We had time to get the electronics gear out again last weekend, so we put together a test circuit for our level sensor.
Leah built our level test rig. The vertical is a hose with a funnel at the top to allow easy filling of water. To her left is the pressure connection to a motorola MPX5010GS sensor. This unit outputs around 5 volts at 50 cm of head pressure. For our purposes we will only need about 20 cm of this range, but the picaxe which will be doing the analog to digital conversion will still see around 100 mls of resolution. We figure there is no need to amplify the output (ie. keep it simple).
On the right hand side of the rig is a tap to let out water and therefore easily vary head pressure for testing purposes. (Thanks Garrett for the inspiration for this test rig 
The circuit we put together has an LM3914 LED driver driving a LED bar display. This will give us a simple visual indicator of 10 LEDs to show a full tun. You can see our prototyping board above and the actual circuit diagram is shown below (sorry about the poor image here – this is a function of uploading to live spaces – please contact me if you want a copy you can read).
We marked out approximate volumes on the test rig, correlating head pressure (height) from the brewing tun onto the hose. The values shown are in litres.
And the circuit worked beautifully. The indicator below shows the ‘tun’ is half full. Now that we have a proof of concept, we can design and build up a printed circuit board.
