are the peripheral accessories that are not mounted
on the main project board. These include the serial
LCD display, the switch, the temperature sensor, and
fan(s). Let’s describe the LCD display and switch
assembly.
Of course, one of the main considerations of any
project is its cost. The most expensive item is the LCD
display, so it pays to shop around. You’ll find them
anywhere from $20 to $60. There are many 2x16
serial LCD assemblies on the market. As long as it
supports the standard 254, one command set, and
receives at 2400 bps TTL serial data, you are in
business.
The selection (at least for me) mostly boiled
down to a display that would provide a nice,
professional looking assembly. I went with for a Scott
Edwards BPI-216 which provides an optional
mounting kit with faceplate and mounting hardware
(BEZ-216) for a nice looking finish. This is important
since both the switch and LCD mount on an external
plastic enclosure, and while cutting out the panel to
make the display window, you’ll most likely leave
ragged edges.
After you have mounted the LCD and switch,
wire them with color-coded wires. Note that the LCD
and switch share the + 5. 12 volt line.
Let’s go over the main board. This board has
both thru hole and SMT components that are
mounted on both sides of the board. Exotic or
ultraminiature packages were avoided; the chips are
0805 sized so they’re large, and the ICs are SOIC
which are the easiest to handle. Assemble the SMT
components first, which all go on the bottom layer
(Figure 3) with the exception of C5.
Make sure that you check the orientation of U2,
U3, and U5; their pin 1 is clearly indicated on the board.
FIGURE 5.
Checking the Circuit
Once you finish the previous step and have thoroughly
inspected everything for poor solder joints and bridges,
proceed with assembling the thru hole devices on the top
layer (Figure 4). Watch out for the proper orientation of
microcontroller U1, the electrolytic capacitors and the
diodes — all which have their polarity indicated with a
square pad — and also that of relay K1. The relay’s body is
marked with a black line which must match the silkscreen
shown in the artwork. Also careful with the connectors;
they are keyed.
Once you have completed the assembly and thoroughly
inspected it for soldering errors, apply nine to 12 volts at
connector J4. Then with a digital multimeter, adjust
potentiometer P1 for a regulator output of exactly 5. 12
volts. The precision of this adjustment will determine (to a
large degree) the voltage readout precision, so perform it
carefully. Once you have done that, check the voltage on
D1’s anode. It should be close to six volts. Lastly, check the
voltage on the negative pin of C4 which will measure close
to - 5 volts.
