Let's Make Robots!

Mr. Basic PCB with motor controller


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Mr._Basic_motor_driver_Schematic.jpg333.35 KB
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During the first Mr. Basic contest some people had trouble driving their motors because of the current required and because of the low voltage provided by Mr. Basic's 3xAA battery holder. I posted a blog showing a schematic for a controller which DAGU has now produced as a replacement PCB for Mr. Basic.

This controller will work on voltages from 3.6V to 7.2V. Higher voltages can be used if the relays are changed. Although designed for Mr. Basic this PCB can be used on any robot and has room for additional circuitry to be added. This board comes as a kit (PCB and components) so you will need to solder it yourself.

motor_controller_board_2.jpg

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Hi, can you still buy this motor controller?
Contact Claudia here: http://letsmakerobots.com/user/3739
I just put mine together. The pictures above really helped to ensure proper component rotation. I can tell ODDBOT is a pro by how his resistors are rotated the same way on each side. Deatils man... details!
It is something I am trying to teach our assemblers. Small things like resistor orientation do not affect performance but makes the product more proffesional. I have been making electronic circuits since I was 12.

Relays, Nice. I used something like this for the original Wall Racers. Works very well. Power from source to motor, straight. I like.

Nice and simple. How much current is the relay contact rated for. Everybody knows relays don't last forever at higher currents. I have seen similar relays rated at 1 to 1.5 amps. DC takes it's toll more then AC on contacts.
The kit comes with relays rated at 2A. If your program turns off power before changing between forward and reverse then your relays should last a very long time.
Why is that? I mean, why should power be turned off before changing between forward and reverse?

When you change from forward to reverse, one set of relay contacts is opened and the other is closed. If current (especially larger currents) is still flowing when this happens, in the instant just before a contact pair opens/just after it closes there is only a very small area between the contact faces through which the current can flow, which makes for a very high current density. High current density translates into high temperatures, which can actually 'weld' the contacts shut, or at least degrade the contact surfaces over time, reducing the performance of the relay.

If you shut the current off a fraction of a second before changing the contacts, then you can avoid this issue altogether, and your relay will thank you for it =)

As far as the gears go, there's always going to be a certain amount of inertia in the system that has to be depleted when changing directions. To forces inflicted on the gear teeth are determined in part by how fast the inertia is dumped, so by changing from FORWARD -> STOP and the STOP -> REVERSE the gears don't take as much of a beating as when going directly from FORWARD -> REVERSE.

Not to mention, if our program turns off power before changing between forward and reverse then your gears should last a very long time too.