CIRCUIT BUILDING FUNDAMEN TALS
AND THE AS TABLE MULTIVIBRATOR
The circuit constructed for this project, an astable
multivibrator, requires resistor-capacitor (RC) circuits
The basic RC circuit contains a single resistor and a
single capacitor. (This was the type used for this project.) In
an RC circuit, after a calculable amount of time, no current
will pass through the capacitor. Current can flow through
a capacitor while it charges, but not once it is charged.
The value, in Farads, of the capacitor times the value, in
Ohms, of the resistor equates to the number of seconds it
takes until the capacitor is fully charged, creating an open
circuit. This time-based functionality is what determines
the period of our clock signal.
Ohm’s law states: V = IR. V is the voltage, I is the current,
and R is the resistance. With regard to circuit construction,
It is important to use Ohm’s law to calculate and place
extra resistors (or higher resistance resistors) at certain
junctions in the circuit. The FPGA input, for example,
functions at 3.3V. Inputting even tenths of a volt higher
than 3.3V could damage the FPGA.
The final piece of the astable multivibrator is the
transistor. For this project, the transistors acted as switches.
Such transistors have three leads: collector, emitter, and
base. The collector is the input, the emitter is the output, and
the base is what is used to control whether the collector and
emitter are a closed circuit or open circuit. When a voltage or
current is applied to the base, the collector and emitter are
a closed circuit. When no voltage or current is applied to the
base, the collector and emitter are an open circuit.
The astable multivibrator is composed of two of an RC
circuit and a transistor, in parallel. LED’s have been added
to the diagram (See Figure 2).
The portion of the circuit with an LED is where the clock
signal/square wave is captured. There are actually t wo
antiphase signals produced by this circuit. Only one needs
to be captured, and it does not matter which. The signal
behaves as the LED’s behave, while one LED is illuminated,
the other is not, then the other illuminates while the other
is off, etc. The theoretical diagram and what was actually
Figure 3. A completed astable multivibrator used with Mojo
Figure 4. Implementation schematic for
of trans people in the U.S. have reported
experiencing harassment, mistreatment,
or discrimination on the job.