Today's article will once again be dedicated to the DS18B20 sensor to answer two common questions. On the one hand, Maxim now produces two different variants, one with PAR and without PAR mode. Secondly, many customers ask how many of the sensors can be used in practice at the same time, which is why we are doing a small test setup to see how many sensors can be operated on only one pin of the nano.
When the sensor came from Dallas, there was only one variant, which supports both parasitic mode and normal operation. Maxim now produces the sensors in the DS18B20 and DS18B20-PAR versions. Both support the 1-wire protocol: The 1-wire protocol has its name, so that only one line is required for communication instead of at least two data lines as usual, the PAR mode has nothing to do with it. So we have three pins, or cables, in normal operation: Data, VCC and GND. VCC and GND should be clear, communication is bidirectional via data. And since electricity must always flow to transmit information, this can logically only be done via GND.
The DS18B20-PAR supports parasitic mode. This is called so, because the sensor can shave the current out of the data line. This means that only 2 lines are required in this operating mode: GND and Data. The VCC of the IC is connected to data via a resistor.
(With the product in the picture above, a resistor was already placed in the sleeve and only two cables were led out of the sleeve.)
Maxim's product policy has created a real mess out of this situation. Since the DS18B20 used to support both modes and this is no longer the case, many retailers have not adapted your product description, did not change the name or at worst, do not know what you are selling at all.
The countless complaints, which are received not only by us but also by many co-providers on a regular basis, are avoidable.
Unfortunately, we found that if the DS18B20-PAR is also referred to as such, sales of the product will decline massively. That's why we use the designation DS18B20 uniformly.
In case of doubt, always ask the dealer back, the execution should not be explicitly indicated in the product description and one cannot do without the PAR mode for his project.
To set up the test: The question of a customer was how many DS18B20s on the nano can be operated on only one pin without any additional power supply. For this purpose, we built the circuit with 10 sensors described in the e-book on one of our breadboards and then wired one by one.
We were able to operate 6 sensors immediately with the nano on a pin and read out with the tester.ino Sketch from the DallasTemperature Libary, here is the result:
I hope today's contribution sheds some light on the subject and saves some of you some of your grey hair. Until the next time :)