through the component selection
routine required to match the design
of the original controller. I wanted to
eliminate the mechanical relay,
open slide switch, and nonlinear
thermistors, as well. I needed something in a small package and finally
settled on the 14 pin DIP PIC16F684.
This chip was selected because it has
eight A/D channels with 10 bit
resolution, which will work well with the
LM35DT linear temperature sensors.
My original idea was to make a
controller for the pump only, but I
later added the digital readout for the
temperatures. This is the reason my
design uses two of the PIC16F684
packages. The schematic and printed
circuit board (PCB) were designed
using software from ExpressPCB (see
Resources). I found their design
software very easy to use, but note
that you should do a line by line check
of your design, as there are no
error detecting features to check your
layout for mistakes.
Your PCB data created in their
software is uploaded to them and the
order is placed for the boards (the
PCB files, as well as the code listing,
are available on the Nuts & Volts
website at www.nutsvolts.com).
Board assembly is fairly easy due
to the DIP devices and through-hole
components. No SMDs are used.
The components in the regulator
circuit (Q5, C1, C2, and C3) are
placed on the bottom side of the
board to allow the board to be
mounted a 1/4 inch from the surface
of the box’s front cover. TB1 is also
mounted on the bottom for better
access to its terminals. See Figures 1
and 2 for photos.
Each LM35DT sensor has the
5V supply decoupled with 0.1 µF
capacitors and a 1.0 µF capacitor in
series with a 10 ohm resistor to
account for capacitive effects of long
wire runs. The compensation is probably not needed in my case, as the wire
runs are only about 30 feet long.
Each sensor output is fed to an
A/D channel on the first PIC16F684
(U1). There, the signals are converted
to digital data, using 2.5V as a reference. The reference voltage is derived
from the R14 and R15 voltage divider
on the 5V supply. The digital data are
compared in software to make the
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