From the forums: Trinket based “String Car Racer”


Thanks to Adafruit forums user HarpDude for sharing this project with us! Check out more here.

Back in the 1980s, my college-aged brother designed a simple motor+battery car that raced along a string between the birch tree and the street-side power pole. For years now, we’ve been improving on the design, keeping it really simple: 1 battery-driven motor, flashing lights, and repurposed materials whenever possible. The primary goal was speed and stability. The car ran at full speed until it found the end of the string — then you either had to be there to catch it or be prepared to repair it before the next time trial.

We talked about making the car smarter so that it could stop and reverse directions, and came up with lots of schemes and circuits to make it possible, but we were more focused on the high-speed “Gump” mode of operation. Run Gump, run!

Fast forward to now. I retired from a technology management job and was challenged by my son to get back into electronic design. What could be better to start than to realize the dream of a simple, autonomous string car that wouldn’t crash at the ends of the string. To keep it simple, I decided to design an initial version with TTL logic and micro-switches as end-of-string sensors. Yeah, I used to design TTL and CMOS back in the day, so I defaulted to that technology.

After shopping around for ICs, I happened on the Arduino web site. Hmm. From there, I gravitated to Adafruit. Here’s a microcontroller with 5 I/O pins and just enough memory for what I wanted. Let’s see if we can wake up those C++ skills, too. Adafruit tutorials were extremely helpful and were essential in the design process.

Long story short, StringCar 3v1 with software revision 4 is now ready to go. The prototype logic board worked so well that I sent the design off to Fritzing to be printed. The software supports two modes today. First is Pong mode where the car accelerates from a dead stop, races to one end of the string, brakes, reverses direction and speeds to the other end of the string — forever. The second mode is Boomerang where the car travels to the far end of the string, brakes, reverses, then stops at the origin, flashing its 5mm NeoPixel blue to remind you that it’s back home. Selecting the mode is simple. Power-on or reset puts you into Pong mode and initially flashes the LED purple before taking off. Boomerang mode is selected by holding down one of sensor switches during power-on/reset. The LED flashes blue to let you know the car will return and stop.

Read more.


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