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!