This piece of test equipment could be very useful for designing and debugging digital electronics. The design is flexible enough that it can be modified to suit your needs.
Click the following link to go to his page describing the build process : http://www.arthropodsystems.com/SquareWaveGenerator/SquareWaveGenerator.html
|Front view showing controls|
|Circuit diagram, just four transistors.|
|Output at 1Mhz on scope|