- Location
- Massachusetts
DUCKMAN said:
I already mentioned I was wrong, but thanks anyway.
Burglar alarm system resistors
- Thread starter peter d
- Start date
- Status
I already mentioned I was wrong, but thanks anyway.
- Location
- Massachusetts
LarryFine said:
Larry it is a series circuit, not parallel. The burgler alarm contacts are wired 'closed' when the system can be armed, an open trips the alarm.
But as has been pointed out to me with the resister at the panel tampering with the circuit is made easier.
This of course is the opposite of Fire Alarm circuits with the detector contacts closing for alarm.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080411-2043 EST USA
I can build a normally closed dircuit with a termination resistor at the far end of the loop. Any deviation from the expected line resistance, defined by that resistor, can then cause an alarm or trouble signal. One could use +/- 5% for a trouble signal and anything greater than +/- 10% could be an alarm. How effective such a system is to prevent a knowledgeable intruder will depend upon time constants and their knowledge.
However, it can be very useful in detecting bad wiring, including shorts or opens, or high contact resistance.
.
I can build a normally closed dircuit with a termination resistor at the far end of the loop. Any deviation from the expected line resistance, defined by that resistor, can then cause an alarm or trouble signal. One could use +/- 5% for a trouble signal and anything greater than +/- 10% could be an alarm. How effective such a system is to prevent a knowledgeable intruder will depend upon time constants and their knowledge.
However, it can be very useful in detecting bad wiring, including shorts or opens, or high contact resistance.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080412-1622 EST USA
ELA:
I simply presentated an example. What actual set points you would use would depend upon many factors.
However, today, vs maybe 30 years ago, I almost always use 1% RN55 1/4 W metal film resistors instead of carbon comp. These are actually less expensive than carbon composition resistors, also a better temperature coefficient, and less variation of resistance with age. In most cases I do not actually need that tight 1% tolerance but it makes no sense to use carbon comp in most cases. Also a tighter tolerance on the resistors used in a product result in a better product within reason. At very high frequencies a standard metal film will have a higher impedance than a carbon comp.
Also you could work with a lower tolerance resistor and simply have an adjustment for the mean value of the resistor used. The ideal way to build a circuit to accomplish the detection goal would be with a bridge circuit.
Ball park you are about $0.01 each in an axial lead resistor, and less in a surface mount in moderate volume.
My resistor useage is mostly in gaging equipment used to make automotive differential axles.
.
ELA:
I simply presentated an example. What actual set points you would use would depend upon many factors.
However, today, vs maybe 30 years ago, I almost always use 1% RN55 1/4 W metal film resistors instead of carbon comp. These are actually less expensive than carbon composition resistors, also a better temperature coefficient, and less variation of resistance with age. In most cases I do not actually need that tight 1% tolerance but it makes no sense to use carbon comp in most cases. Also a tighter tolerance on the resistors used in a product result in a better product within reason. At very high frequencies a standard metal film will have a higher impedance than a carbon comp.
Also you could work with a lower tolerance resistor and simply have an adjustment for the mean value of the resistor used. The ideal way to build a circuit to accomplish the detection goal would be with a bridge circuit.
Ball park you are about $0.01 each in an axial lead resistor, and less in a surface mount in moderate volume.
My resistor useage is mostly in gaging equipment used to make automotive differential axles.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080412-2013 EST USA
Using a Zener, an approximately constant voltage device, as the end of line terminator requires an approximation to a constant current source as the exciter. Using either a resistor or a Zener as a termination requires some degree of filtering to minimize noise problems.
Both of these terminations will allow you to detect a shorted or open line. Also one could build a resistor into each switch so that if it is a normally closed system that a jumper across a switch would cause an alarm.
.
Using a Zener, an approximately constant voltage device, as the end of line terminator requires an approximation to a constant current source as the exciter. Using either a resistor or a Zener as a termination requires some degree of filtering to minimize noise problems.
Both of these terminations will allow you to detect a shorted or open line. Also one could build a resistor into each switch so that if it is a normally closed system that a jumper across a switch would cause an alarm.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080413-0942 EST USA
ELA:
What are the primary causes you encounter for false alarms?
What kind of sensors are you using, mechanical contact like a plug into a socket, contact type limit switch (Microswitch), magnetic switch, reed type magnetic switch, ultrasonic motion, IR motion, photocell, inductive, capacitive, RF, and I suppose others?
Typically how many sensors on one wire pair?
Would a system that used a single wire pair, and could identify the location of an open circuit within the range between two sensors be of any great value? To some degree this system could be self checking.
.
ELA:
What are the primary causes you encounter for false alarms?
What kind of sensors are you using, mechanical contact like a plug into a socket, contact type limit switch (Microswitch), magnetic switch, reed type magnetic switch, ultrasonic motion, IR motion, photocell, inductive, capacitive, RF, and I suppose others?
Typically how many sensors on one wire pair?
Would a system that used a single wire pair, and could identify the location of an open circuit within the range between two sensors be of any great value? To some degree this system could be self checking.
.
ELA
Senior Member
- Occupation
- Electrical Test Engineer
I was speaking in general terms that a 5-10% change in resistance is not very large. I prefer to have a larger noise margin.
I have done a lot work with EMC compliance for CE marking and when you subject a system to radiated RF and conducted RF a good noise margin as well as filtering -as you mentioned are important.
Older fire systems are notorious for being activated by keying up an RF source near to them.
I was just making a general observation that 10% is a small change to be detected as an alarm. Not that it cannot be done but I am more comfortable with something like + 100% for a open circuit warning and -75% for an alarm. Depending upon the termination resistance value used.
For system reliability it is good not to have too tight of a tolerance to allow for component variations, interconnection and line resistances etc.
Alarm systems are not my primary focus. I work in the industrial controls area and larger voltage thresholds and noise margins are a good thing.
I have done a lot work with EMC compliance for CE marking and when you subject a system to radiated RF and conducted RF a good noise margin as well as filtering -as you mentioned are important.
Older fire systems are notorious for being activated by keying up an RF source near to them.
I was just making a general observation that 10% is a small change to be detected as an alarm. Not that it cannot be done but I am more comfortable with something like + 100% for a open circuit warning and -75% for an alarm. Depending upon the termination resistance value used.
For system reliability it is good not to have too tight of a tolerance to allow for component variations, interconnection and line resistances etc.
Alarm systems are not my primary focus. I work in the industrial controls area and larger voltage thresholds and noise margins are a good thing.
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