Datalogger

When I installed Solar Hot Water heating, I wanted to know just how much electricity my hot water cylinder was still using so I could accurately gauge the effectiveness of the solar panels. There are a number of ways of doing this, but I decided to simply record when the hot water cylinder turned on and off. I used a PicAxe datalogger and a current clamp sense to safely and accurately measure this.

Data logger front view

Data logger attachment

The data logger wakes up every 15 seconds and checks for a change of state in the current flowing through the hot water cylinder element, as sensed by the simple current clamp (400 turns of wire on a steel U-bolt). This keeps current consumption low (< 1 mA) and gives a switch on/off resolution of about 1 minute which is good enough for me. The PicAxe datalogger has a realtime clock (3 Volt lithium battery good for about 10 years) and I am using a 32 kB EEPROM which is capable of storing about 4,000 samples (each sample has 8 bytes of data). At about 100 samples a week there is plenty of time before it is necessary to upload the data to the PC.

The circuit diagram is shown below (click on to display full size). Full details including PicAxe code provided in this PDF document (userid: open, password: sesame). Since it has been in operation, I have found that long term reliability is better with 4 x 1.5 Volt batteries. As 3 x 1.5 Volt = 4.5 Volts starts to drop the PICAXE keeps working, but at around 4.2 Volts the RTC interface stops working and returns values of hex FF when interrogated. The components are designed to work at 5 Volts.

The results of the data logger are crucial to determining the effectiveness of my solar panel, see Solar Water Heating page.