Voltage drop switching relays?

[Joe in Bendigo]

Hi,
I need a design or some design help to build a circuit for security systems
fault finding.
First a little explanation:
A number (up to 6) of PIR (passive infrared) sensor are wired in series and
have normally closed relays inside them. When one of them is triggered, the
contacts open, setting an alarm for the circuit, which then dials out on a
phone line to a control room who sends someone out to find the cause of the
alarm.
The problem is that these sensors are only on at night, and are in a
different room each. A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the secuirty
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....
We really need to know which room the alarm came from - see the dilemma?

Now, rewiring each sensor separately would be current proactice, but isn't
an option for various reasons.
I can put a resitor across the relay terminals in each sensor without
compromising its effectiveness. I thought that if I put a DIFFERENT resistor
across each sensor, then when one is triggered, I would get a SPECIFIC
voltage drop (for EACH sensor) in the circuit!
What I need is a way of triggering a separate relay for each voltage dorp
value (or a small range either way), so that I can record the events in each
room to try and determine the cause.
So I need a circuit which latches or opens a relay for each of a small range
of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?
Your help would be very much appreciated!
Cheers,
Joe in Australia

 

[logized]

Hi,
I need a design or some design help to build a circuit for security systems
fault finding.
First a little explanation:
A number (up to 6) of PIR (passive infrared) sensor are wired in series and
have normally closed relays inside them. When one of them is triggered, the
contacts open, setting an alarm for the circuit, which then dials out on a
phone line to a control room who sends someone out to find the cause of the
alarm.
The problem is that these sensors are only on at night, and are in a
different room each. A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the secuirty
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....
We really need to know which room the alarm came from - see the dilemma?

Now, rewiring each sensor separately would be current proactice, but isn't
an option for various reasons.
I can put a resitor across the relay terminals in each sensor without
compromising its effectiveness. I thought that if I put a DIFFERENT resistor
across each sensor, then when one is triggered, I would get a SPECIFIC
voltage drop (for EACH sensor) in the circuit!
What I need is a way of triggering a separate relay for each voltage dorp
value (or a small range either way), so that I can record the events in each
room to try and determine the cause.
So I need a circuit which latches or opens a relay for each of a small range
of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?
Your help would be very much appreciated!
Cheers,
Joe in Australia

Joe,
You could try using a Microchip PIC 16F628 microcontroller
http://ww1.microchip.com/downloads/en/DeviceDoc/40300c.pdf - these are cheap
and have voltage comparators, could be programmed to activate relays etc as
required.
An alternative way to detect which sensor was triggered, would be to modify
each relay circuit so that the relay operates for a different amount of time
when triggered (by experimenting with changing capacitors in the relay
circuit for example).
You could then use the PIC microcontroller to measure the relay operate time
to determine which sensor was triggered.

Dave

 

[Phil Allison]

The problem is that these sensors are only on at night, and are in a
different room each. A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the security
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....

We really need to know which room the alarm came from - see the dilemma?

** Dear Joe,

instead of wasting you time chasing stray cats and strange sources of IR
energy why not SOLVE the silly problem ???

Replace those *PITA* PIR sensors with dual sensor IR / microwave ones.
These will only trip the alarm when BOTH sensors indicate there is cause to
do so at the same time. All the usual sources of false alarms ( moths, cats,
rats, passing car headlights, swinging doors or curtains in the wind etc )
will trip only one or the other type of sensor - but not both.

If you can find some microwave sensor types available cheaply - then use
them wired in parallel with the PIR ones. Locate them covering the same
space and *voila*.




............ Phil

 

[John Crighton]

Hi,
I need a design or some design help to build a circuit for security systems
fault finding.
First a little explanation:
A number (up to 6) of PIR (passive infrared) sensor are wired in series and
have normally closed relays inside them. When one of them is triggered, the
contacts open, setting an alarm for the circuit, which then dials out on a
phone line to a control room who sends someone out to find the cause of the
alarm.
The problem is that these sensors are only on at night, and are in a
different room each. A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the secuirty
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....
We really need to know which room the alarm came from - see the dilemma?

