Nuts and Volts - January 2008

■ FIGURE 2. The schematic of the single channel camera timer. The two channel version has a second relay and diode connected to PICAXE-08 pin 4, or pin 3 on the IC.

camera. The two channel version has full control over a camera and is best for cameras with power-save features that may shut it down during a mission. However, the two channel camera timer is not limited to operating a single camera. It could operate two separate cameras inside a BalloonSat or a single camera along with a second device that a relay can operate.

THEORY OF OPERATION

Upon powering up, the PICAXE blinks its indicator LED (connected to pin 1) five times showing its readiness, then goes into a wait loop until the shorting block is pulled off the commit pin. The commit pin pulls pin 3 to +5V and the shorting block shorts it to ground. After detecting the removal of the shorting block (that is, when pin 3 goes high), the PICAXE signals its detection of the removal by blinking the indicator LED five more times. Then, the PICAXE enters an infinite loop where it energizes the relay by setting pin 2 high and turning on the indicator LED by setting pin 1 high. The LED is a troubleshooting tool that indicates when the PICAXE is triggering the relay to take a picture.

After a one second pause, the PICAXE sets pin 1 and pin 2 low to open the relay and shut off the indicator LED. When the PICAXE turns off the relay, the magnetic field contained within the relay’s coil collapses and creates a potentially damaging reverse current through pin 2. To protect the PICAXE from damage, the 1N4001 diode shorts the relay’s coil to reverse current. After a pause, the PICAXE again sets pin 1 and pin 2 high to then take the next photograph.

The two channel camera timer

operates the same way, except that its two relays are fired in sequence. The first relay attaches to the camera’s power switch, allowing the PICAXE to power up the camera. After a short pause, the second relay (which is connected to the camera’s shutter switch) fires to take a picture. After the camera has time to save the image to memory, the first relay is fired once more to shut down the camera.

BUILDING THE ONE AND TWO CHANNEL CAMERA TIMER

version) are strain relief holes for the timer. Wires connecting the timer to the camera and battery pack pass through the strain relief holes to prevent normal use breaking the soldered wires off of the PCB. The battery holder for the timer can either be a nine volt battery snap or a four cell AAA holder.

The commit shorting block is a short loop of wire terminated with small crimps and mounted inside a plastic housing. A cord tied to the wire loop of the shorting block exits the BalloonSat airframe and is attached to a brightly colored tag.

Begin assembling the timer by soldering the lowest lying components (the resistors and diodes) first. Solder an eight pin IC socket to the PCB rather than the PICAXE to protect it from the heat of soldering. I recommend using snappable headers for the programming header and commit pin (it’s cheaper to purchase a long row of headers and snap them off as needed). Watch the orientation of the LM2940 voltage regulator and the diode(s). If the diode is soldered backwards, it will short out every time the PICAXE fires the relay.