rain detector heater

[jgershonw1]

I am making a rain detector using a leaf sensor. Basically it's a pc board with closely spaced parallel traces, which starts to conduct when it's wet. To make it effective, I want to heat the backside of the board to dry the detector/trace side. The board is approximately 2x4 inches. Any suggestions for a heater, than can efficiently heat the backside?

 

[Dippy]

I'll make no comments on corrosion or AC vs DC, or better methods for the same result, but how about a resistor or resistors to heat it?
They come in all shapes, sizes and power ratings.
You can do the calcs.
I can't imagine the overall effect will be very efficient.

 

[BeanieBots]

Rather than a heater, how about a fan to blow dry it.

 

[Dippy]

And maybe rub in a bit of gel too.

 

[moxhamj]

You may end up with a very effective device to convert metallic copper into copper oxide if you are not careful. Bare metal, of any sort, will happily convert back to its oxide form when water is present, and especially when water and electricity are present. Exceptions of course are Gold and Platinum.

A simple resistor could do the heating. A 5W or 10W resistor. Say you have a 5V supply and you want 5W of heat, then W=IV so you need a 5V 1Amp supply. V=IR so V=5, I=1 so R must be 5 ohms. Closest is 4.7 ohms. Turn it on and off with a mosfet or transistor using one of the standard picaxe control circuits in the manual.

This should work great for a few months. Then the copper will start corroding, the resistance will go up, and the readings will become less useful.

Even though gold and platinum don't corrode, there is another cheap and cheerful way of detecting conductivity, and that is to use a huge amount of iron and accept that corrosion will happen but plan that it will be slow enough not to worry about. Eg a couple of big steel wood screws eg 1/2" in a piece of wood 10cm apart.

 

[BeanieBots]

Even gold will "dissapear" after a while with acidic rain water and a current flow.
Stainless steel with an AC drive should give quite a long life.

 

[Dippy]

jg, you've opened a nice big cannoworms now

well, i was trying to keep it on target by not diverting you from your desired design .... but how about capacitive or optical.

I really think you will have corrosion problems with your planned method.
And how much heat will be required to evap off all the droplets from a 2x4 inch piece of board?
I don't know if you access to a gold plating plant, like Dr-Ac does, so that maybe a problem...

So, get some stainless steel rod, beat it flat and stick it to some PCB.

 

[hippy]

On the heating question ...

2" x 4" is quite large, and isn't PCB fibreglass quite a good insulator, not very heat conductive ?

One solution could be to take (at least ) half your copper sensor prongs and conect them to the tab of a 78xx regulator, drop quite a lot of current so it heats and the prongs themselves work as a heatsink. That probably gives better contact heating than you'd get with heating resistors etc.

I've got a 12V SLA to 6V camera convertor using a 7805 which drops 6V+ at 2A and makes for a very nice handwarmer / handburner using a 1.5" x 3" x 0.25" aluminium finned heatsink.

 

[eclectic]

Not a clue if it might help, but

1. A sloping board.
2. Picaxe detects conductivity.

3. Capillarity matting drains /siphons off the water.

e

 

[BeanieBots]

Nice heater Hippy. Ideal for UK winter photography.
Might want to upgrade to a switching buck regulator for summer.
Would also double the SLA life.

 

[jgershonw1]

Thanks for all of the suggestions and comments. I was thinking of making the board myself using a double sided board, but the corrosion is going to be a problem as mentioned already. The leaf sensor boards that you can buy are gold plated and cheap enough, so I may go that route. They are actually alot smaller than the size that I stated. Somebody suggested using a 5 watt light bulb, but that didn't sound like the most efficient way to transfer the heat.

 

[Dippy]

Why not use one of those aluminium bodied resistors bolted to the PCB?
The ones made by Arcol and many others are designed to bolt to something flat to conduct the heat away.

When I've tried them stuck to aluminium the heat is 'transferred' quite nicely.
The Arcol ones are anodised so that forms an electrically insulating layer, but you can add a plastic film or, if you fancy spending 20p, some proper heatsink insulating silicon sheet stuff.
It would be much better than a bulb.
And you may be able to do it more cheaply with those square section ceramic power resistors.

And there are other power resistors in a TO220 package too.

hippys idea could be convenient, just depends on your setup.

So many options..... depends if your budget will stretch to a quid.

 

[jgershonw1]

Why not use one of those aluminium bodied resistors bolted to the PCB?
The ones made by Arcol and many others are designed to bolt to something flat to conduct the heat away.

When I've tried them stuck to aluminium the heat is 'transferred' quite nicely.
The Arcol ones are anodised so that forms an electrically insulating layer, but you can add a plastic film or, if you fancy spending 20p, some proper heatsink insulating silicon sheet stuff.
It would be much better than a bulb.
And you may be able to do it more cheaply with those square section ceramic power resistors.

And there are other power resistors in a TO220 package too.

hippys idea could be convenient, just depends on your setup.

So many options..... depends if your budget will stretch to a quid.

I think this is probably the best way to do it. It's inexpensive and will be easy to mount.

 

[nbw]

You could use stainless steel TIG welding rods - 1.6mm in diameter. Cut them to any shape you need. Use a flux to tin the ends so you can solder to them, don't forget the heatshrink. AC is best, but you can get away with DC for a long time if you don't pulse the rods continuously. For a rain detector, maybe pulse for a 2ms burst once every 5 min? That will greatly extend the life of the rods - a 910mm one cost me 42 cents (about 15p).
Enjoy!

 

[BCJKiwi]

You could try cycle spokes. Most are stainless and have a thread and nut at the rim end - makes it easier to mount.