Servo - July 2009

always include a 1K resistor in series between the two pins. At 5V, this limits the current flow to 5 mA which prevents any potential damage to your project.

Then We Get to the Meat and Potatoes

When your new EZ range finder arrives, you can use the schematic in Figure 3 to install it on your TankBot’s breadboard. The only aspect of the circuit that requires explanation is the 100 µF electrolytic capacitor across the EZ1’s power and ground lines and the 100Ω resistor in series with the Vcc line to the EZ1. This is the combination that MaxBotix recommends to minimize the power line noise created by our servomotors. This noise can be considerable and can have a negative effect on the reliable operation of the EZ1, so don’t forget to filter the power line noise in any project that includes an EZ1.

Figure 4 is a photo of the EZ1 circuit installed on the TankBot’s breadboard. As you can see, I have also left all the IR object detection circuitry on the board — when you have finished working with the two EZ1 programs that we will discuss shortly, you can experiment with combining the IR object detection and ultrasonic range finding functions into one program. (You didn’t think you were going to escape without a homework assignment, did you?)

We will be using two programs this month: EZ1Term.bas and EZ1HotSauce.bas. Download both of them from the SERVO website ( www.servomagazine. com) and print them out for reference in the following discussion. Our first program (EZ1Term. bas) is a simple test for the MaxSonar range finder. It repetitively inputs the analog distance data from the EZ1, doubles it to convert it to inches (as

discussed earlier), and outputs the data to the PICAXE Programming Editor’s Terminal Window. Once you have installed the EZ1 circuit on your TankBot, just download EZ1Term.bas to the bot’s 14M, open the

Terminal Window (under the

PICAXE menu, or just press the F8 key), and observe the results. Don’t run the TankBot’s motors yet — we’ll get to that shortly.

To test my EZ1 setup, I left the programming cable attached to my PC and placed my TankBot on the floor with the 0 end of a 12 foot tape measure aligned with the front of the EZ1 sensor. For an object to detect, I used a handy hot sauce bottle (Doesn’t everyone have several of them laying around?) because its 3. 5 inch width is as close as I could get to the 3. 25 inch dowel MaxBotix used for its testing of the EZ units (see www.maxbotix.com/down loads.html).

Using this setup, I learned a couple of interesting things. First, the distance data reported by my EZ1 was consistently two inches less than the actual measurement. I’m not positive about the cause of this discrepancy but the five volt regulator on my BrainAlpha board is actually putting out about 4. 9 volts, which may account for it. In any case, I took the easy way out and just added two to all my readings (but not in the version of the program that you downloaded)

Figure 3. Schematic of the EZ1 rang-finding circuit.

now my reported distance to an object is consistently within one inch of the actual measurement, out to a distance of eight feet. Between eight and 10 feet, the EZ1 data is about two inches off.