Servo - December 2015

Either of these could be used to drive the twin-motor robot chassis back in Figure 1, using all four of the PICAXE 08M2 outputs. One limiting factor is that only pin C. 2 can use the

do: sertxd(#pinc.3, 13,10): loop
‘‘‘ display pin c.3 value

PICAXE’s PWM function to automatically control motor speed. If you need more PWM channels, you can move up to a 14M2 or 20M2 — both of which have more GPIO pins and can plug directly into this breadboard.

Now we have just enough knowledge to be dangerous.

All four of the PICAXE 08M2’s output pins will be used to control four inputs on a dual H-bridge to drive two motors. We still have input-only pin C. 3, to which we have previously attached an IR receiver to read IR remote signals. We can certainly put that sensor here again and control a robot with an IR remote.

A better alternative is to use an IR obstacle sensor module which will let a robot avoid obstacles and drive around autonomously. This ~$2 module emits an IR signal which gets reflected by nearby objects and sensed by a built-in 38 kHz IR receiver. As a bonus, we can still use this sensor to receive IR remote control codes, albeit at a reduced range. Be sure to get the right four-pin sensor shown in Figure 6. Search your favorite sellers for “IR infrared obstacle sensor module Arduino smart car.” Order a few; they are handy and the range varies between modules.

We could round up all the parts mentioned and make a traditional differential steering 2WD robot chassis, but let’s go for instant gratification. Instead of tracking down the parts and waiting a week or more for shipping, let’s just hack a common RC toy. The same principles of interfacing H-bridges apply. I selected a $10 RC car at Walmart which should be universally easy to find. Ordinarily, it would not be my first choice, but the price was right and (provided you can find the same car) it will be a simple matter to follow along and make the mods shown here. You won’t win any DARPA Challenges, but you’ll have some fun and learn a lot of fundamentals for this car’s $10 asking price:

Pros: Cheap, widely available, and mechanically complete. Uses 3xAA batteries, which will also power the breadboard. Drives like a car, so it’s easy to go straight. Fairly durable, large rubber tires can be used outdoors. Quick hack; no mechanical building, just disassembly.

I haven’t found much hard info on this module, but it has a 555 timer generating pulses sent to an IR LED. I was expecting to see 38 kHz, but I saw closer to 42 kHz on the modules I tested. We won’t use the Simple soldering, uses existing H-bridges, only requires four 220 ohm resistors to connect to PICAXE. RoboCar excels at running fast simple geometric patterns like this star: www.youtube.com/watch?v=_gbU_iMH388.

ENable pin, just the GND, +, and

Cons: Sloppy steering linkage won’t turn very sharply forward.

OUT pins. I used a servo extension cable to connect to the sensor. The sensor’s output goes to input pin C. 3. The pot closest to the removable (but don’t!) jumper works best turned fully clockwise (frequency adjust), and the other pot controls sensitivity (clockwise = more sensitivity). We’ll use the sensitivity pot to adjust our triggering distance which will vary greatly based on the color, reflectivity, size, and geometry of the obstacle.

There are two LEDs on this sensor; the one marked ‘Pled’ is a power on indicator, and the other ‘Sled’ is the sense indicator for when reflected IR is sensed. This is an Sharper turns in reverse. Won’t turn in place like a differentially-steered robot. The steering is not proportional; an ungeared motor slams steering full left or right and stalls. Under-geared drive motor drives car too fast for precise motion, no braking; car coasts a bit after turning the motor off. “Shoot through” is possible, where you can destroy H-bridges from a programming error! Traction grip varies between carpet and tile floors; you may get different steering results on different surfaces. Speed and path repeatability are dependent on battery condition.

“active low” device, so its output pin is normally high (logic 1) and goes low (logic 0) when triggered. There’s a small transition range where the The Walmart car is a New Bright “RC Sport” 1/24 scale, “Full Function Radio Control,” No. 2423, SKU# 50211 02423 as shown in Figure 7.