A few months ago I was feeling inspired to create a nice countdown timer.  With the next Dorkbot open mic only days away, I finally had the motivation to actually put it together.

The main challenge was driving the LEDs with a constant current, because they need about 10.5 volts across the several series LEDs.  I wanted to run from 12 volts, so there wasn’t much voltage left over for the normal current limiting resistors.  Instead, I used this opamp circuit.

The project runs from a Teensy 2.0.  The code is very simple, using the SPI and Bounce libraries for the hardware interfacing.

Source: http://www.dorkbotpdx.org/blog/paul/big_7segment_countdown_timer

Source: http://www.dorkbotpdx.org/blog/paul/big_7segment_countdown_timer

Source: http://www.dorkbotpdx.org/blog/paul/big_7segment_countdown_timer

Source: http://www.dorkbotpdx.org/blog/paul/big_7segment_countdown_timer