Line Follower Sensor Plus
5-channel IR line sensor plus front (?) IR proximity sensor plus bump switch in one package for about $3.15 USD shipped (Oh, plus tax now!). Might need a bit of rework since the front IR sensors hit the wall before the microswitch. 
But an interesting little sensor array, all wired up to a convenient connector. https://www.ebay.com/itm/223263216001
I for one, would love to see a Boebot/Activity bot with high speed servos do impressive speeds on a line following course. It would take a sensor like this to do so.
BTW, there are those who consider line-following as the "holy grail" of robotics: https://www.wiltronics.com.au/MeetEdison/ , about a quarter way down the page.
But an interesting little sensor array, all wired up to a convenient connector. https://www.ebay.com/itm/223263216001I for one, would love to see a Boebot/Activity bot with high speed servos do impressive speeds on a line following course. It would take a sensor like this to do so.
BTW, there are those who consider line-following as the "holy grail" of robotics: https://www.wiltronics.com.au/MeetEdison/ , about a quarter way down the page.
Comments
@Whit @"Ken Gracey" @"Carol Hazlett" @"Duane Degn" @xanadu
I just ran a "robot building" class at church Sunday afternoon with eight father/child teams. They built these simple $5 line follower robot kits. TTYTT I prebuilt half of each robot beforehand (PHEW!) with most of the tricky parts (IC and polarity parts: caps & LEDs) done. That way first-timers could actually build the robot in a two-hour session. Some people had never built anything or even soldered before (I provided irons and all tools) but people had a blast and dangit if 5 of the 8 people finished their working robots onsite. I took the 3 non-op robots home to debug and return to the builders. Kids (boys and 2 girls!) had a blast doing the mechanical stuff, even if most dads did the soldering.
The weak spot on these kits is that the photocells & LEDs hang way down unprotected, so one bump knocks & bends them, and it stops working. But for this class, I designed & 3D printed a plastic block to support and align everything, it worked GREAT. Smartest thing I did, along with prebuilding some of the robot. I got pretty good at it after building TEN ( had to have backups). Also smart: I videoed every step (switch installation, resistor installation, motor/wheel assembly, etc) and had each step looping on a big screen TV, as the builders built.
I was so doggone busy that I got NO pictures, but it was AMAZING. I'm always blown away how much adults and kids like these silly little line followers. And people want to build more stuff, I took a few other kits like LED blinkies, FM radio and metal detectors and they liked everything I showed. Kids love leaded solder, more to come!
My old video below, just to show these bots in action.
kit building (many types) is fun and satisfying.
no matter how simple or complicated the kit is you gain experience from it.
I am a kit builder ( radios, ham radios, and associated electronic tools and gear)
I enjoy teaching young people on kit building and safe and proper soldering.
i get a lot of pleasure just seeing the surprise on their face when something they built works great.
Agree to all that. I've been a ham radio guy for 40+ years. I bet kids would still enjoy building a crystal radio. We live very close to the antennas of 50 KW AM radio station KNX: https://en.wikipedia.org/wiki/KNX_(AM) That signal is so strong it overpowers everything else. I hear it coming through sometimes on audio amplifiers. It wouldn't take much of an antenna to pick up that station on a crystal radio. Like maybe a SPOON. Just need to find a cheap bulk source for those high-impedance crystal earphones.
What about 1k units, @ US$ 1.75 apiece?
https://shop.miniscience.com/navigation/detail.asp?id=CH905MP
NYCE! Now that's being a hoarder's hoarder! Actually I just found a teardown at https://www.petervis.com/electronics/crystal-earpiece/crystal-earpiece.html and I'm considering MAKING earphones from piezo disks, which are cheap and plentiful. Will probably fab one up to verify function and then 3D print a bunch.
The first ones I've ever had in my hands (circa 1960) had a really hard earpiece (meant to be soft, but...), causing some ear pain and scars, after as many hours of use as the batteries where able to last.
My father and my granpa ever warned me to spare my ears, but they where also concerned about the TCO of the AM receiver, since the 1.5 V cells where not any cheap, at the time.
@erco - Great story! Nice mod on the sensor, too.
Bridgekeeper: STOP! Who would cross the Bridge of Death must answer me these questions three, ere the other side he see.
Sir Erco of Inventoy: Ask me the questions, Bridgekeeper. I am not afraid.
Bridgekeeper: What... is your name?
Sir Erco of Inventoy: My name is Sir Erco of Inventoy.
Bridgekeeper: What... is your quest?
Sir Erco of Inventoy: To seek the Holy Grail, um, I mean, to build a line following robot.
Bridgekeeper: What... is your favourite colour? (English Spelling)
Sir Erco of Inventoy: really dark black, not all black will work all the time, sometimes the black from a printer isn't black enough, and needs to be on another color (plain old U.S. spelling) so there is good contrast...
Whit,
A Monty Python classic!
Good sir Whit knoweth of which he speaks, as he peers into the blackness of my soul.
Black lines matter!
(please don't ban me for life, I couldn't resist)