The circuit above above makes use of the CMOS 4017 decade counter IC. Each depression of a switch steps the output through 0 - 9. By coupling the output via an AND gate to the next IC, a predefined code has to be input to create the output.
http://www.zen22142.zen.co.uk/Circuits/Switching/diglock.htm
Digitally programmable current sources that feature automatic trimming and retain the setting despite power-down cycles are useful in applications such as RF- and laser-communications drivers. This circuit is particularly suited for setting the drive curr
http://www.edn.com/article/CA193194.html
http://www.web-ee.com/Electronic-Projects/projects/digital_echo/index.shtml
This is a hardware and software solution to monitor the status of an engine in real time. In addition, information relating to the overall health and systems of the vehicle can be displayed.
http://www.nyx.net/~smanley/tach/tach.html
Designs a Digital Filter and draws the frequency response!!!
http://dolphin.wmin.ac.uk/filter_design.html
pseudo-random signal generating circuit ideas
http://www.geofex.com/Article_Folders/LFOs/psuedorandom.htm
Tutorials Fundamentals of Digital Logic
http://www.williamson-labs.com/480_logic.htm
Explores the logical path from the elementary BPSK modulation scheme, to QPSK, to OQPSK, to MSK, and finally to GMSK modulation.
http://www.emc.york.ac.uk/reports/linkpcp/appD.pdf
If you have a digital SLR camera, it is easy to construct a pinhole lens that will allow you to try and experiment with pinhole photography.
http://www.dennisonbertram.com/hackmaster/2005/02/digital-pinhole-lens.htm
converts the change in resistance of a potentiometer into a digital value without using an expensive A/D converter
http://www.edn.com/archives/1997/050897/10di_04.htm