Some sensors MAY actually work. This does not discard what i wrote before (I tested them all), but: For some reason some of the sensors gives invalid response to ‚Power supply’ command. They always respond as they are parasite-powered. Some devices (ex. mine) do check that and reported them as invalid. However I tested all ‚Invalid’ with osciloscope. Some of them gave some response, so i took the deeper test.
If you’re planning to order cheap DS18B20 sensors from AliExpress – keep in mind that irrationally low price (less than $0.8 / pc) smells really fishy.
Manufacturer price for 1k is $1.18.
I recently ordered
two six lots from two different sellers:
http://www.aliexpress.com/item/10pcs-lot-DS18B20-TO-92-18B20-chips-Temperature-Sensor/32615902919.html (Keeps data line low)
http://www.aliexpress.com/item/DS18B20-DALLAS-TO-92-Programmable-Resolution-1-Wire-Digital-Thermometer-new-and-original-free-shipping/32460503179.html (No response for presence)
http://www.aliexpress.com/item/10PCS-LOT-DS18B20-TO92-18B20-TO-92-Temperature-sensor-New-and-Origianl-DS18B20/32353012700.html (No response for presence)
http://www.aliexpress.com/item/Free-Shipping-10PCS-DS18B20-TO92-Sensor-18B20-TO-92-Digital-Thermometer/514174063.html (All works!)
http://www.aliexpress.com/item/Free-Shipping-10pcs-lot-DS18B20-18B20-TO-92-DALLAS-MAXIM-100-New-Original/883530054.html (Keeps data at +1.0V)
http://www.aliexpress.com/item/Free-Shipping-20pcs-DALLAS-DS18B20-18B20-18S20-TO-92-IC-CHIP-Thermometer-Temperature-Sensor/32236763433.html (All works)
In first case I received 4x DS18S20 and 6x something else (not really sure what, but definitely not DS18B20)
In rest none was DS18B20 sensor. Instead i got one NPN transistor, one PNP transistor and 8x
something else (thyristors actually).
The easy test:
According to DS18B20 datasheet, there should be an input diode somewhere between VCC and GND. If you connect the sensor in the opposite way it should give a short.
To test it, I simply used multimetr with diode check mode, connected between VCC and GND.
For real sensor it should ‚detect’ the diode.
For fake of course it would not.
How to recognize fake parts?
As for today (18.05.16) I see some common attributes of all fake parts:
They have DALLAS marking. Normally, when ordering from china, I got parts with ‚7Q-Tek’ or others. Some working parts have DALLAS also. But no DALLAS marking increases chances for working product.
- Some fakes had +919AF serial code, none worked. The +233AA – some good, some dead
Surface with marking is ‚sandy’, not glossy. Its probably because of removed marking
- The diode test (above)
- Test for any communication