My sister in law does epic children’s parties (a couple of years back I had a lot of fun playing Snape and being mean to all the kids who weren’t in Slytherin) and this year I was given a request to build a game for an upcoming Indiana-Jones themed party.

Remember that bit at the very beginning of Raiders of the Lost Ark where Indy has to balance the idol with a bag of sand to avoid the traps going off?

“Can you build a game that does that?”

So I started thinking along the lines of using an Arduino coupled with a weight sensor which triggers an attached Raspberry Pi to play a tune when the idol is balanced, all packaged up inside a suitably rock looking biscuit tin:

The idea would be the idol would be swapped, the Arduino would detect the swap, determine if the weight was different and then set off an alarm and rumble motor if it was. If it was the same weight it would play the Indiana Jones music.

The Arduino has lots of analogue pins that can read changes in voltage, and the Pi can take care of the music and sound effects – it would also be possible to  hook it up to a TV to play the appropriate bit of the film, as chasing down seven year olds with a giant boulder / dart traps / snake pits might not go down well with their parents.

Initially I looked at using a load cell sensor similar to what’s found in electronic scales, however I discovered that the resistance generated by these sensors to be really tiny, and it’s not just a simple matter of hooking them up to an Arduino. Note – since I built my initial version Sparkfun have actually brought out a handy load-cell amplifier kit that should make this possible for version 2!

So instead I opted for a much simpler force sensor – these are widely available and their resistance can be easily measured, but they’re also a bit inaccurate. For the purposes of the game and to add a bit of random variability I decided to go with this simpler route.

My arduino circuit is as follows – it includes an LED to indicate if the pressure sensor is detecting the idol properly. The Arduino communicates with the Pi over serial to then trigger the appropriate sound file. I ran out of time to include the rumble motor but it would be quite easy to add one.

Adafruit have a tutorial along with some suitable code which can be used to send the force sensor output to the Raspberry Pi  via serial:

/* FSR testing sketch. 
 
Connect one end of FSR to 5V, the other end to Analog 0.
Then connect one end of a 10K resistor from Analog 0 to ground
Connect LED from pin 11 through a resistor to ground 
 
For more information see www.ladyada.net/learn/sensors/fsr.html */
 
int fsrAnalogPin = 0; // FSR is connected to analog 0
int LEDpin = 11;      // connect Red LED to pin 11 (PWM pin)
int fsrReading;      // the analog reading from the FSR resistor divider
int LEDbrightness;
 
void setup(void) {
  Serial.begin(9600);   // We'll send debugging information via the Serial monitor
  pinMode(LEDpin, OUTPUT);
}
 
void loop(void) {
  fsrReading = analogRead(fsrAnalogPin);
  Serial.print("Analog reading = ");
  Serial.println(fsrReading);
 
  // we'll need to change the range from the analog reading (0-1023) down to the range
  // used by analogWrite (0-255) with map!
  LEDbrightness = map(fsrReading, 0, 1023, 0, 255);
  // LED gets brighter the harder you press
  analogWrite(LEDpin, LEDbrightness);
 
  delay(100);
}

This also lights up an LED to give useful visual feedback – the Pi then just listens on the serial input and triggers the according sound effect.

Assembled the sensor looks like this (the force sensor is under the lid of the coffee tin):

Inside my ‘Idol’ (which was a papercraft model printed on shiny paper), was a counterweight made from a stack of pennies – this is reasonably heavy and I was able to adjust the sensitivity of the circuit to detect the presence (and absence) of the Idol.

To build the pedestal I used an octagonal quality street plastic box, covered in paper mache and spray-painted with stone effect spray paint. The arduino and pressure sensor were contained inside a metal coffee tin.

On detecting the Idol being removed, the Pi plays the sound sample “It belongs in a museum” from Raiders of the Lost Ark, if the sensor is successfully fooled it plays the Indiana Jones theme!

After a bit of play testing we found that balancing the idol was very hit and miss – although a handy rock nearby seemed to do the job most of the time. Ideally this would use the much more accurate load cell to measure the weight of the idol and compare it to the bag of sand after a button press. I still have all the parts, so I’m just waiting for the next time someone in my family holds an Indiana Jones themed birthday party.