Power and data transmition on 2 wires.

Forum 2005-2010 (read only) Hardware Interfacing
1

G'day all.

My pool light broke (cause me fat mate decided to use it as legde). So as you do I thought -- I know I'll stick in an arduino, a bunch a rgb hi intensity leds, a temperature sensor, a motion sensor and a microphone in that thing.. and create the ultimate intelegent pool light I never needed.

Well It all looks do-able The question is: there be only 2 wires running from the lights 32v psu through the pool to the light, how can I piggy back bi-directional data transfer on the beefy 32v power line.

I needs the data transfer so the pool lights color can be co-ordinated with the colored lighting I use elsewhere and so data about pool usage and temperature graphs can be sent back.

I have a few ideas on how this could be done, and it seems like a lota fiddly work. Is there somfin out there as what does this kinda job? Is there a arduino project doing somfin like this that I could not find.

Cheers for any info,

-Wazza

2

Looks like a job for a good old fashioned modem. AC couple the signals into the power line and the DC won't be an issue. You should be able to pick up a pair of old modems cheap.

3

"good old fashioned modem" may be too big a device to cram in a pool light fitting.

Also Id have to turn the unit off for 4 seconds and then turn it on again everytime the conection whent dead. But I do love the sound those things make while handshaking.. it make all the connection dropouts worth while.

4

Why not add wireless to the project? Seems like adding XBee to the project would be the perfect solution.

5

I had assumed wireless comunications would not be possible underwater.

I thought all the extra um conductivity, and reflectence from the water surfaces (including pool edges) would muff every thing up. I know that remote control subs need to work at very low frequencies (around 20-30kHz)... and the is an asumption of no reflectance from big objects.

I had a small remote sub once, it worked in the sink, but not the pool. And it is dead now anyway(cheap China import).

A quick back of the napkin assuming my main problems involve signal atenuation from my (saltwater) conductive poolwater; and surface refraction from the pool top (not sides and bottom);

Attenuation of radio signals in water (dB/metre) = 0. 0173 sqrt( f * q)
where: f = radio frequency (hz)
q = water conductivity (M ohm per metre)

Ok I stick my multimeter in the pool and find 150 K resistance per cm.
that is 15 M per metre. This is less conductive than ocean water which is around 4 M pm in temperate oceans. So my mesurment is in the right ball park.

Now xbee gives a 900MHz and 2.4Ghz solution.

for 900MHz: 0.0173sqrt(900000004) = 328.24 dB/m
for 2.4GHz: 0.0173sqrt(900000004) = 1.6950e+003 dB/m

So the 900MHz is the least imposibly far fetched solution and my light is 70cm under water so . about 230db of attenuation... still not a good result.

Ok add to this signal loss by refraction at the water surface... (I call this rlaws)
going by the standard formula:

rlaws (dB) = -20 * log [(7.4586*10-6) * sqrt(f / q)]
where: f&q are as they were before:

Hey there is a surprise higher frequencies are atenuated much less by surface refraction. Also the salt reduces the refraction problem; Finally something going my way.

So I get 57dB atenuation due to reflectance at the surface.

Ok Im feeling all in all I need a lower frequency (but not to low to make the surface refraction a problem)... plug both formulas together and solve for minimum combined distortion and I get... a frigen headache... Its so bed time.

Anyway I figure the standard xbee will not cut it. But I know there are some "underwarer enabled" wifi devices around and I am not sure how they get over these hurdles.

I dont suppose anyone would care to test an xbee in a salt water pool for me (I dont have a xbee) to check my calculations... I saw a web page saying xbee could go underwater to a certain distance and I assume that is a fresh water scenario.

6

I assume this pool light is in the side of the pool wall. Sure the transmission will be limited by the water as well as the concrete wall. SO you may not be able to receive data from the pool in your house but at a closer range you should be able to pick up data.

Now isn't there power cable running threw a conduit to the light housing? If there is room you can always disconnected the light, attach a cord to the cable and pull the cable out threw the conduit. Then attach the added data cable you want to use and pull it all back threw the conduit with the cord.

7

Wait, yes, I could transmit the signal strait into the side of the pool that is 40mm of water in the pool light housing (only 13 db atenuation)... But I don't know how to calculate the reflectance and refaction of the chance in medium from water to fibeglass then fibreglass to dirt, then find the atenuation caused by 90cm of earth. (not transmiting strait up due to a pool rim and filter so is longer than the 70cm water path.)

Um, does anyone know the calculations for this (I only read half a book on geophysics and have forgotten most of it) but I assume there is a geophysisist out there who does this every day... The earth is sand and varies in water content quite dramatically.

I assume the atenuation in earth would be similar to the calculation for air, when the sand is dry and getting close to water ain the wet, and probably imposible if there is some iorn or something conductive / salty in the soil).

As for refaction I assume the water/fibreglass is the main problem... will dig around in some books to see If I can find the appropriate calculation, which I may have to fudge because there is a copper addative in the fibre glass used in my pool.

8

Um as for yanking the wires, if that was possible I would have done it.... :slight_smile:

9

If it where me this would give me a good excuse to buy a set of XBees. Then i would trow together a quick test program and set it up and see how good the range is.

10

"good old fashioned modem" may be too big a device to cram in a pool light fitting.

a "good old fashioned modem" are still being made today, I got a external usb model for 2$ at a computer scrap store and its external dimensions are just abit larger than a pack of cigarettes

noone said you had to get a 1978 hayes

11

Total (existing) pool light internal area is about 35mm at its highest. and about 20cm in diamiter (domed). Adding a modem + circuitry is possibly too much, given most of the room will be given to the leds and the (lillypad or similar) arduino. I think many modems are still encumbered by size and tend to die after a couple of years.

If I am building something for myself I dont want to pull it apart in two years time to re-build.

I just remain un-convinced on the modem idea... but the xbee through the wall idea has me thinking now.

12

It would "seem" possible to simply do a binary transmition by varing the voltage going in via a 2v voltage drop (over a small diode..) (30v low and 32v high) and use some circuitry to trigger the rx pinn acordingly... the retun current could then have a hevier load imposed on it via action of the tx pin and a bit of circuitry... The same setup on the other side and presto com port over the power supply.

La la la I am in a fanciful world now... But wait, is this possible?