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Any suggestion on how to build home-brew rain sensor for car? I've found one which shows this particular sensor is just pcb plate (or maybe there something on it I can't see).

enter image description here

What is the difference between built in sensors and home-brew ones like above? I would like to make something which is very close to built-in. Other question is where to place it?

Later I can connect it to AVR and collect data.

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    \$\begingroup\$ dust and corrosion will affect impedance greatly over time. but essentially impedance due to moisture is the key to detecting rain. Apple products contain chemical moisture dots imbedded around their products to detect out warranty violations due to moisture ingress which change colour permanently. Electronic moisture sensors must have low leakage due to dust collection/ If self cleaning from rain , better but must have a solder mask to prevent corrosion. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 7 '12 at 19:10
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    \$\begingroup\$ A better solution uses IR refection off the glass at 45 degree angle. If the glass is wet, there will be more scattering of light and a drop in reflected value. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 7 '12 at 19:13
  • \$\begingroup\$ @Richman Very interesting. Is it possible to make one with IR? Can you give more details on which IR transmitter/receiver to use, how to place? Should I place one inside and one outside? It would make things complicated... \$\endgroup\$ – Pablo Dec 7 '12 at 19:17
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The rain sensor that you show isn't a very good one. It can detect normal tap water fairly well, but not rain water.

It works by detecting an electrical short between the two sides of the sensor. Tap water and most river/lake water (not rain) is electrically conductive. When a drop of water hits that sensor then it will conduct electricity across it. The reason why there is that "dual interlocking comb" pattern on the PCB is to increase the chances that a single drop will touch both sides.

The reason why it will not detect rain is this: pure water is NOT electrically conductive! The water must have dissolved minerals in it to become conductive. It is possible to submerge entire PCB's into pure water and have them function just fine (until the water is contaminated). Rain is basically pure water. There are some things in it from dust and pollution, but it is pure enough to cause sensors like the one you show to be unreliable at detecting rain.

Most commercial automobile related rain sensors use some sort of IR system. I will let you google that one yourself. You can easily find all sorts of diagrams, drawings, and even aftermarket sensor kits for your car.

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  • \$\begingroup\$ I tried to google before coming here, but can't find IR based diagrams or DIY tutorials. Only few commercial sensors on ebay etc.. I would appreciate if you could share one or two of such links. Maybe I am using wrong keywords. \$\endgroup\$ – Pablo Dec 7 '12 at 20:41
  • \$\begingroup\$ Here are three links to get you started: vishay.com/docs/81039/81039.pdf, en.wikipedia.org/wiki/Rain_sensor, and raintracker.com/how_rt_works.php. \$\endgroup\$ – user3624 Dec 7 '12 at 21:01
  • \$\begingroup\$ Doesn't look like IR solution is feasible to achieve without mentioned component. \$\endgroup\$ – Pablo Dec 8 '12 at 0:11
  • \$\begingroup\$ @Pablo There are also capacative sensors for this, but I don't know anything about those other than they exist. \$\endgroup\$ – user3624 Dec 8 '12 at 2:25
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You can indeed use the sensor shown with rainwater.

Water has a high dielectric constant so if you drive AC across it, you will find its capacitance changes. How you use that is up to you - you could use it in an L-C oscillator, and measure the frequency change (it should drop) as rain lands on it.

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  • \$\begingroup\$ If the PCB tracks are the plates, they will have a very small area, and the area between them can easily be filled with dirt. I'd suggest using a grid made of partially uninsulated wires instead of a PCB. \$\endgroup\$ – Oskar Skog Feb 19 '17 at 22:01
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You can use some kind of drum and a microphone as well as minimal amounts of digital signal processing done on a microcontroller to detecting the sound of falling rain drops.

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You can use that or a similar pattern on the inner layers of a 4 (or more) layer board, and sense the capacitance. That solves the corrosion problem. I implemented this some years back in an AVR design where I used one pin to sense the voltage on the capacitor, and another pin through a large resistor to charge and discharge the cap. In a tight loop, I set the output pin high and waited for the input to read high, then flipped the output low and waited for the input to read low. That operation was repeated a large number of times, and then measured against a timer register to see how much real time had passed for N operations. The shorter the time, the less capacitance.

For testing I used a single drop of white glue, and I was able to watch it dry over a period of 30 minutes or so. Just one drop on several square inches of PCB area.

You're measuring the bulk dielectric constant of the surrounding material.

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