAC TR-7 or the DEI equivalent (not sure of part number).  These little modules should be able to take your signal and create pulse outputs from it - and I would imagine they would use less current then a standard Bosch style relay.



-------------
Kevin Pierson



Posted By: orbitald
Date Posted: August 12, 2008 at 1:05 PM

The sensors only have four contacts (I didn't get the ones with the tamper alarm):

1) +12V
2) -12V

And

3) Closed Circuit Protective Loop
4) Closed Circuit Protective Loop

The circuit 3-4 goes open when a person is sensed.

And I'm checking into the PAC device - looks promising!

Thanks,

David




Posted By: megaman
Date Posted: August 12, 2008 at 7:18 PM
when you say "-12v" do you mean Ground?



Posted By: orbitald
Date Posted: August 12, 2008 at 7:44 PM
Oh - my mistake.

Should be GND for #2

D



Posted By: dualsport
Date Posted: August 14, 2008 at 9:07 AM
You could do it with just a single resistor and a BS-170 MOSFET.

With all your PIRs wired in series, one end of the string would connect to ground, and the other end connected to a positive voltage source through a 1M ohm resistor, and also to the input of the transistor.
The transistor output (source and drain) would be connected to your MP3 player contacts, with the drain connected to ground and low side of the contacts.

When all the PIRs are closed, the transistor input is kept at ground, which turns off the transistor. When any PIR opens up, the ground connection is lost, and the 1M pullup resistor pulls up the voltage going to the transistor gate. With 12V on the gate, the transistor turns on, and closes the contacts on your MP3 player.

This uses a minimum of power since there's no relay coils involved.
Are your PIRs being run off battery power also? Wouldn't seem like a battery will last too long if that's what you're using to power those up also-

Should be pretty simple to wire up-




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