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Tinkering with an RC Car (4)

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Lets use what we learned on the previous page to go a step further. We will remove the rc control board from the car and use it to control another device.

Supplies needed:

  • Basic Tools
  • SPST relay - 3 volt coil (or a relay that matches with your car) Any relay such as a SPDT or DPST or DPDT can be used.
  • 3-AA Battery holder
  • Small perf board (optional but it makes things easier)
  • Misc wire
  • Motor or other device and a power supply that mates with it. In our case we have a 12V battery and a 12V windshield wiper motor
  • Optional 12" x 12" piece of plywood, hardboard, particle board or MDF board for mounting
  • Optional mounting screws and non-metallic spacers to mount rc control board and optional perf board

Remove the control board and wiring from the RC car - clip the wires as long as you can and leave them on the board. Make sure you know which leads are the battery feeds to the board. Double check the polarity. Generally red should be positive and black negative but it always pays to check things out with a voltage meter before taking things apart. Take the antenna loose also. Use screws and mounting non-metallic spacers to mount the rc control board and perf board to a piece of wood.

1)  Wiring Diagram 2) Completed Setup 3) Close up of Control Board, Perf Board and Batteries mounted on wood 4)  Side view of mounted boards 5) View of the back of the perf board showing soldered conections

Image 1 is the complete wiring diagram. Image 2 shows the final look of the project. Image 3 is a close-up of the board and control wiring. Image 4 is a side view of the mounted pc boards showing the spacers used to hold the pc boards off of the wood. Image 5 shows the wire solder connections of the components on the back side of the perf board. It is not obvious in the picture but the relay is actually plugged into an IC socket to which the wires were soldered. The IC socket was soldered into place and then the relay was plugged in. This protects the relay from possible damage due to overheating while soldering.

If you study image 2 closely you may notice that the wiring for the motor is slightly different from the wiring diagram. The wiring diagram shows running the positive connection from the battery through the relay. In the image the positive battery terminal is connected to the motor and the negative terminal is run through the relay. There is also a wire running from the negative battery terminal to another connection on the motor. This relates to a special feature of this particular motor. This motor will always stop in the same position.

Video: Radio control Relay device in operation

Important Note: This example was put together to demonstrate how to use the inexpensive rc car controller with a relay. The 3v (coil voltage) relay used in this demonstration is only capable of switching 2 amps (at up to 24V). The 12V windshield wiper motor used in the demonstration draws slightly more than 2 amps of power when there is no load on the motor. Under load (actually moving something) the motor can use up to 7 amps of power which would overload the small relay used for the demonstration. If a team wishes to try this they need to make sure that they find a relay that is capable of switching the current their motor (or other device) will be using. If you cannot find a suitable relay then you could consider doing a two step process where you use a small relay that works with the rc receiver to activate a larger relay that is rated at the capacity of the device you wish to control.
Additional Notes: The example wiring shows the three AA battery case permanently attached to the rc car receiver. The battery case in the example has a 9 Volt battery connector and power is disconnected by disconnecting the wires from the battery case. If you do not use a connector that can be easily removed from the batteries pack you will want to install some type of switch in one of the power leads to the receiver board. The receiver draws power even when the transmitter is not in operation. If you do not provide the ability to turn off the receiver board your batteries will run down very quickly.