IoT for Swimming pools: Zwave temperature sensor


Temperature sensor DS18B20 is a great product! It's cheap and internet is full of cheap waterproof cables with DS18B20 sensors. The sensor works in wide range of temperatures, from -55C to +125C. That happens to be the normal operating temperature of Finnish people. I need temperature sensor for yard temperature and in Helsinki area the lowest temperatures can be -25C. The lowest I've been in was around -40C( in northern Finland). And the Finns have sauna's. The highest temperatures in sauna can be +110C, DS18B20 should handle that as well. The I need to be able to measure the temps of my swimming pool. DS18B20 cables are mostly waterproof, so it works there also. Great product!


The sensor uses 1-wire-protocol that only needs two or three wires. One for ground, one for data and additional third wire for extra power for more cable length. You can add several 1-wire sensors in one network. Every 1-wire device has an unique address.

Fibaro Universal Sensor (Zwave) can read up to four DS18B20 1-wire sensors. The product is only a board, without any case or power (battery or transformer ).


The device is powered with two wires at the bottom, GNP & P. I bought simple 9V transformer and soldered a DC connector for it to GND and P wires.

For 1-wire I decided to use simple 3,5mm stereo plugs. GND was ground of course and I used tip connector for data and middle for power. It doesn't matter of course which ones you use, just be consistent ;)

Using stereo plugs for connecting 1-wire also made connecting three temperature sensors a lot easier. You don't have to solder 3+3 stereo plugs. Just solder 3+1 and then simply buy stereo plug dividers.



My current "case", a plastic bag. The Universal sensor is inside the bag and it has two connectors outside, the power and stereo audio.

Then I bought two pieces of (electrician's) 2m plastic pipe, so that the one with the smaller diameter could go inside the larger one. (I could not find straight pipe that was three meters long ). I filled the larger one with sand and made some bends so that it fits in the corner of my swimming pool. Painted the pipes with white epoxy water-resistant paint and waited for 6 days paint to dry. 



There are three sensors inside in different depths. The pipe is open because the sensors are water-proof. I might seal it later with silicon.


The measurements go to Raspberry Pi3 with Home Assistant python-based home automation system. The data is recorded to InfluxDB time-series database and Grafana is used for reporting graphs.

Right now the sensor temps are: 20,9C , 20,7C and 20,2C. I keep my pool water at about 21C because it nice to have it a bit chilly when you go swimming while having a sauna. Because I'm a Finn :)

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