74HC165 Wire Length!!

[Suraj143]

I want to connect 8 switches to 74HC165.The problem is switches are placed in 100m length.

Will this 74HC165 can detect this signal?Supply is 5V.

 

[Mikebits]

Bad idea. I mean, not a good plan, just...um bad idea.

 

[Suraj143]

Then what do I have to do?

 

[Mikebits]

That much wire is a big lightening bolt detector, or some other form of zap noise, not to mention the other myriad of problems you will encounter. How did you get stuck doing this design?

 

[Suraj143]

Hi Mike there is a building & it has 8 rooms.Each room have a switch.We want to display the room number when somebody has press the button.

The displaying stuff we can do.The only problem is the input switches wire length.Don't know whether this 74HC165 IC can detect that signal.

 

[Suraj143]

What about adding 8 opto couplers to the inputs of the 74HC165 & drive opto's with 12V via resisters?

 

[JimB]

The opto coupler is a good idea.
You need to be carefull with the power supply for the switches and optos, a separate supply from that used for the 74HC etc and logic would be ideal.

JimB

 

[Ubergeek63]

5KV RS232 transceiver chips are less expensive than optos ... put the HC165 at the far end and drive out clk,ld, and bring back d. at that distance they are still good for 2400bps

 

[Hero999]

I don't see the problem with a huge length of cable.

It should be fine as long as they're some diodes at either end, Schmitt triggers are used and the speed is kept low enough.

If ringing is causing many pulses then connecting a resistor to match the impedance of the cable is probably a good idea. You obviously won't know what the impedance is so you'll need to experiment but I'd start with a 600R resistor across the input at the end of the cable.