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I have two DS18B20 temperature sensors connected via a OneWire bus to an arduino. I noticed that these sensors get hot, when permanently powered and decided to just turn them on for measurment.

First attempt: Define a pin as output and feed the sensors. Because I received only one value (first sensor on bus), I assumed that the current delivered may not be high enough and utilized a transistor (+resistor) as switch for the main 5V line to feed the sensors from the mains (as before). The result is the same. I only get the value of the first sensor. When I connect the sensors directly, I get both values.

I tried up to 4 seconds delay after switching the bus on and again after reading the values (before switching the bus off again), with no luck. Only the first sensor answers.

Do I have a timing or a current problem here?

UPDATE

Wiring (working): Two DS18B20 in parrallel with a 4,7k resisor between DATA and VIN. One Arduino pin used, Sensors directly powered by power-supply (same as arduino)

Wiring (not working): Broke the VIN line and added a transistor as switch between power-supply and sensors VIN. Used a second pin and a resistor to power up the line.

The transistor is a TC124E (npn) logic level transistor. I have the feeling, that this boy is not providing enough current to the sensors, which in addidtion have five meter cables attached...

UPDATE II It is not the current. I replaced the transistor with a mosfet (70T03H) and the result is exactly the same: When switched, only one sensor answers . When I connect the gate directly to a +5V line, I get both values as expected.

Kinda think, that the switching of the pin irritates the bus or the specific device.

UPDATE III (Solution)

I tracked this down to a mixture of

  • Different resolution settings on the sensors
  • my code not waiting long enough to get the sensor with the higher resolution to setup (on-to-read time). Giving the bus slightly more time to setup before any reading is accepted, solved my problem.
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4
  • Please add the wiring used.
    – Mikael Patel
    Commented Aug 21, 2016 at 12:22
  • Cable length won't lead to sensor heating but may cause sufficient distortion of signals to prevent recognition of all the devices.
    – JRobert
    Commented Aug 21, 2016 at 12:59
  • @JRobert I know :), but the cable "consumes" current and I beg this is my problem her. Going to heat the iron and report the outcome.
    – moestly
    Commented Aug 21, 2016 at 13:01
  • Or to be more accurate, the current in a long cable could cause a voltage drop (current in must == current out at any point on the bus). But yes, testing with a short bus is a good first step. Also see my (ASCII) schematic for a parasitic bus with strong pullup, in my answer below.
    – JRobert
    Commented Aug 21, 2016 at 13:04

1 Answer 1

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If you were trying to wire your bus in parasitic mode, here is what I am using:

OneWire Bus in parasitic mode:

+------------+     +5v            +---------+     +---------+
|    MCU     |      |             | DS18b20 |     | DS18b20 |
|            |  R_pu = 4.7K       | g  d +5 |     | g  d +5 |
| Strong-    |      |             +-o--o--o-+     +-o--o--o-+
| Pullup   > o------o               |  |  |         |  |  |
|            |      |               +--|--+         +--|--+
|            |      |               |  |            |  |
|    Data <> o------o---------------|--o------------|--o------ - - -
|            |                      |               |
+------------+                      V               V

I run at least three sensors this way on an approximately 6 meter bus, completely reliably.

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  • Nice Work. Upvote for that, even If it is not my wiring.
    – moestly
    Commented Aug 21, 2016 at 13:20

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