Now, rewiring each sensor separately would be current proactice, but isn't
an option for various reasons.
I can put a resitor across the relay terminals in each sensor without
compromising its effectiveness. I thought that if I put a DIFFERENT resistor
across each sensor, then when one is triggered, I would get a SPECIFIC
voltage drop (for EACH sensor) in the circuit!
What I need is a way of triggering a separate relay for each voltage dorp
value (or a small range either way), so that I can record the events in each
room to try and determine the cause.
So I need a circuit which latches or opens a relay for each of a small range
of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?
Your help would be very much appreciated!
Cheers,
Joe in Australia

Hello Joe,
since you have asked for any ideas, how about
an opto coupler across each closed relay contacts.

If a closed relay contact opens, the opto operates and
causes a small separately powered circuit to unlatch (delatch)
or a small separately powerd relay to unlatch itself.
A little led would show that the latch had been released.

That would be six little modules each powerd with its own
9V battery or whatever and two little push buttons on each
module to set or reset the latch the next day.
A bit of food for thought.

Regards,
John Crighton
Sydney

 

[scada]

Hi,
I need a design or some design help to build a circuit for security systems
fault finding.
First a little explanation:
A number (up to 6) of PIR (passive infrared) sensor are wired in series and
have normally closed relays inside them. When one of them is triggered, the
contacts open, setting an alarm for the circuit, which then dials out on a
phone line to a control room who sends someone out to find the cause of the
alarm.
The problem is that these sensors are only on at night, and are in a
different room each. A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the secuirty
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....
We really need to know which room the alarm came from - see the dilemma?

Now, rewiring each sensor separately would be current proactice, but isn't
an option for various reasons.
I can put a resitor across the relay terminals in each sensor without
compromising its effectiveness. I thought that if I put a DIFFERENT resistor
across each sensor, then when one is triggered, I would get a SPECIFIC
voltage drop (for EACH sensor) in the circuit!
What I need is a way of triggering a separate relay for each voltage dorp
value (or a small range either way), so that I can record the events in each
room to try and determine the cause.
So I need a circuit which latches or opens a relay for each of a small range
of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?
Your help would be very much appreciated!
Cheers,
Joe in Australia

Using different resistors to pull different Relay voltage coils could be a
problem. The relay pick up currents will be higher than the holding
currents, which would make it hard to define different voltage levels. Use
your resistor idea, but input that to an Analog to Digital converter then
input that to the serial port of a PC and have a program do data logging.

 

[Ken Taylor]

Using different resistors to pull different Relay voltage coils could be a
problem. The relay pick up currents will be higher than the holding
currents, which would make it hard to define different voltage levels. Use
your resistor idea, but input that to an Analog to Digital converter then
input that to the serial port of a PC and have a program do data logging.

The OP is referring to putting resistors in the relay change-over terminal
connections, not in series with the coil. While a comparator would probably
work, the false triggering would continue, as Phil has pointed out. The
system is a bodge and should be replaced - that would almost certainly be
less than the cost of continued call-outs.

Ken

 

[Bushy]

G'day Mate,

A number (up to 6) of PIR (passive infrared) sensor are wired in series and
have normally closed relays inside them. When one of them is triggered, the
contacts open, setting an alarm for the circuit,

So modify the circuit to only trigger on two pulses repeated within a
certain time. The sensor will trigger several times on most people, while
the cat won't set them of anywhere near as often.

which then dials out on a
phone line to a control room who sends someone out to find the cause of the
alarm.

Maybe set to re-ring every time it is triggered and the common ones would
then require two calls to trigger a callout. Sort of like train the operator
to be a computer program. Each call is only a few cents! As long as you
don't give the Kiwi operators more than half an hour for lunch, 'cause it
takes to long to retrain them!

The problem is that these sensors are only on at night,

Easy, put a bit of black insulation tape over the light sensor and they will
work as happily in the daylight!

and are in a
different room each.

How much do you want to spend???

A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the secuirty
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....
We really need to know which room the alarm came from - see the dilemma?

Are you sure, is there another way round like add a couple of cheap
black/white radio or cable cameras and switch between them?

