Difficulty Level = 5 [What’s this?]

Like many parents, we make hand-made costumes for our kids instead of buying cheaply-made (and expensive) costumes based on licensed characters. This year, my youngest son wanted to be a robot. My wife did a great job making the costume, but I just had to add some cool electronics to take it to the next level.

The electronics are nothing fancy — a simple Atmel ATtiny13 microcontroller that interfaces with two 74HC595 shift registers to light up LEDs randomly. The technical details are below, but in the process of building this, I was really struck by how electronics and computing are being embedded into everything. This week I was making a TODO list and one of the items was “finish source code for robot costume”.

OMG, now the costumes we make have source code.

This is a great example of how technology is becoming increasingly ubiquitous. Ten years ago, this would have been far beyond my reach. But in 2010 I can build this easily. The microcontroller cost $1.04, the shift register chips are $0.25 each, the resistors are a penny each, and the LEDs probably average $0.20 each. Definitely less than $5 for everything.

Technical Details

Here’s the schematic:

Schematic for robot costume circuit

Here is a closeup of the circuit board. There is a piece of clear acrylic protecting it. I ran out of 16 pin IC sockets, so the shift registers are in 20 pin sockets.

Circuit on front of costume. ATtiny13 microcontroller and two 74HC595 shift registers.

And finally, the simple source code that runs on the chip. I use CrossPack for AVR development. I use avrdude and a Bus Pirate to upload the code onto the ATtiny13 chip.

/*
 * ATtiny13 driving two 74HC595 shift registers
 *
 * Randomly turns on/off output pins of the shift
 * register ICs.
 * A random number of outputs are set high, then
 * a random time delay occurs.  Then the cycle
 *  repeats.
 *
 */

#include <stdlib.h>
#include <avr/io.h>
#include <util/delay.h>

#define DATA PB0
#define CLOCK PB1
#define LATCH PB2

int main(void) {
  int d;
  char n;
  char i;

  // set DATA, LATCH and CLOCK pins to OUTPUT
  DDRB |= (1 << DATA);
  DDRB |= (1 << LATCH);
  DDRB |= (1 << CLOCK);
  PORTB = 0;

  for(;;) {
    // choose number of LEDs to light up.
    // n will be between 4 and 16
    n = 4 + (random() % 13);

    for(i=0;i<16;i++) {
      // for each LED, probability of it being lit
      // is n/16
      if ((random() % 16) <= n) {
	PORTB |= (1 << DATA);  // set DATA pin high
      } else {
	PORTB &= ~(1 << DATA); // set DATA pin low
      }

      // toggle shift register clock pin
      PORTB |= (1 << CLOCK);
      _delay_ms(2);
      PORTB &= ~(1 << CLOCK);
    }

    // once we've shifted out all 16 values, toggle
    // the latch pin.
    PORTB |= (1 << LATCH);
    _delay_ms(2);
    PORTB &= ~(1 << LATCH);

    // delay random amount of time between
    // 100ms and 500ms
    d = 100 + (random() % 400);
    for(i=0;i<d;i++) {
      // _delay_ms function must be called with a
      // constant value, not a variable!
      _delay_ms(1);
    }

  }
  return 0; // not reached
}