I used a nutrient doser for a few years and found it good. When it died, I looked to replace it and found the prices had gone through the roof. Knowing a bit about electronics, I decided making one would not be too hard. Now it is 18 months later and I have a box which can store a years worth of data, has a 7 inch touchscreen with which it can show graphs etc to the user. It monitors tank temperature, air temperature, RH%, PH, EC and currently can output 8 x 12v signals to operate relays or low power devices. Sensors are on a serial bus so the possible number of them is more than you could need, same with the pumps. Currently I use serially addressable pumps accurate to 0.5ml or 1% (whichever is larger) which can dose normally, or dose over time (e.g. 20ml over 10 minutes) or dose constantly (e.g. 5ml every 10 minutes).
As the sensors and pumps are on a serial bus, you can have up to something like 112 devices on the bus, with multiplexing you can have closer to 1000. That would take some more inventive coding but the electronics is totally possible.
The sensors are lab grade and very accurate. Here is a photo of the PH graph when the doser is controlling the nutrient solution for a while, if you note. 5.8 is at the bottom of the graph, and 5.9 is at the topā¦
The jaggies on the line are where the doser is adding PH upā¦
As you can see, it is capable of keeping the PH (and also other readings) to a much smaller range than most dosers can even measure. I will be calling it The MicroDoser, of course 
It will currently auto adjust dosing to match your desired accuracy range, I want it to notice when it is not dosing enough and increase dosing automatically also. If given access to a drain+fill facility, it can reduce your EC by itself and make sure your nutrient solution never goes out of the ranges you set.
It will monitor and adjust your air temp, RH%, water temp, EC, and PH all in one box, no need to connect to a PC.
There are options for adding DO and CO2 sensors so it could manage those also. Even if by just doing a tank refresh if Do is too low. With more outputs, it could regulate anything it can have a sensor for. The plan is to give it a drag and drop programming interface so you can fully customise it to your purposes. Even if you wanted to look after your fish with it, hehe. AFAIK it will be the only doser that could monitor and check both your fish and your plants in an aquaponic system and take care of bothā¦
Currently I use VNC to connect to it but the plan is to make an app for your phone which can show graphs and let you control all features from anywhere on the planet with internet.
I hope to bring it in at a price point slightly less than the current offering from Autogrow systems, the Intellidose (which has far fewer features, a crappy LCD screen and needs to be connected to a PC to program) which AFAIK is the current best offering for smaller growers (not guys in warehouses or greenhouses).
As I have a development team of just one person, things slow down when life happens. I hope to have more time and cash to devote to it this year.
