Pin the Heart on the Robot

My son is the middle of transitioning from the firefighter phase to the new robots phase.  Everything up until a few weeks ago was firefighters firefighter firefighters.  Now, it’s all about robots.  So for his upcoming 4th birthday party, we HAD to make it robot themed.
When searching for activities, my wife came up with a brilliant idea of doing “Pin the Heart on the Robot.”  The idea, just like “pin the tail on the donkey,” is to have a big poster of a robot and have the children try to pin a paper heart appropriately on the robot.  But this got me thinking – what if we could make this more, well, robot-like?  What if we could make the heart actually light up when they pin the heart in the correct place?
My requirements were to have a circuit that can be built that is small, light to fit in a 4 year old’s hand and be sturdy enough to survive a drop (I mentioned these were 4 year olds, right?).  The device needs to have at least one LED that will somehow light up when it is successfully ”pinned” within proximity of the robot’s heart.
For this project, I decided on a Hall Effect sensor.  This tiny little package can be switched on an off in response to a magnet field.  This is sort of the same principle that the iPads use to turn the screen on when the cover is removed.  The US1881 that this project uses is a “latched” sensor.  This means that when the circuit is triggered, it stays on until the opposite pole of the magnet is applied.  So by embedding a magnet either on or behind the poster, the LED will turn on when the heart is stuck in the right spot.
Bill of Materials
  • 1 US1881 Hall Effect Sensor (SparkFun part #COM-09312, or Digi-Key part #US1881EUA-ND)
  • 1 big ass magnet (K&J Magnets part #BY0X02)
  • 1 10mm Red LED (SparkFun part #COM-10632)
  • 1 CR2032 coin cell (DigiKey part #P189-ND)
  • 1 20mm coil cell holder (DigiKey part #103K-ND or part #3003K-ND)
  • 1 56ohm resistor
  • 1 10Kohm resistor (optional: the datasheet for the US1881 recommends this, but I didn’t end up using it. Worked fine without it. YMMV)
  • 1 protoboard (about 1″ or 1.5″. E.g. Sparkfun PRT-08814)
  • 1 poster with a robot image


1. Follow this schematic:

It’s pretty simple really, just 4 components.  Note that with this setup, the LED will initially be ON.  You’ll need to “turn it off” by holding the magnet up to it.  The schematic could probably be modified to use a transistor like a 2N3904 to switch the LED the “right” way, but I was too lazy.

2. Print out a big poster with a robot image.   Etsy has some great robot images available for sale.  My wife found this lovely design for Retro Robots from cloudstreetlab.  Then I uploaded the image to and printed out a large 24” x 36” poster. (Disclaimer: I work for  This was the perfect size to provide a nice big target for a 4 year old.


3. Put up the poster and place the Big Ass magnet behind the poster where the heart should be.  Remember, the magnet in the parts list above is pretty damn strong (hence the “Big Ass” moniker).  Do not place this near a CRT TV or hard drive.  It will cause issues.

4. Put some double stick tape or a roll of masking tape on the back of the Robot Heart so that it will stick to the poster.

5. Blind-fold a kid, give them a Robot Heart, and see if they can pin it on the heart.  The kid who is able to pin the heart and have the LED light up wins!

Here are pictures of the finished board, complete with crappy soldering.


You could get crafty can actually cut out the protoboards into heart shapes.  Also, adding a few dummy capacitors, resistors, or extra wires might dress this up a bit, too. It’s a good chance to get creative.

Unfortunately, due to an ordering snafu, I didn’t end up getting enough parts for all the kids coming to the party.  Therefore, we’ll probably have to save this for a future birthday.  If anyone does use this, let me know how it went.

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