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A small Motor Controller

I often play with my Fischertechnik kits and generally my models employ one or two small dc motors and a few switches. This project will drive two motors and you can connect up to four switches to it.

The motors can be turned on and off as well as change their direction. This requires four control lines. The other four lines are available for switches or other purposes. The relays I picked for this board happened to be in my junk box and can carry up to 1A. Other relays can of course be used, but be warned. I could not find very many 5V relays. Some 6V relays are suitable, but they are at the limit of their design. Some will work correctly, others will just move a bit but not make contact because the voltage is too low.

Circuit diagram
The circuit is straight forward. The motors have their own supply shown at the top right. Relay C1 and C3 turn the motors on or off, C2 and C3 change the direction of the motors. The four coils are connected to the lower four lines of a ULN 2801 type darlington driver. The four free lines are brought out to a socket. Two LEDs indicate that the respective motor is active. There is a link to select between user port power supply or printer port. If you use the printer port option your motors must use the same voltage as your relays. You also need to connect link 2, in order to unite the two 0V lines. The user port option completely separates the two circuits and any voltage up to 30V can be used for the motors.

Other circuits are possible, but this one makes it easy to fit an indicating LED and has comitted lines for on/off and left/right. A refinement might be to fit two-colour LEDs and show to show on/right - on/left, with one indicator.

Below is a proposed PCB layout in gif format. If you have an Acorn computer, download the draw file here. The pinout of the relays I used is rather peculiar, so if you use your own, the layout will have to change somewhat. The built circuit is below the pcb.


The component side

The final annotated illustration shows the board connected to a model which uses two motors. The switches are not connected as yet.

I have written a small program which will demonstrate the operation of the board. it is called MotorCon and you can find it on the pd section of my site or click here. At a later date I plan to include a programming feature so that you can drop a script file onto the program which will then drive your motors accordingly.

As usual, if you have any problems send me an e-mail and I will do my best to help you.

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