Feed Back through series circuit

[rauthr80]

I am using one series ciruit having no of switches connected in series through two wire cable. I want to take feedback in control room, that which switch is operated, how it can be done? In this case I want use same pair of cable that is used for connection in series circuit.

 

[IRstuff]

You can tell if one or more switches are open, by placing large resistors in parallel with each switch. By judicious choice of resistance values, you can encode the switch information and measurement of the net resistance will tell how many switches are open. This is how alarm switches are configured to distinguish between a normal switch open or a open circuit altogether.

TTFN

 

[OperaHouse]

You could also use multiple diodes with the forward voltage drop. Use numbers like 1, 2, 4, 8..... The same procedure as with resistors. Start with looking for the highest bumber and work back. Is there an 8 in the number, is there a 4 in the number, is there a 2 in the number, etc. An easy job for a PC or PLC. Above four switches creates accuracy problems.

 

[Skogsgurra]

I agree, diodes are a good solution. But there's a catch. If you have long wires then you usually get 60 or 50 Hz stray voltage into your circuit. This gets rectified by the diodes and creates a hard-to-interpret mess.

There are (at least) two solutions; either use two diodes "anti-parallel" instead of a single diode or put a capacitor across the switch/diode part of the circuit. Select the capacitor to be around ten times the cable capacitance. A 100 m (330 ft) cable needs something like a 100 nF capacitor.

Gunnar Englund

http://www.gke.org