Nuts & Volts - February 2004

Stamp

by Jon Williams

Putting the Spotlight on BASIC Stamp Projects, Hints, and Tips

Stamp Applications

Makin’ It Motorized

From discarded toy — to mobile robot!

Figure 1. The Pololu Motor Controller (right) with a BS1 module.

Idon't know about you, but I'm still exhausted by last month's column — wow, that was a real workout, wasn't it? I do hope you found it useful though. This month, we're going to go a lot easier, but still have a bunch of fun! And after its terrific comeback the last couple of months, we're going to have that fun with the venerable BASIC Stamp 1.

There's no denying that personal robotics is one of the fastest growing aspects of hobby electronics. There are clubs all across the globe devoted to it, and each week seems to bring another television show with robotics as its centerpiece. An easy way of getting started in homegrown robotics is to convert an existing motorized toy. They're all over the place, in fact, your family may have a few left over from the holidays that no longer seem interesting. Why not spice up an old toy with a brain you can program yourself? Okay, then, let's do it.

There are BS2 examples of this kind of robot everywhere; I wanted to see if there was enough space in the BS1 to pull it off. As it turns out, there is with room to spare, which means we can add more "intelligence" to our robot once we get in going.

Figure 2 shows the schematic for our simple robot controller. Two pins are used to communicate with the Pololu controller (SMC02), two others for our bumper inputs

(using our standard "safe" circuit). The first control line to the SMC02 is the serial connection. The SMC02 will automatically detect baud rates from 1200 to 19.2 kBaud. As our top-end limit on the BS1 is 2400, that's what we'll use. The second line controls the Reset input to the SMC02. Using the hard reset line will let us stop both motors at once when required without having to send serial commands.

While interfacing to the SMC02 is simple and straightforward, it is very specific. Let's have a look at the set-up portion of the program:

Micro Motor Control

E l e c t r o n i c s NUTS & VOLTS

F o r

E v e r y t h i n g

Since virtually all motorized toys use small DC motors, and controlling them requires full-time PWM that we can't do natively with the Stamp, we'll use an external motor controller. Pololu Corporation makes and sells components devoted to small robotics and their Micro Dual Serial Motor Controller is perfect for our task of converting a motorized toy. The SMC02 accepts instructions through a serial connection and will control two motors (speed and direction). Motor voltage can be from 1.8 to 9 volts, with currents up to one amp per motor! This little dude rocks. And little is accurate; you can see what it looks like in Figure 1 next to a penny and a BS1-IC module.

Let's get right into it. A typical first robotic project is a "bumper bot" — the kind of robot that when it bumps into an obstacle will turn away and then keep moving.

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Setup:
  HIGH SOut
  GOSUB Reset_SMC

...

Reset_SMC:
  LOW Reset
  PAUSE 1
  HIGH Reset
  RETURN

The first line sets the idle state of the serial connection. Since we're using a "true" mode, this is important so that the start bit (high-to-low transition) of the first transmission is properly detected. Next, the controller is reset by taking the Reset input low and holding there for a millisecond before retuning it high. The Pololu docs suggest that one millisecond is overkill, but I found that at least one millisecond was required for proper operation.