Reducing wasteful solar battery drain
Reducing solar battery consumption during low output solar days.
While solar energy offers many benefits, one downside is using storage batteries to power devices at night or during rain. This affects pool pumps and garden sprinkler systems run by well point or borehole pumps. A cheap rain sensor can solve this, but these sensors corrode quickly due to the DC voltage across the electrodes, rendering them ineffective after a few weeks. (electrolysis).
Very popular budget rain sensing device.
Sensors – capacitive vs a pure DC system
The sensor uses a comparator, with the threshold voltage set by the trimpot on the PCB. Copper strips, nickel plated, are prone to corrosion, resulting in a short service life of a few weeks. Gold plated modules are better. The image below shows an improved design with a low wattage heater and gold plated electrodes. Despite its reliability, lifespan is affected by the DC applied to the electrodes and pollutants in the rain.
Gold plated rain sensor with heater.
Capacitive Sensors
If the DC arrangement is not reliable, what is the solution? Based on my experience and extensive reading, there are two types of sensors that perform better than the former sensor type. They are (1) the capacitive sensor and (2) the more expensive but highly reliable prism-type rain sensing device.
Capacitive type rain sensor
The Capacitive sensor – using alternating current
A basic capacitor consists of two plates separated by an insulator, often air. It blocks DC since there’s no circuit continuity, but allows alternating current (AC) to flow, with the resistance known as reactance. This current increases with frequency increase or if the frequency is fixed, with capacitance increase. Rain changes the capacitance, altering current flow. These sensors can also detect a frequency shift if so designed when raindrops are present. To prevent false triggers from condensation or icing, they typically include a heater.
Many DIY enthusiasts prefer capacitive type sensors. A common issue is using plates too far apart, resulting in small capacitance and ineffective triggering. The spiral design is popular, as shown in the photo. KEMO, based in Germany, is a reputable manufacturer. The image provided is an authentic device, not a pirated version. While more expensive, its quality surpasses many cheaper imports.
The capacitive type sensor is the direction many DIY enthusiasts follow. I believe a failing point in many of the experiments is that the designer is relying on plates too far apart – this would make a for a very small capacitance and may not trigger effectively. The spiral design shown in the photo is the more common design. One or the better manufacturers is KEMO, a company based in Germany. I have taken the liberty of adding an image from their collection – this is the bone fide device and not a pirated version. It will be more expensive solely because the quality is superior to many of the cheaper imports.
Image credit to KEMO – The KEMO designed capacitive M152K rain sensor.
KEMO rains sensors which use gold plated electrodes – DC and capacitive types
Kemo is well established and recognised company in the electronics sector, their kits available worldwide. Good quality and won’t break the bank. You can find KEMO here.
The Prism sensor
Our last sensor type is the prism type rain detection sensor system, most often found in the automobile industry. I am not going to re-invent the wheel, much has been written about this type of detection system and the go to website for this is Yoctopuce
The common and garden home weather station bucket system
I have not covered the most common rain sensor, the bucket system used mainly for irrigation systems, as it should be used with a Microcontroller along with other sensors. In the next article, I will discuss my design, which includes a solar panel, LDR, bucket, and capacitance rain sensors, aimed at reducing current draw during low light conditions.
Please give me feedback in the comments section.