“Never pass up a teachable moment.” As a Boy Scout leader of many years, this has become one of my many mottos. Any time you have the opportunity to grab a child’s attention and focus that attention on something more educational than the latest video game then grab it and have fun. My son’s 6th birthday party recently became one of those moments.
My wife (aka social planner) told me we were having a circus themed party with lots of different game stations (ring toss, face painting, a balloon artist, clown, etc.) and asked what I wanted to do. First thing that came to mind was a soldering session. Sadly, I don’t recall ever going to a kids birthday party that included something educational. So, I was breaking new ground here among our friends.
To make things a little more interesting, I decided to not just have them solder a few things and move on but instead I wanted them to build something fun they could take home with them. The idea for a “birthday badge” was born.
The idea was to create a badge that would encourage interactions among the kids at the party but would also be fun once they got home. Because I expected 20+ kids at the party, it also needed to be fairly cheap to build.
After stirring around a few ideas, I finally decided on a design for a badge that would create interactions by having the badges detect other badges within close proximity. This would give the kids the opportunity to get up close and force some conversation.
Each badge has both an infrared (IR) transmitter and an IR receiver. The IR transmitter uses an IR LED that operates as half of an astable multivibrator circuit. The other half is a red LED. So, it appears that only the red LED is blinking. The IR & red LEDs alternate at about a 2Hz blink rate.
The Eagle CAD files can be downloaded here.
The receiver uses a phototransistor in a Darlington pair configuration with a transistor to detect and illuminated another red LED when IR light is detected. The phototransistor will also detect some ambient light which is why you see the LED dimly illuminated when in a lit room or outside.
After finalizing the design, it was time to go to the breadboard.
During breadboarding, I found that my calculated blink rate wasn’t quite what I was getting in reality. I played with different values for R2 & R3 until I finally got the blink rate I wanted (about 4Hz). I then built a second transmitter on a different breadboard just so I could test the power output of the transmitter and the sensitivity of the receiver.
I didn’t quite get the range I was hoping for but since I was limited to a single 3V coin cell battery, I decided a range of 6-8” would be sufficient.
Now, it was time to design the actual PCB board. I decided that I was going to model the PCB after a clown’s mouth to stick with the circus theme. After several nights of routing & tweaking the design, I finally ended with this…
This was my first PCB to have manufactured. So, I called on some friends for advice on where to have it done. Pete Brown recommended DFRobot.com and I went with them. DFRobot has a really good step-by-step guide for producing the files needed from Eagle to provide them with. A couple hours later and the files were uploaded. Time to wait…
A few days after I submitted the files, I got an email back with a picture from DFRobot warning me that the silk screen on my “open hardware” logo was striped on some of the boards but they were otherwise perfectly functional. They went ahead and shipped them but told me to let them know if they needed to do anything to make it right.
I expected about 20 kids at the party. Unfortunately, you can only order PCBs from DFRobot in quantities of 10, 50, 100, or 200. It really wasn’t much more expensive to order 50 than 20 so I went with that option and decided I would use the extras to teach a soldering class at NashMicro. Because of this buffer, I wasn’t at all concerned about the bad silkscreened logos on some.
Eleven days later, I had the PCBs in hand! If you order from DFRobot, be sure and choose the DHL option. It’s only a few dollars more than parcel post is much faster than the usual three weeks delivery from China via parcel post.
The boards were in perfect condition minus the screen print problem I had already been warned about by DFRobot. Overall, I was very pleased with my experience with DFRobot and I will definitely use them again in the future and highly recommend them to others.
Oh, the best part…the PCBs ended up costing me $1.80 each. Unfortunately, I had to pay for the 10cm x 10cm size although they’re actually only 5cm x 10cm because they don’t offer 5cm x 10cm as an option. I also paid a little more for red PCBs and the DHL shipping. If you had more time then I did and didn’t care about the color, you could definitely get the boards made cheaper.
While I was waiting on the boards to arrive, I went shopping for the parts. Since these were basically disposable projects, I didn’t really have to worry about quality. As long as they blinked for a few hours during the party, I would be satisfied. I found as many of the parts in bulk on eBay as possible. Here’s the parts list and sources.
| Part | Qty | Unit Price* | Cost |
| 1 | $0.38 | $0.38 | |
| 1 | $1.80 | $1.80 | |
| 2 | $0.04 | $0.08 | |
| 1 | $0.12 | $0.12 | |
| 1 | $0.08 | $0.08 | |
| 3 | $0.28 | $0.84 | |
| 2 | $0.02 | $0.04 | |
| 2 | $0.02 | $0.02 | |
| 1 | $0.02 | $0.02 | |
| 2 | $0.13 | $0.26 | |
| 1 | $0.14 | $0.14 | |
| 1 | $0.28 | $0.28 | |
| TOTAL | $4.08 |
Now with all the parts in hand, we were ready to party! Because I knew it would take too long for the kids to assemble all the parts and I didn’t have the patience to teach SMT soldering to 5 year olds, I did all the assembly except the resistors before the party.
Me & my oldest son, Ian Jr., had a little soldering party of our own the night before the kiddos arrived. In retrospective, if I were doing this again I probably would make all the parts to be assembled beforehand as SMT and used an oven to reflow rather than hand soldering all those thru-hole parts.
(yep, in our pajamas…)
Here’s what’s involved in constructing a Birthday Badge. A demonstration of it’s operation is at the end.
The day of the party was a huge success. We had 20 or so badges floating around on both kids & a few adults before the day was up. I heard from one of the kids today (a week later) that her badge is still blinking! I haven’t discovered yet how long one will blink but I’m going to start that test. I suspect that it will go for a month or better. I’ll update when I have scientific evidence.
We only had one badge that didn’t work properly the first time after assembly. I believe it had a bad red LED but the kid didn’t want to stick around and fix it since the blinky (transmitter) side of the circuit worked fine and mom was ready to leave.
It was a great day and I’ve heard from more than one parent that their kid’s favorite activity was building the badge. Mission accomplished. Maybe a few new engineering careers were inspired.
