Hi Fred, My name is Kam Leang and I'm a graduate student in Mechanical Engineering at the University of Utah. I TA a class called Mechatronics and each year we give students an open-ended design project of designing and building an autonomous robot using your Handy Board as the base controller. We've been using the Handy Board for 3 years now and it has been very sucessful! Ever since we started using the Handy Board, we've been experimenting with various motor driver circuits to drive larger DC motors requiring more current. For the past year and a half since I've been the TA, we've been using a bipolar "push-pull" motor driver circuit that I've included as an attached file (mtrdrv.pdf). The circuit works great and allows the Handy Board to control a DC motor in both the forward and reverse direction! The circuit is simple and has a low parts count. It's been so successfull that I've even had PCBs made for the class! I was wondering if you would be interested in posting the circuit on your Handy Board site for other to use. If you are, then the following information below simple outlines the circuit and its basic features. Thanks, Kam Leang kleang@eng.utah.edu *************** Bipolar Motor Driver Circuit Instructions ************* Bipolar "Push-Pull" Motor Driver Circuit, Kam Leang 1998 kleang@eng.utah.edu The following information pertains to the bipolar "push-pull" motor driver circuit for the MIT Handy Board. The following circuit has no guarantees so use it at your own risk! Description: The bipolar motor driver circuit is designed for the MIT Handy Board to drive larger current consuming motors connected to the motor output ports of the HB. The circuit is simple and cost under $3 to construct. So far, the circuit has worked well with Tamiya motor/gearbox sets avaialable from Edmund Scientifics (www.edsci.com) and Mondotronics Robot Store (www.robotstore.com), and a few RC car motors. Other DC motors will work, so experiment! How it works: The circuit is simple. The 510 Ohm resistors connected to the base of both transistors limit current from the HB motor output port. A positive output from the HB will cause the NPN (TIP120 Darlington Power Transistor) to turn on, allowing current to flow from +V to ground. This will effectively cause the motor to turn in one direction. As the output from the motor output port of the HB becomes negative, the PNP (TIP125) transistor turns on allowing current to flow from ground to -V, which causes the motor to turn in the opposite direction. The 1N4004 diodes are used as flyback diodes since the motors are inductive loads. The 1k ohm resitor between the negative (-) motor output port of the HB to ground is used to eliminate problems associated with HB resets. For some strange reason, without that resistor, the HB sometimes resets itself. At this point, I don't why. Construction: Construct the circuit as shown in the "mtrdrv.pdf" schematic. The power supply that seems to work well is a 7.2V RC car battery, similiar to the ones available from Radio Shack (#23-230, or #230-330). Since the bipolor motor driver circuit requires a bipolar power supply, a center tap on the RC car battery is required. Cut open the back of the battery exposing the center tap connection between the two cell arrays. Solder a wire with the equivalent guage as the original terminal wire (black and red) to this location. The center tap is now the ground terminal of your battery pack power supply. The voltage measured between ground and the red or black lead will be +-3.6v, respectively. Now, your RC car battery pack is bipolar! Connect the appropriate polarities to the circuit. Note that the bipolar power supply is only +-3.6V, therefore motors that demand higher voltages requires a higher voltage source! Good luck! Kam Leang 1998 kleang@eng.utah.edu Parts List: Qty Component Price ----------------------------------------- 1 TIP120 (NPN) $1.00 1 TIP125 (PNP) $1.00 2 1N4004 Diode $0.50 2 510 Ohm Resistor $0.10 1 1K Ohm Resistor $0.05 ------- Total $2.65 Vendors: Jameco (www.jameco.com), Mouser (www.mouser.com), and Hosfelt Electronics (www.hosfelt.com).