Can i share +ve and GND for multiple DS18B20 ?

craigcurtin

Senior Member
Guys,

I have a CAT5 cable run of about 70 metres (stupidly only pulled one when we did it and now no more access)

I currently have 2 x DS18B20's running fine from a 40x1.

I want to exapnd this to 3

My thought was to use two of the lines (or even two pairs) for +ve and GND and use the other lines for data lines to the DS18B20's

The +ve and GND could be broken out at a central point at the end of the run and then a standard 3 wire run to each DS18B20

Is this feasible to do - or am i missing something ?

If i was to do it this way would i put the 4K7 resistors at the far end of the wire (near the sensors) - or back at 40X1 ?

Craig
 

Technical

Technical Support
Staff member
This should be ok, but is a long run. Resistor position is best by experimentation, but you may want to reduce to smaller e.g 2k2 on this long un.
 

moxhamj

New Member
re "I have a CAT5 cable run of about 70 metres (stupidly only pulled one when we did it and now no more access)"

Should be fine for one or two more sensors. But the planets may still not be aligned and it may become necessary to add more things at the end of your wire. A display. Some leds. A video camera. I have been down this path and ended up sending 12V down one pair (with a local 5V reg) and data down another pair and using a picaxe at the other end to do the sensing and encode into a Serout packet. At the very least, put a plug at the end of the cat5 run rather than hardwiring it in!
 
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hippy

Senior Member
It should work with a common +V/0V to all sensors. Get it working on a short length of cable then on the long run. Start with a single sensor, then add the others.

I'd go with Dr Acula on adding LED+R's at the far end so you can see that power is on. What resistor value and where as Technical says will be a bit of trial and error, you may even find an R at each end works best. You might find that uF reservoir caps at the sensor end plus decoupling caps help as well. Putting 12V up the Cat5 and a local regulator at the sensor end may also help.

Alternatively put a PICAXE at the sensor end and use serial to send the data to the master. The Dallas one-wire (TM) interface is quite high-speed and there should be a lot less problems with much slower serial.
 

ylp88

Senior Member
I recall that the characteristic impedance of Cat5 line is around 100 ohms (you might want to check this...), so if you run into problems it might be worth trying to stick a 100 ohm resistor at both ends of the data lines to eliminate any signal reflections that may occur due to improper terminations of what is actually a transmission line.

ylp88
 

craigcurtin

Senior Member
Thanks for all the answers guys

It is in the back of my mind longer term to move a Picaxe out there and just send the results to the master.

In the first instance I will experiment with the share GND and +ve and report back as to mix of resistors and positioning etc

regards

Craig
 
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