■ SCHEMATIC 1.
Schematic for the
physically connects to a corresponding brush that rubs
against a metal strip embedded into the track, which
carries the same electrical potential when power is
applied. In Figure 1, the brushes are connected to the
motor terminals. It is between the brushes and the motor
terminals that we will connect the microcontroller circuit.
As Schematic 1 and Figure 2 show, we must connect
our new microcontroller circuit between the two brushes
and the DC motor. Schematic 1 shows the voltage
regulator at the top, the half-bridge drive circuit with FET
drivers to the right, and the PIC12F683 microcontroller,
photo transistor, and programming header to the lower
left. Figure 2 shows the slot car after the
microcontroller drive circuit was built and
installed on to the slot car. The circuit board
can attach to the car’s frame using a few small
(and I do mean small) drops of super glue.
In Schematic 2, D1 and D2 are the infrared
(IR) LEDs that are read by the photo transistor
on the slot car (Digi-Key part #QEE123-ND).
The 100 ohm (0603 sized) resistors can also
be purchased through Digi-Key.
Let’s sit back and think about exactly what
we want to do. We want to run the motor
in the forward direction, with the ability to
control the speed and to electrically brake or
stop the motor. How can we drive a brushed DC motor to
accomplish these tasks? By using a half-bridge drive circuit.
You can also use a full-bridge drive circuit, but to keep the
overall circuit small, simple, and less expensive, a half-bridge circuit will give us exactly the performance we want.
This type of circuit will also give us the ability to drive the
motor forward and electrically brake/stop the motor.
However, for this application, I found only one major
drawback with using the half-bridge drive circuit — as there
are two MOSFETs controlling the switching inside the half-bridge circuit, there is a possibility of both MOSFETs being
switched on (conducting) at the same time. This situation
will create a condition where current will flow through
■ FIGURE 2. Slot car with microcontroller
drive circuit installed.
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