I have a circuit like this:

Vcc(5V supply) -> 4.7K R --> LED ---> Push Button Switch S1 --+

Ground <----------------------------------------------------------+
24V Supply ------------------------->24V AutoLamp L--------------+

Gnd <-------------Transistor driver-------------------------------+


Switch S1 and L1 are located at a distance of approx 10 metres from supply point.

A four core cable ( 3 core cable with shield, with shield used as fourth conductor) is used. Shield and one wire of this cable are connected to switch.Other two wires are connected to the lamp. ( This cable has to be used since this board is a retrofit into an existing installation)

When S1 is pressed, LED conducts and glows. The junction of LED and S1 is connected as input to a 74LS123 Monostable to generate a pulse. This pulse, ORed with some other pulses is taken as RESET signal to 89C51. So When Switch is pressed , microcontroller gets reset.89C51 could also get RESET due to some other inputs.

Microcontroller software first keeps the Lamp Off, checks if RESET occured due to S1 and if yes, turns the lamp ON , by driving a transistor and then does some other control work.
If S1 was not pressed, then it looks at some other inputs.

This is working fine, as long as my shielded cable length is about 2-3 metres. It also works when I use a four core unshielded cable (of any length)

When shielded cable is used and when the cable length is of the order of 10metres, LS123 starts generating multiple pulses and 89C51 gets multiple RESETs.

Theorising that this could be due to induced current on S1 wires when Lamp is turned ON, I put a 0.47uF capacitor across S1 input and the problem was solved.

This board is to be installed at multiple locations. Now the problem is , different field locations appear to be needing different values of capacitors (probably because cable lengths are different). It is clear that cable capacitance is coming into play.

Any suggestions: what exactly is happenning and how to solve this problem?


Thanks in Advance
KG