back of the garage and wired to the garage door opener
with 14 gauge wire.
Figure 1 shows a photo of the prototype that is still in
operation. (The circuit is simple enough to build and use
on a prototyping board.) Figure 2 is a printed circuit
board (PCB) version that is more professional looking and
easier to hook up. Figure 3 is the schematic, and Figure 4
shows a wider angle with the sequencer connected to the
garage door opener, garage battery, truck, and ATV.
I chose the Parallax BASIC Stamp 1 microcontroller,
mainly because I had one from another project and it is
easy to use. It also has an onboard regulator, brown-out
circuitry, and a reset input pin. The Stamp reads an 0834
Analog-to-Digital Converter (ADC) which is covered in the
BASIC Stamp applications manual.
Because the ADC inputs can only measure up to five
volts, the battery voltage is divided by three, using a
resistor divider with an isolated 10K eight-pin Single In-line
Package (SIP) resistor network. Since the resistors in the
SIP are made using the same process, their values are all
very close to each other. The tolerance is 2%. This can
also be done with discrete resistors, but with less accuracy
unless the resistors are matched.
■ FIGURE 3. Schematic.
This load is on the battery all of the time; the current
draw should be as low as possible. I used a 10K SIP —
with three of the resistors in series — which uses 0.45 mA
( 13.5/30K). A higher value SIP resulted in less accurate
readings due to the higher input impedance on the ADC
input. The open circuit voltage of the solar panels can be
as high as 23 volts; the ADC input for them is divided by
four. It uses all four resistors in the SIP. During the load
test of the solar panels, dividing by four keeps the voltage
on the ADC input less than five volts. Power for the Stamp
and ADC is supplied by the battery(s) through diodes D4,
D5, or D6. That way, it doesn't matter which output is
connected to a battery or left open.
In order to reference the ADC to ground, P-channel
MOSFETs (Q1, Q2, and Q3) are used. The MOSFETs need
very little current to be turned on, and have a very low on
resistance. It is a P-channel IRF 5305 which has a built-in
bypass diode. The bypass diode will allow a charged
battery to push current to a charging battery when the
MOSFETs are off. The purpose of diodes D1, D2, and D3
is to block this current. They are Schottky MBR2535CTs
which have a lower voltage drop than standard diodes.
The diodes are rated for 30 amps, and the MOSFETs are