If you are a coffee lover, you want to stop the shot as soon as all the good coffee flavour has been extracted from your coffee, and definitely before all the nasty flavours begin to come through (known as over-extraction).
This can be see by watching the coffee flowing, and switching off the flow just as the chocolate brown coffee colour disappears from the shot. This point is known as when blonding starts, as the coffee flow is now a straw yellow colour.
The point at which blonding starts is around 15 to 30 seconds of flow, depending on the coffee variety and type of roast.
The trouble is that it is a pain to time the shot every time, or to bend down and look up at the coffee flow to see when the blonding starts, as this all takes place under the overhang of the front of the machine. Commercial machines use a flowmeter to measure the shot volume, and stop the pump automatically when the required shot volume is achieved. Fixing the time of the operation of the pump gives a good approximation of the volume of the shot, at lower cost.
The coffee shot controller monitors the operation of the water pump, which provides the water pressure to force the flavour out of the coffee. Whenever the pump operates for more than 10 seconds, the controller times the operation of the pump. The time at which the pump is switched off is stored in memory.
The next time that the user wants a shot, they only have to flick the coffee switch briefly, and the controller will read the value out of its memory and will run the pump for only that amount of time.
Shown below is the controller installed inside a Gaggia coffee machine. All interconnections are provided with silicone insulated wiring due to the high ambient temperature.
The controller monitors the pump switch volts via an opto isolator. The opto led can withstand 50mA, and a minimum of around 5mA is required to turn the transistor on. For 230Vac (325V peak), a pair of 10k resistors in series, and a diode in series to protect during the negative cycles, are sufficient. The peak led current is (325-0.7-2.0)/20k = 16mA. Every 20ms (for 50Hz mains) the transistor in the opto isolator will be switched on and this can be detected by the micro-controller.
The choice of micro-controller was based on the need for EEPROM memory to store the required time for the pump to operate, and on board ADC for possible future design changes. The choice was either 12F675, 12F683, 12F1822 or 12F1840. All of these cost comfortably less than £1.
The main issue with the construction was to provide isolation of the micro-controller circuitry from the mains voltages present on the pcb. For the present application this is not an issue, as there are no external connections which are accessible to humans. However, there are connections available on the pcb for external sensors (flowmeter, temperature sensor, front panel mounted potentiometer, etc) and these may need to be isolated from the mains in future projects using the same pcb.
The prototype is complete, with no modifications required to move to final production. The anticipated market is the enthusiast coffee maker owner who has completed other modifications to their machine such as adding PID a controller. The next steps are to produce in volume and generate interest through coffee forums.