REVIVING AN ANDROBOT BOB: Part 2
Sonar MUX board (top).
and only one at a time. The schematic
presented here will do just that.
It only requires 5V, ground, and
three data lines to select the appropriate
relay. It uses a 74HC138 and a 74HC14
inverter to select, and a ULN2803 to
drive the relays. To determine which
relay will be energized, we must decode
Sonar MUX board (bottom).
the signals for relay select. This is what
the 74HC138 chip does well.
It is a three to eight decoder and
depending upon what pattern is applied
to its inputs A0, A1, and A2, you will
get one of the outputs (Y0 through Y7)
low and the remaining will be high. We
will use the active low to activate a relay
for a particular transducer.
That will be great for the first five
(0-4), but what happens when the bit
pattern is accidentally set to 5-7?
Remember that one transducer and only
one must always be selected. That is
why we take the signal for A2 to switch
the last relay and ensure that all others
are switched out of the circuit. The
inverters from the 74HC14 convert the
active low to become an active high.
The active high goes to the ULN2803
chip which drives the relay. It contains a
set of eight Darlington transistors and
has protection diodes built in.
As a visual indication, a small LED
was placed next to each relay so it is
easy to tell when it is energized. It only
takes wiring a small LED and current
limiting resistor across the coil of each
relay. It is best to select a resistor for
the LED so that it can be clearly seen
but not overly bright. Anything more is
Schematic of custom
sonar MUX board.
46 SERVO 03.2008