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Tearing down the simple and effective Mr. Coffee

Posted: 08 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:GUI? phenolic board? FR-4? transformer? microcontroller?

Mr. Coffee is very simple in operation. The heating unit boils the water, which bubbles through the ground coffee. All the electronics for the display, user buttons (just three of them), mode-setting slide switch, indicator light, and heater control are on a 3x11 cm PC board (figure 4). To reduce cost, this is a single-sided phenolic board (not the more expensive FR-4 laminate) with through-hole components on both sides, and jumpers for the equivalent of circuit tracks on the bottom where needed.

What I liked about the design was the bottom, unclad side (figure 5). All the connections are clearly labelled, with meaningful designations such as Heater Relay, Line(L), Line(N), S4 Brew, CR1, and AC. The small off-board transformer for the board's circuitry power sub-system was also marked; it was a 120 VAC/9 VAC, 100 mA centre-tapped unit. In operation, Mr. Coffee is open loop: The microcontroller senses the switches and turns on the heating coil relay, and the coil heats the water. (It is rated at 900W, by the way).

Figure 4: The clad top side of the control PC board has about half the components.

Figure 5: The unclad bottom side of the board also has some through-hole components, and all connections and components are clearly labelledwhat joy.

In short, the board is a hobbyist's delight. You can get useful parts from it, certainly. But even better, you can use this as the basis for a cost-effective heater controller, using the control board and the heating element. It would take a little work to add a temperature sensor with adjustable setting for closed-loop operation, along with a comparator to control the heating element relay, but it could be done.

You could also use it to experiment with different on/off control approaches and issues (such as PWM, duty cycle, or thermal time constants) and see how these factors affect temperature stability, undershoot, and overshoot. Maybe that would lead to exploring more advanced algorithms such as the ubiquitous proportional-integral-derivative (PID), adaptive and predictive techniques, or even just analogue control (proportional) for the heating element instead of a simple but effective on/off relay.

What really impresses me about this sort of product is how it manages to meet all the basic market requirements, and at low cost. (Given the retail price, the BOM must be around $15-20.) It has to operate from the AC line, deliver substantial power and control, be safe and shock-free, and include a microcontroller, basic display, and low-voltage supply for the electronics.

Have you ever taken apart a basic consumer product to see how it is made? Have you seen any surprises? Have you seen anything negative that "shocked" you?

About the author
Bill Schweber is an editor for EE Times and a registered professional engineer.


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