Switch detection over long distances

BeckettM

New Member
The purpose of this post, is to pass on some ideas for the detection of a switch closure at long distances.

It is based on a circuit that I use for the trip system at a ski field, which operates in a normally closed system, in order to detect trips and cable faults.

In reading several posts here and at other places, it always intrigues me why people switch volts, rather than switching grounds.
When switching volts, any short in the cable usually results in damage or other problems, and can result in other problems related to power supply overload. :eek:

There is also the issue of exernal interference, which if received can pass directly into the supply line, and result in undesirable consequences.


The part circuit attached is in use and is based on a 12 year old version that used a 12v source. The new one has been tested with a 1kohm loop and still operates (although the LED is very dim).
The system is running at a distance of 1km, (so the loop is actually 2km), using 4 core underground telephone cable.


The design incorporates a 24vAC Varistor across the input, in order to help minimise any lightning damage that could occur if the switch became open.
(Just remember to include a large earth connection, and suitable copper tracks to pass it.)

The diode and capacitor were part of the ability to source from an external supply, and the full board contains 8 inputs. They are not necessary but it is good practice to include them.

The LED was for a visual indication, other than the ones provided on the Latching Board.
Again it is not necessary for the circuit to work, but is good practice.


One of the other advantages of using a normally closed system (open to pass data) is that the chances of external interference, is reduced and only becomes relevant when the switch is open.


Using a "shorted to ground" system also allows that the switch could simply connect to the local earth, rather than have to come all the way back to the source. (just remember to ensure you connect your negative supply to earth)

Hopefully this will give some readers some other ideas, or will assist those pondering if such things are possible :)

Cheers
Mark
 

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