Detecting Multiple Switches Remotely

i can help you reduce one wire out of four. use an encoder. there will be three lines to represent the six switch states. A decoder at your end will decode the switch states.

prateek

pstiady wrote:

Surely 30m of telephone or alarm multicore is quite cheap?

--
Melodolic Spielberg

A typical problem.

OK

I'm confused. 30 meters of UTP networking cable has 8 conductors and is dirt cheap. I bought a 1000 ft box of Cat5 at a nickel a foot.

Again why would you need separate cables? I grant you that Cat 3 telephone wire is a bit cheaper, but not by much.

More complexity.

The simplest way to do it is to user 30m of Cat 5 which will cost way less than $10 USD and wire each switch to a line. I would advise using an optoisolator at the receiver for each line in order to protect your local circuit from noise picked up from the 30m of cable.

Next I'm just thinking out loud. A cheap and reliable way to accomplish this with two conductors is to use a current loop. The idea is to emit a constant current source (Something along the lines of 50ma using a LM317 and a resistor) Then each switch would switch in a resistor that loads the circuit with a specific resistances causing a specific voltage drop across the circuit. Using a sense resistor back at the current source, convert the current into a voltage. The voltage would represent the combinations of switches thrown. The easiest way to read that would be a microcontroller with a ADC, like some

18 pin PIC or small AVR part.

The remote end would be nothing more than switches and resistors.

Just an idea.

BAJ

Thank you very much. The idea of a current loop is really inspiring.

Correct me if I'm wrong, but I think there are 2^6=64 possible states, are they?

Patrick

Prateek wrote:

where's the 12V coming from.

cat5 is fairly cheap and has 8 conductors.

it could be done with 2 conductors.

where's the 12V coming from. what sort of an output do you want?

Bye. Jasen

The 12V is coming from the power supply. The output would be an indicator such as LED. I am thinking of using DAC - ADC. Will this work? Does anybody have a good suggestion what DAC and ADC that will serve this purpose?

Patrick

so you have a 12V supply at the signal end and want to operate 6 indticator lights at the far end of the cable ?

the switches can they be wired any way, or is there a requirement that they are connected directly to the 12V supply ?

if you're allowed to rearrange the switches I can see a simple way to reduce it to 4 wires. if 12V is also available at the switch end that'd help too. if about 12V AC (or less) is available even better.

Bye. Jasen

Six switches, each with two states, represents a total of

2^6 = 64 states. You need one wire per switch plus a common ground if the information is encoded in binary format.

A parallel multiplexor isn't going to save any wires, but a parallel to serial converter would.

You could use DPDT switches and have the second pole add/delete a resistor or capacitor in parallel with the R or C in a square wave circuit at the remote end. The output of the circuit could be used to gate an RF signal onto the two wires that carry V+ and gnd. At the decode end, detect the RF and gate that into a PIC. Program a lookup table that converts the pulse rate into which switches are on or off.

Or use the time it would take to create the above kludge to make a few extra bucks, and install the necessary cable to do it with a simple circuit, low cost, and high reliability.

Ed