www.servomagazine.com/index.php?/magazine/article/november2010_Eady
PHOTO 2. A set of ACSC04-41SURKWA-
F01, 4511, 4042, and 2N7002 datasheets
along with an ohmmeter is all we need to
figure out how this industry-standard set of
electronic components is wired together.
7. 5 and 25 volts AC or DC. A full wave bridge rectifier
mounted just below the power jack in Photo 1 guarantees
the correct DC input polarity and rectifies any AC input
voltage. Be careful if you use an AC source supply as you
must consider that applying 24 VDC is much different than
applying 24 VAC at the eM8’s power input jack. A nine-volt
DC wall wart is safe and our power source of choice.
The eM8 design reserves the least significant nibble of
the PIC16F882’s PORTA for analog use. AN0 and AN1 have
variable gain Microchip MCP6032 op-amp front ends. The
op-amps are configured to provide gains that range from
unity to 10. The analog circuitry can be seen just to the
right of the 7805 voltage regulator in Photo 3.
On the digital side, eight digital I/O pins are supported
in the original eM8 application. The I/O pins can also be
routed to buttons and LEDs which are located on the break-away segment of the eM8 main printed circuit board (PCB)
under the lens in Photo 4. In fact, the LED
display electronics shown in Photo 2 are also
mounted on a break-away PCB segment.
An ICSP adapter, a MCP9701A
temperature sensor, and a five-volt OMRON
There is plenty of information about the eM8 hardware
that can be ascertained from simple observation and the
eM8 Specifications Summary document. For instance, I
verified that SOT-23-packaged parts marked as 12W are
MOSFETs and SOT-23-packaged parts marked as A6t are
diodes by using the Summary’s description of the break-away relay PCB. A pair of A6t diodes are connected in
parallel and attached in parallel with the G6K relay coil. A
12W MOSFET looks to be switching the ground side of the
G6K’s relay coil.
The more I stared at the eM8, the more I learned
about it. The LM317 adjustable power
supply circuitry is standard stuff and so is
the 7805 voltage regulator design. I’m
really tempted to pull out the ohmmeter
and start tracing through the CMOS latch
and buffer circuitry. That’s not going to
happen since once we dive into the source
code, I’m sure that many more of the
eM8’s hardware secrets will be revealed.
PHOTO 3. Stare at this long enough and the eM8’s major electronic
subsystems will magically appear.
It’s Written in
Assembler!!
That it is. The eM8 source code was
composed with PIC assembler source. I am
not surprised as the eM8 and its code are
intended as teaching tools. If you’ve never
tried writing a PIC program using assembler
mnemonics, you haven’t lived. The cool
thing here is that if you have reservations
about PIC assembler and/or C, you’ll be
52 SERVO 11.2010
