AHAB - Trigger device

For AHAB, I built a reusable trigger device using an Atmel ATtiny85 microcontroller. The circuit provides an input button, an output LED, and a switchable current shunt. This circuit was used in two roles on Ahab:

Camera trigger: This simple program briefly triggers the output once every 7 seconds, using the LED to indicate run state. This was used to trigger the digital still cameras.
Secondary cutdown device: This program allowed the user to specify a timeout in 10 minute intervals. After the specified time had elapsed, the output will be active for 60 seconds. This was used to sever the payload after a set time period.

The circuit is straightforward, and provides in-system programming using Atmel's standard 6-pin programming header:

Circuit schematic

For the camera, EXT was wired to the active pin on the camera shutter button, and the camera and timer circuit's grounds were tied together. When the trigger was activated, the camera button line was pulled low through the transistor.

For the cutdown device, EXT was wired to one end of a coil of nichrome wire, and the other end was tied to Vcc. When the trigger was activated, current flowed from Vcc, through the nichrome, and then through the transistor to ground. This caused the wire to glow red-hot, severing the line.

The software was developed on an Intel Mac using AVR GCC. I used these scripts to build a modern version for my computer. AVRDUDE was used to download the software to the device.

Browse the source or download a tarball (5kB).