Project
+ 5V
9VDC
+_
K3-9 k3-7
K3-4 K3-9
5K Ohm
COMPUTER
K3-15
SIMM 100
J4-232
J2-1 K3-28
SERIAL PORT
MINI RELAY
D1
100 Ohm
C
B T3
E
D
G T1
S
D
G T2
S
7.2V RC BATT. +_
www.digikey.com A manual complete
with schematic and parts list for
the SIMM100 can be found at www.
lawicel.com
The responsibilities of the interface
board are: measure the divided battery
voltage created by the test-load, then
transmit measured data to the serial
port of the computer, and also control
the connection of the load and ADC to
the battery. Figure 5 shows a connection diagram of the analyzer. FETs T1,
T2, and T3 are controlled by the interface board to connect/disconnect the
load and ADC (analog-to-digital converter) input from the battery under test.
The interface board will connect the
load by turning ON T1/T2 and the ADC
using T3 after it receives a “Go” control
word from the PC to start testing.
When the test is done, the PC will
send a control word to disconnect the
ADC then the load by turning OFF the
transistors. It is important to automatically disconnect the load because
batteries can be damaged if discharged
beyond their cut-off voltage (more on this later). During
testing, measured voltage is fed into the microcontroller’s
10-bit ADC, processed, and then sent to the computer via
a RS232 level-shifting chip. The SIMM100 has a ground
plane that greatly reduces interference and promotes
stable analog-to-digital conversions making this board a
perfect control solution.
Bascom-AVR is a “Basic”
language compiler from MCS
Electronics that was used to program
the interface board. MCS Electronics
provides a generous demo that is
only limited by its 2K byte program
size. The program for this project is
fully supported by the demo version
and will easily fit in the size limit. You
can download your own demo at
www.mcselec.com I used the
STK500 development board from
Atmel to program the chip, but in-circuit programming is supported
by the SIMM100 when done with a
compatible programmer.
Listing 1 shows the small amount
of Basic code that is needed to support the project requirements. The
program only fills six percent of the
controller’s memory, making the
ATmega16 a little bit of an over-kill
for this application.
0.3 Ohm 50 W
0.3 Ohm 50 W
ALUMINUM
HEAT SINK
Figure 5. Connection diagram.
The Interface Board
The interface board (Figure 4) is a SIMM100 PCB
from Lawicel that is fitted with an ATmega16 AVR micro-controller from Atmel. I received my SIMM100 PCB from
MCS Electronics ( www.mcselec.com) and populated it
with components purchased from Digi-Key at
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44
JULY 2005
