To measure the camera response to 0.1%, a range of 1000:1 is needed. This requires at least a 10 bit counter (2^10 = 1024). A constant current source is required to drive the LED so that its luminosity is constant. With those specifications in mind, I came up with a design.

Parts:

• ATTiny2313 (slightly overkill)
• LM317 voltage regulator (acting as constant current source)
• LM340 5V regulator
• 2N3904 NPN transistor
• 1KΩ 25 turn trimpot
• 10KΩ resistor
• 0.1μF cap
• 0.22μF cap
• 10 position DIP switch
• DC power jack
• 2 position screw terminal
• 3 x 2 header

Schematic and Board:

This was my first time using Eagle, but thanks to Sparkfun’s tutorials it was easy. Links to Eagle files are at the end of the post. I used DorkbotPDX’s PCB service to order the boards. The price was amazing, only $10.85 (that includes shipping!) for three double-sided 1.6″x1.4″ PCBs.

Code:

/*Attiny2313 LED timer
 *V1.0
 *Jordan Horwich
 */

#include <avr/io.h>
#include <avr/interrupt.h>

uint16_t initialDelay = 0xffff; //initial delay before LED turns on. Used for convenience.
volatile int var = 0;
uint16_t top = 0x0000;

uint8_t input = 0x00;

uint8_t i;
uint16_t k;

int main(void)
{
  cli(); //disable global interrupts
  TCCR1B |= 1<<CS11 | 1<<CS10; //divide clock by 64
  TCCR1B |= 1<<WGM12; //put Timer/Counter1 in CTC mode
  OCR1A = initialDelay; //initial delay, default 2^16 cycles, or ~4 seconds
  TIMSK |= 1<<OCIE1A; //enable timer compare interrupt

  DDRD |= (1<<4); //set PortD Pin4 as an output
  PORTB |= (1<<7)|(1<<6)|(1<<5)|(1<<4)|(1<<3)|(1<<2)|(1<<1)|(1<<0); //enable internal pull-up resistor on DIP switch inputs
  PORTD |= (1<<6)|(1<<5); //enable internal pull-up resistor on DIP switch inputs

  //read and store DIP switch values
  i = ~PINB;
  k = (i<<2);
  if((PIND & _BV(PD6)) == 0) {
    k |= (1<<1);
  }
  if((PIND & _BV(PD5)) == 0) {
    k |= (1<<0);
  }
  top |= (k<<6); //scales 16-bits to 10-bits by shifting k 6 places left

  sei(); //enable global interupts

  while(1) {
  }
}

ISR(TIMER1_COMPA_vect) //Interrupt Service Routine
{
  PORTD ^= (1<<4); //xor toggles LED
  OCR1A = top;
  if(var == 1) {
    TCCR1B &= ~0x07;
  }
  else {
    var++;
  }
}

 
The 10 position DIP switch is used to input the desired LED on-time in binary. Each position represents a bit with bit-0 on the far right closest to the power jack. The current is set from 1mA to 500mA by adjusting the trimpot. As the code only reads the DIP switch once when the ATTiny first powers on, DIP switch is changed and then the reset button is pressed to change the time interval. This becomes very tedious if you need to cycle through all 1024 values. The code could be rewritten so that the time interval value is stored in the EEPROM and is automatically incremented after every reset. The ATTiny’s clocks and counters are set such that the shortest time interval is 0.004096 seconds, and the longest time interval is 4.194304 seconds (0.004096 * 1024).

Results:

The first interval tested in the video is 1111111111, or about 4.19 seconds. The next is 01111111 (~2.10 seconds), then 0011111111 (~1.05 seconds), then 0001111111 (~0.52 seconds), and I think you can figure out the rest.

Files:

• Eagle schematic, Eagle board, main.c and makefile (zip)

See more pictures of this project on flickr.

As I thought about it more, I realized an iPod would make a perfect speaker. The headphone jack could be the audio input, the hold switch could be the power switch, and the scroll wheel is the perfect size and shape for a speaker.

Parts:

• 2.25 inches (diameter) speaker
• switch
• mono audio jack
• 10kΩ potentiometer
• 10μF capacitor
• Altoid Tin Amplifier
• gutted iPod

Construction:

I attached the power switch, audio jack, and potentiometer with epoxy and hot glue, using white credit card plastic to cover any gaps. Following the advice of a comment on the Altoid Tin Amplifier Instructable for gain control, I connected one pin of the potentiometer to pin 8 of the LM386 and the other pin of the potentiometer to the 10μF capacitor, and the other pin of the capacitor to pin 1 of the LM386.
      

Then I glued in battery contact springs and fit the speaker. Luckily the width of the iPod is perfect for a AA battery.
      

Then I assembled the Altoid Tin Amplifier. This was my first time etching a PCB and I was delighted with the result. I used the toner transfer method to print the design on the PCB, and ferric chloride to remove the unwanted copper.
      

After soldering everything I turned it on and… success! It works and is surprisingly loud for such a small speaker.

I liked the final result so much that I built another one! I didn’t have another broken iPod, but I was able to buy the front and back panels from WeLoveMacs.com. When I received the panels I was happy to see a “warranty void if removed” sticker. I couldn’t resist.
      

See more pictures of this project on flickr.

A few years ago I attempted to put a camera in a small rocket. I had always had the idea in the back of my head, but I never knew how to do it. As soon as I saw this Make Magazine article I knew I had to try.

I mostly followed the advice in the Make article. The major problem I encountered was what camera to use. I didn’t want to risk damaging anything expensive, so I opted to use a $30 CVS “one-time-use” video camera. The Camera Hacks forum has useful information to get around CVS’s security and make the camera reusable.

I removed everything but he bare essentials of the camera to reduce its weight. Following the instructions on the Camera Hacks forum, I soldered headers onto the camera for a USB connection.

I attached the camera to the rocket by gluing standoffs into the nose cone, and screwing the camera into the standoffs.

With the help of jbgizmo.com I built a cheap lightweight altimeter that uses a pressure transducer to calculate altitude.

With a hole drilled in the nose cone the final result looks like this.

I launched the rocket once with the camera and once without. Both launches had the altimeter. The launch with the camera used a D12-5 engine and reached an altitude of 420 feet. The launch without the camera used a E9-4 engine and reached an altitude of 835 feet.

See more pictures of this project on flickr.