To allow me to keep an eye on the status of fermentation within the demijohns while they are fermenting I designed a fermentation bubble counter.
The purpose of this small electronic board it to count the bubbles rising the air lock in a demijohn, As long as all the brewing/fermentation parameters are similar when I brew the exact same beer I would expect the number of bubbles and rate of bubbles at a particular time during fermentation to be the same. So far this does seem to be the case.
At the very least it should give me an idea of when a stuck fermentation may be occurring and also help me to decide when fermentation is slowing/stopping and I should take a gravity reading.
The actual electronics used are quite simple, An infrared led is placed on the board at a location where it shines through the thin part of the airlock where bubbles must pass.On the other side of the airlock is placed an infrared photo transistor which constantly detects the infrared passing through the water in the airlock.The output from this photo transistor is then fed into a comparator IC so that a nice signal output is available.
When air passes through the airlock the infrared beam is deflected for a fraction of a second and the signal from the photo transistor drops. This then turns off the signal from the comparator. This output is then fed into the Pokeys57e board which counts the amount of these pulses.
The graph of the bubbles is always available on the android application. An example graph of a brew which has only just begun fermenting is shown here.