June
2006, Issue 191
Measurement
System for Weight and Dimensions
Renesas
M16C Platform Design Contest 2005
MOTHERBOARD
DESIGN
I
used an SKP16C62P evaluation board for the Weasure prototype
because I knew its peripherals (display, buttons, and
LEDs) would be useful for development and the finished
prototype (see Photo 3). To interface the evaluation
board with the scale, I designed a motherboard PCB with
sockets aligned with the expansion pins on the SKP16C62P
board (see Figure 1). The board has resistor networks
and connectors for the photocells and a connector for
the scale. Its MAX233 RS-232 serial interface chip provides
the proper signal levels for talking to a PC’s serial
port.
|
Photo
3—The Weasure board includes an SKP16C62P board.
The MAX233 is under the SKP16C62P board. The ribbon
cables are connected to the photocells on the measurement
frame. |
|
(Click
here to enlarge)
|
Figure
1—The motherboard routes the sensor information
to the SKP16C62P board and the serial information
from it. |
A
9-V battery (or an optional wall-wart power supply)
powers the DW-36XP scale. The 9-V supply from the scale
is connected to the Weasure motherboard, where a 7805
regulator supplies the 5-V supply for the SKP16C62P
board, the MAX233, and the resistor networks (see Figure
2). The PWM output from the scale is connected to the
M16C/62P’s timer B4 input pin.
|
(Click
here to enlarge)
|
Figure
2—The power supply, scale, and photocells are connected
to the SKP16C62P board. A 7805 regulator allows
the circuit to be powered from the scale’s 9-V supply.
The serial interface is used to send measurements
to your PC. |
Because
the DW-36XP scale is obscured by the Weasure’s measurement
platform, the controls and display on its front panel
are replicated on the Weasure board. A switch on the
motherboard duplicates the On/Off switch. The zero function
is duplicated by S1 on the SKP16C62P board. This button
resets the scale’s zero point. Like the other functions
on the DW-36XP’s front panel, this is easily duplicated
in software.
I
modified the scale with a connector for the 9-V supply
and on/off and PWM signal outputs. I tack-soldered wires
to the scale’s control board and routed them to the
connector. Conveniently, a cutout on the back of the
scale’s enclosure was easily adapted to a connector
for attaching it to the Weasure motherboard.
