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Materials needed:
1 set of .25 Watt resistors
1 quad comparator
4 PNP bipolar transistors
5 NPN bipolar transistors
5 photoresistors
4 AA bateries
1 battery case (optional)
2 SCR
1 breadboard
1 RC car
laser pointer

Time requirements:
4 hours

Step 1: Basic Breadboard Instructions



A quick refresher on the basics of breadboards.

Step 2: Laser Pointer Detetection

The basic way to detect the laser pointer, is to use photo-resistors.  The photo-resistors pictured above have a resistance of 10 kilo-Ohms in darkness, and a resistance of 1 kilo-Ohm when the laser pointer is shined on them.  This provides a very good way to detect the light.  When building the circuits later on in this project, all of the resistors on the high side of the voltage dividers should be replaced with one of these photo-resistors.

Step 3: H-Bridge Basics

The H-Bridge is a very commonly used circuit to control DC motors.  It allows the user to spin the motor forward and reverse.  This is very useful for this project because RC cars have one motor for steering and one motor for propulsion.  You will need to be able to make both motors go forward and backward.  The basic principle behind the H-Bridge is the motor will spin in the forward direction when the High Side Left and Low Side Right switches are closed.  The motor will spin in the reverse direction when the Low Side Left and High Side Right switches are closed.  For other combinations, the motor will not move.

Step 4: H-Bridge Construction

This is a schematic of the H-Bridge for this particular project.  The 1K resistors will limit the current into the transistors and will prevent them from burning out.  The two High Side transistors are PNP transistors, and the two Low Side transistors are NPN.  They will act as the switches from the previous step.  With no input, the PNPs act like open switches and the NPNs act like closed switches.  By applying a 5 volt signal to the forward input, the Left High Side switch will close, and the Left Low Side switch will be forced open, leaving the appropriate configuration for forward motion.  The reverse input works the same way.

Step 5: Comparator Construction

In order to apply a 5V signal, it is helpful to use a comparator.  The quad comparator used in this project has four individual comparators.  Because we are building two H-Bridges, we will have four total inputs.  We will use one comparator per input.  The comparators function by taking an arbitrary input voltage, and if it is above a certain threshold, outputting 5V.  We can design the threshold to be whatever is convenient for the project.  This will make it easy to supply the inputs to the H-Bridges.  Pictured above is a basic comparator schematic.

Step 6: Drive Circuit

Now that you understand the basic components, you can assemble the drive circuit.  On the upper half of your breadboard, assemble the circuit shown above.  Note the H-Bridge circled in red, and the comparator circled in yellow.

Step 7: Steering Circuit

Now, directly below the drive circuit, assemble the steering circuit, as pictured below.  The steering circuit is a little different because the wheels need to lock to the left, right, or center.  This is because you won't be able to apply the laser pointer to both the drive and steering circuit at the same time.  By locking the wheels, the car will be able to turn.  In the image above, the circuit circled in red is the left locking mechanism, and the circuit in yellow is the wheel centering circuit.

Step 8: Assembled Circuits

The final circuit should look like this.  Notice the cones of paper over the photo-resistors.  These cones shut out most of the ambient light, improving performance.  Also, notice the area left for power at the rear of the car.  Make sure the batteries will be placed above the rear wheels (the drive wheels) this will provide extra down force on the drive wheels, resulting in more torque.

Step 9: Power

To supply power to the motors, connect four AA batteries to the power rail on the breadboard.  This will supply power the the entire car.  The case pictured above is optional, but it makes it the project much easier.  You can build your own case if you want, but be sure to connect the batteries in series.

Step 10: Results

CONGRATULATIONS!  The car is now finished.  It should look similar to the image above.  Notice the homemade battery case.  Now, test the modified car out.  If you are experienced in printed circuit board layout, you can also do this project on printed circuit boards, allowing the car's body to fit on top.  Just make sure to cut hole for the photo-resistors.

Have fun.
<p>interesting project, but also turn right or left the car depends on the sensor?</p>

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