Adafruit 1528-1592-ND

Support Q&A
1

------Question for 381 Please Put your question below------

In late April, 2025 I purchased 2 of these devices. The doc says that they have a DS18B20 sensor which is supposed to be quite accurate. That doesn’t match my experience. I have connected one device to a Shelly plus universal following this youtube video https://www.youtube.com/watch?v=qtt0W_CP294 and Shelly doc. I have this sensor, a Sensor Push HT1 and a bi-metal thermometer and they all read different values, sometimes up to 10F different. Additionally, I saw the 18B20 go crazy one day bouncing from ~35F to ~110F within 2-3 minutes, bouncing up/down/up/up/down continuously. It isn’t connected in parasitic mode and I didn’t use the 4.7K resister. I have read that there are a lot of 18B20 conterfits, but I think that DigiKey and Adafruit should be reputable. Is there something that I should investigate?

2

Hello ccrislerathome,

Welcome and thank you for your post in the TechForum.

I’m not familiar with these but I’m going to do some research on this and get back to you as soon as possible.

3

Hello @ccrislerathome,

I do not believe you received a counterfeit device. As stated on Adafruit’s product page, they’ve been testing for fake devices for years now (see the screenshot below). That being said, if you have an Arduino handy and would like to double check for yourself, I would absolutely encourage you to do so.

image
image1003×302 25.4 KB

Since you’re using a device like the Shelly Plus Uni to interface with the sensor, troubleshooting is going to be difficult as it’s not intended for development/debugging purposes. I do, however, have a few questions that may help sniff out any problems.

  • Did you receive the Adafruit sensor with three wires or four wires (see the last points in the screenshot above).
  • You’re using a Shelly Plus Uni but the video instruction are for the Shelly Uni. Did you check for any wiring differences?
    image
    image549×267 28.3 KB
  • You said your sensors “all read different values, sometimes up to 10F”. How are these tests being conducted? Are the sensors right next to each other? What’s the average difference between these values? Can you provide graphs of these sensor readings over time?
  • This is indeed strange. When I see behavior like this from sensors I play with, it’s typically because of an improper electrical connection or an issue with the firmware. Like I said before, since you are using the Shelly, there’s not much that can be done to debug the firmware other than make sure that your device is up to date. It would be nice if you could connect the sensor to an Arduino (or similar) to verify its functionality through more transparent means. Regarding the former point, however, it would be helpful to ensure that your connections to the Shelly are correct and sound. I assume you used solder and heat shrink as was done in the video? Perhaps check for bad joints and/or broken wires.
  • Finally, you purchased two of these sensors. Have you tested the other one to see if you get the same behavior??

Because the wiring diagram for the Shelly doesn’t include a pull-up resistor, I think it’s safe to assume that you indeed don’t need the 4.7K resistor that was included with the sensors. It should be included on the Shelly itself. I can’t say for sure, of course, since I don’t have a schematic nor do I have the device in my hands. You may consider trying to verify it yourself, though, as a sanity check if anything…

4

Thank you for your very helpful response. I really didn’t think that either DigiKey or Adafruit would be dealing with counterfeit devices.

I have the original doc from Shelly for the shellyplusuni, which is where you extracted the wiring diagram. Yes, I followed that diagram. The connections are soldered and have heat shrink. No, I haven’t tested the other sensor and don’t (yet) have an Arduino. The sensors are located within 12” of each other. The SensorPush is behind a sun shield, the ds18b20 is not, so I would expect it to be warmer when the sun shines but it isn’t. Both the ds18b20 and the SensorPush HT1 claim significant accuracy.

I don’t have complete confidence in the Uni. I have had a LOT of trouble with the signal strength. I ran it over the summer with a weak signal but it did stay connected to my network and updated the temp data. I had installed it as a test setup. When I decided to use it I sealed it inside an electrical box. I then noticed that there was a firmware update available, so I updated it. The firmware update reported a 20 dBm signal strength boost, which doesn’t give me a ‘warm and fuzzy’ feeling.

I use these sensors to control a greenhouse with HomeAssistant. Here is some temp data from both sensors. When the temp is in the low 60’s (F), the 2 are close. They do change in the same direction over a minute, but the ds20b18 changes more, but probably still within tolerance. The delta isn’t worrisome. However, later when the GH warmed up, they differed by 5 F, which I don’t like. I didn’t have a chance to compare these readings with the bi-metal thermometer. I don’t know what my next step should be. I don’t like calibrating at a single point (32F/0C), but I don’t have another reference temperature.

Device Temperature Time
shellyplusuni_temp 63.5 2025-11-03T05:00:00.000Z
shellyplusuni_temp 63.28 2025-11-03T05:00:07.352Z
SensorPush HT1 63.61 2025-11-03T05:00:00.000
shellyplusuni_temp 74.084 2025-11-04T15:20:24.834Z
shellyplusuni_temp 74.3 2025-11-04T15:21:04.836Z
SensorPush HT1 78.35 2025-11-04T15:20:50.891Z
5

Hello @ccrislerathome,

Thanks for the additional information. Let me give my thoughts on a few points.

12 inches can be quite significant depending on the environment. I haven’t spent much time in greenhouses, but I imagine it could be one such example. Consider placing the Adafruit 381 right next to (i.e., touching) the SensorPush and keep them both behind the sun shield. In this scenario, I would expect them to report similar temperatures over time.

Keep in mind, though, that the 381 is encased in a waterproof stainless steel tube. It will be much slower to changes in the ambient temperature compared to the SensorPush. This may also explain why direct sun exposure doesn’t raise the temperature readings as expected. Uncoated stainless steel reflects thermal radiation and the tube may not be well-coupled to a warmer surface. How is the 381 mounted? Secured to a surface, laying on a table, dangling in mid-air?

From what I can tell, the Uni is sold as a bare (unenclosed) circuit board with flying leads, no mounting holes, and a terse user manual with no mention of ESD precautions. Many potential issues could arise when using this board and most of them would be difficult to diagnose. I’m not saying it’s a bad product or that you can’t get good results from using it. I’m just saying you should be prepared to deal with unexpected results. Luckily, it does appear to be a popular product with a robust community of support.

You should definitely do this. Comparing the behavior of both sensors will have provide great insight into any potential issues. Especially if you use another interfacing device like the Arduino.

I have to say, these data points don’t lead me to believe there’s an issue with the 381 sensor. Like I mentioned, these are two different styles of sensors in non-identical environmental conditions. A 4.3°F differential may actually be correct.

Digital instant read thermometers are quite common and inexpensive. One experiment I might run is to place the 381 probe into a cup of water, wait several minutes, and compare the 381 reading of the water temperature with that of the instant read thermometer. Try several different water temperatures within the spec of the DS18B20.

Hopefully my ramblings have given you a few ideas. My overall advice is to run multiple tests, collect a sufficient amount of data, and try to isolate the issue. I understand it’s difficult given a lack of debugging equipment. But I encourage you to get creative and be persistent.

Best,
Matt