■ The Smart A/C
after the 8M receives a “compressor on” order from the 28X1.
It then will go on steadily after the delay time is achieved and
as long as the 8M continues to receive a compressor on order.
At this same time, the 8M issues a signal to activate RLY1.
The reader may notice there are several parts listed, such
as RLY2, RLY3, LED4, LED5, RESN, DIPSW1, and SEN4,
which are shown on the schematic (and circuit board) but
are optional. These parts were designed into the circuit
board for future expanded capabilities of the controller.
The prototypes of the Smart Controller use
For higher potential accuracy, you can tweak the voltage
regulator so it puts out exactly 5. 12 volts. Alternatively, you can
modify the program a bit. For instance, if the supply voltage is
exactly five volts, you will want to multiply the average reading
you get from the 10-bit A/D converter by 5,000 and then divide
the result by 5,120. The result is the binary number of the
temperature (in half degrees Fahrenheit). In general, multiply
the average reading by the EXACT supply voltage in millivolts
and then divide this by 5,120.
44 October 2008
professionally made, double-sided printed circuit boards
(PCBs) with plated through holes, but without
silk-screening. These boards were made by expresspcb.
com. I also took advantage of their free schematic and
layout software, both of which I can recommend.
While most parts in this design are standard size with
.1” pin spacing, if you use the foil pattern provided with
the files on the website (or purchase a PCB through
www.magiclandelectronics.com), make sure you either
use the exact pushbutton switches called for in the parts
list or only substitute switches that have nearly identical
I recommend that the reset switch, the optional DIP
switch, and all the connectors (J1, J2, etc.) be mounted
on the foil side and the other components on the normal
If your particular case has good air circulation, SEN1
(room sensor) can be mounted directly to the board.
If not, it is best to attach a short three-conductor cable to
it and place the sensor outside the case, like I did.
The air flow sensor is constructed using a tiny PCB.
Make sure the 100 ohm resistor (heater) contacts the
sensor. A tiny bit of epoxy cement can be used here.