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.

We have ethernet

A bit of chopping and soldering and I have an Ethernet socket on the NXP. The chopping was an old D-Link 530-tx network card. I also discovered it has an 93LC46 1024-bits serial electrically erasable prom on it. That could be an interesting device one something sometime. It's not even had its legs trimmed off so it will go in a 8 legged socket (although I snapped a leg off de-soldering it!

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.

Regulation

I have decided to go with a single 12v power supply instead of the dual 5v / 12v I was using. I needed a regulated 5v for the Atlas Scientific sensors so I rolled it all into on. The 12v goes into the control board and thr CPU can take the 5v Vcc from there up the 15-Way D-Sub. I've still got a problem because added the temperature sensor means I need 16 channels! So I'm going to have to think my way round that. Worst case I shall use the shell of the D-Sub as an extra pin and link that to Gnd.

I've added the 5v regulator to the board. I don't know how many amps I will be drawing to run the semiconductors. I hope it's not much because it is that current times approx 7v that I will be dumping out of the regulator. The regulator is maxed at 1.5A anyway so at most I will be dumping 10.5W and only consuming 7.5W ! I'm pretty sure that 1A is unlikely. If it becomes an issue I can go back to split supplies but I bet they do the same. The light I want to use for a 3ft sqr grow is 250W so it's a small proportion. Perhaps I can water cool it :)

Here's the circuit without ICs and just a regulator

I have tested the 5v rails and they are registering 4.9v on the multimeter, unloaded. My Multimeter is cheap so I don't know if it is out of calibration or the regulator is only producing 4.9v. No matter. It is more important to have a constant voltage than an exact one.

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.

Time to start soldering

The breadboard design testing phase is over. I'm confident I can commit some Veroboard and a few of DIP sockets to build what will be a "Double Quadruple Half-H Driver Controller" board with TTL control and 8 outputs (+ve and gnd) that can be up to 36V and two gangs up to 2A per gang.
The inputs are 8 Activate lines and up to 4 Enables plus Vcc1 & GND and Vcc2 & Gnd.
I'll keep Vcc2 on it's own socket - don't want 36V hanging around the TTL lines, so that leaves 14 connections. Just right for a 15 pin D-Sub

SO I ordered 10 male & 10 female 3.5mm jacks.

I've got some 15 Way D-sub sockets so I got a couple of 15 Way D-sub 2 row male plugs. Hopefully I'll be able to use an old VGA cable for the wire, or else some ribbon cable.

Now I need to work out a Vero board layout

I found an online tool for designing stripboard. Annoyingly you can't save it when using Firefox which, obviously, I found out after designing my layout with it. It maybe can in Chrome, I'm not sure. It says Chrome works better so perhaps this is what was meant, I haven't tried it.

Anywhere, here's a screencap of the design with some annotations.

This is the view from the component side with the tracks superimposed through. The tool lets you do it differently but I'd already done it by the time I worked that out. The tool isn't excellent but at least it did what I needed.

The beauty of doing it with a designer is that there's something to test against before adding the components, which was fortunate because I had a couple of tracks conducting when the should have been isolated.

It's taken years to get this good at soldering ;) and naturally without three hands, I managed to solder the first socket in one column of holes the wrong way and the worst way, so one of the link wires is under the small socket, no big deal.