Now, rewiring each sensor separately would be current proactice, but isn't
an option for various reasons.
I can put a resitor across the relay terminals in each sensor without
compromising its effectiveness. I thought that if I put a DIFFERENT resistor
across each sensor, then when one is triggered, I would get a SPECIFIC
voltage drop (for EACH sensor) in the circuit!
What I need is a way of triggering a separate relay for each voltage dorp
value (or a small range either way), so that I can record the events in each
room to try and determine the cause.

A couple of local cameras and a vcr, or a cheap old 486 laptop, a modem, a
powerpoint(?) and a webcam to work out which sites have cats and which ones
don't? You can pick up a 486 for a couple of sixpacks, and a cheap webcam
from Kerri-Anne http://sites.ninemsn.com.au/minisite/Mornings/default.asp
you can even get the second one for halfprice, but you could go to your
local retail outlet and work out a bargain on last weeks model they are
stuck with..........

Or the cheap way, a cat trap?

So I need a circuit which latches or opens a relay for each of a small range
of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?

You can use two voltage comparator circuits at each relay to compare the
voltages at each point. Should find a circuit in the 741 op amp cookbook!

Hope this helps,
Peter

Your help would be very much appreciated!

I knew it would be, and it gives really nice fuzzy glow, oops, how many
drugs have I had?

 

[Soeren]

Hi Joe,

What I need is a way of triggering a separate relay for each voltage
dorp value (or a small range either way), so that I can record the
events in each room to try and determine the cause.
So I need a circuit which latches or opens a relay for each of a small
range of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?

Take a look at the LM3914 LED voltmeter (LEDs could be replaced with opto
couplers or perhaps drive reed relays directly.


--
Regards,
Soeren

* If it puzzles you dear... Reverse engineer *
New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>

 

[Joe in Bendigo]

Thank you all for your input!
Some of your advice has made me rethink the issues and others I have
followed up by reading the sources you suggested.
This suggestion of using an LED voltmeter appears the most easily managed,
since I'm clearly no electronics expert.
I may even drive tone generators from the LED outputs instead and plug their
outpu into the microphone input of a laptop and record any triggers onto
file - so we can visually see where the trips occur and how often.
Cheers!
Joe

 

[Kirk Shirley]

Hi,
I need a design or some design help to build a circuit for security systems
fault finding.
First a little explanation:
A number (up to 6) of PIR (passive infrared) sensor are wired in series and
have normally closed relays inside them. When one of them is triggered, the
contacts open, setting an alarm for the circuit, which then dials out on a
phone line to a control room who sends someone out to find the cause of the
alarm.
The problem is that these sensors are only on at night, and are in a
different room each. A common fault for me to have reported is that there
were frequent "single hits" (short infrared events that triggered one of the
PIRs) in a circuit and the the circuit was isolated to stop the secuirty
guard driving to the site lots of times without ever finding anything.
Occasionally it is a heater that was left on overnight and it's thermostat
has been cycling. Other times we suspect a cat roaming buildings at
night....
We really need to know which room the alarm came from - see the dilemma?

Now, rewiring each sensor separately would be current proactice, but isn't
an option for various reasons.
I can put a resitor across the relay terminals in each sensor without
compromising its effectiveness. I thought that if I put a DIFFERENT resistor
across each sensor, then when one is triggered, I would get a SPECIFIC
voltage drop (for EACH sensor) in the circuit!
What I need is a way of triggering a separate relay for each voltage dorp
value (or a small range either way), so that I can record the events in each
room to try and determine the cause.
So I need a circuit which latches or opens a relay for each of a small range
of 6 voltges: eg. 3V to 3.5V - 4V to 4.5V - 5V-5.5V etc.
Anyone have any ideas?
Your help would be very much appreciated!
Cheers,
Joe in Australia

Dear Joe,
I agree with an earlier post of replacing them. If you don't want to do
this the simplest circuit is to use some "tattle-tales" used in Air
Conditioning. These are just some very low current fuses that open when your
relay contacts open..
Kirk

 

[Larry]

Dear Joe,
I agree with an earlier post of replacing them. If you don't want to do
this the simplest circuit is to use some "tattle-tales" used in Air
Conditioning. These are just some very low current fuses that open when your
relay contacts open..
Kirk