Waterproof temperature sensor housing

I had some plates made at work. They are 60mm x 60mm x 10mm copper that is then tinned via electroplating. The electroplating is to stop a layer of copper oxide forming on the surface and acting as an insulator. Between each layer I have inserted a gasket made from a bicycle inner tube. The hole in the middle is 18mm and is the cavity in which the LM35 temperature sensor and an LM358 op-amp with appropriate resistors (270k / 27k) for setting the gain (1 + 270/27 = 11).
I tested it for waterproof and all went well.

Solder on

That's the bulk of the soldering done. The 5v voltage regulator, the Nand Gate and L293D Half-H Drivers are in position. The control lines from the NXP are soldered in with a ribbon cable socket, the 3.5mm jacks for 12v outputs and a 3.5mm jack for the 12v supply are all in place. I added the extra GND line to make the D-Sub 16 lines. It didn't all work first time because a tiny sliver of solder was connecting the 5v rail and the enable line. Took me a while to find it but the fact that I did intermediate testing meant I knew it was the newest soldering I had to re-inspect. I could really do with better inspection equipment - the light isn't bright enough and maybe a magnifying lens or camera would be a benefit.

Here it is with the NXP up and running. The PSU star connector is 12v in.

Temperature and op amps

As I said, turns out the conductivity sensor has no built in temperature sensor is in, which is a pain as I have to make a waterproof thermometer. No matter, I have the sensors - LM35. My LM358 Op Amps arrives (5x) and with them my 5v regulators L7805CV.

Even though the function for calculating the gain of an op amp is easy Vout / Vin = 1 + Rf / Rg, finding resistors from the box of resistors you have takes a bit of time.

I ended up choosing 2 x 270k for Rf and 55k for Rg. I don't know if 2 x 27 and 5.5 would be better. But I ran it in the Livewire simulator and it worked so we'll see.

Now I have the regulator I am going to include that too.

As usual the circuit is online. http://www.circuits.io/circuits/6588

G = 1 + Rf / Rg = 1 + (270k + 270k) / 55k = 540k / 55k = 10.8182

Using this ratio we can see that we top out at just over 30 ° C. An unlikely value for a tank of water in the UK

A quick test with that circuit and I get 1.5v in my nice cosy house.

° C	LM35: 0.01V per ° C	LM358: * 10.8182
0	0.00	0.00
5	0.05	0.54
10	0.10	1.08
15	0.15	1.62
20	0.20	2.16
25	0.25	2.70
30	0.30	3.25

Now the problem I have is how to make this :

waterproof!

Knocking some sense into it

I finally got the Atlas Scientific conductivity sensor all wired up.

Turns out I have slightly misunderstood the documentation which says "temperature corrected reading". But it turns out you have to tell it the temperature and then it responds with an adjusted reading. Luckily I have a bag of LM35 temperature sensors. Of course, this does give me the problem of making it water proof but that should be too much trouble. I could be doing without the added complexity but hopefully it's not too much to extra. The datasheet says that its output is 0 mV + 10.0 mV/°C

The analogue in pin on the NXP is in the range 0-3.3v so if we use that raw it is the range 0-330°C which is an order of magnitude away from perfect, so I'm going to have to add an op-amp, they are only £1.88 for 5! I'm actually a little bit excited about this :)

The test code, as usual, is on my account at mbed.org

I have written code to read the sensor, and pump readings out to Ethernet when ready. It is time for bed so this will have to wait until a suitable time. I also need a stable5vsupply for the sensor so I have ordered so 5v regulators too, I think I have a couple of 5v zeners for now.