Wednesday, May 20, 2020

Day 104, 105 - Wemos D1 Mini

Tuesday (104) - More of the same overcast w/occasional showers. At 56°F, the temperature is the same as Fairbanks right now.

The Wemos D1 Mini microprocessors arrived today so I can start playing with them for my "SmartRV" project. I first connected one to the Raspberry Pi using one of the USB ports. This allowed me to upload a minimal configuration which included WiFi info and static IP address along with other network info. After that, it no longer needs to be plugged into the Raspberry Pi. Right now it is plugged into a phone charger and I can access the device using WiFi through the web server on the Pi.

Since I don’t have any sensors or relays yet (they should arrive tomorrow, Friday, or Saturday) I just played around with the built-in LED. From my phone connected to the Home Assistant website on the Pi, I can turn the LED on/off and vary the brightness using PWM. This whole process took about 20 minutes which included soldering the header pins to the D1 Mini.

Power for the boards will be from some tiny buck converters that are rated at 3amps at 5VDC with a 12VDC input. And they are really tiny. 0.5” x 0.7” and can be mounted directly to the circuit board. The first project will be temperature monitoring for the freezer and refrigerator. And an almost identical one will be monitoring the temperature in the wet bay.

Wednesday (105) - The temperature sensors I decided to use are DS18B20 digital sensors mounted in waterproof SS probes. The specs say that they are accurate to 0.5°C. Kind of overkill for my application but they were only $12 for 5 sensors. The sensors are designed to work on a buss so multiple sensors can share a single data line. Each sensor has a unique 64-bit address. The other option was the popular DHT sensors but they weren’t in the waterproof probes.

The disadvantage to the DS18B20 sensors is that I needed a 4.7KΩ pull-up resister. I don’t happen to have any lying around here (I do have a package at home!) so another Amazon order for 1,350 assorted resistors for $12. This is one of those times when I miss places like Radio Shack. Since I didn't have any resistors right now, I went ahead and tried out plugging the sensors in. The board has ~100KΩ pull-up resistors built-in but the spec sheet for the sensors said to use a 4.7KΩ resistor. And it looks like they work without the pull-up resistor. The next step is to "package" this so it fits behind the plastic panel on the refrigerator including the buck converter. For now, I'll just run the wires past the gasket. I may also run a temperature probe behind the refrigerator so I can monitor the temperature of the ammonia coils.

Of course, I had to see if it was going to work. The breadboard is taped to the door and the sensors are draped on the shelves. This is starting to get fun!

I have made airline reservations to head back home on June 4th. I also was able to schedule doctor and dentist visits for then. This is shortly after the expected end of the Alaska 14-day quarantine for out of state travelers. Hopefully, it won’t get extended again!


  1. Replies
    1. Thank you! This was supposed to be a project for the stay-at-home period but Amazon was de-prioritizing shipping non-essential things such as this.