The deadline for this 555 contest came up very fast, so here I am wiring my blog post for it's entry right around 2am. Over the past few weeks I have been thinking and thinking, trying to come up with an awesome design that used just tons of 555 timers, but with lack of time this month and after seeing some of the other designs posted around I decided to go for something simple.
The design I am submitting is a level that can be used for any purpose where you need to find a level surface in respect to gravity. Typical levels have a clear tube full of a liquid and an air bubble that you have to visually watch. This design uses two separate audio tones at different frequencies that when a level surface is found, the two tones exactly match producing a single uniform tone. With this design, you can listen for a level surface instead of watching. The video has a brief outline of it's function.
When designing it my first idea was to generate two individual frequencies and mix them together, the results being a sounds similar to DTMF tones you hear on a phone. After testing this I realized that it didn't work as well as I would have liked, it was very difficult to accurately detect the 'true' portion of the level. The resulting design ended up working quite well using two tones that are switched back and forth by a 555 as an oscillator controlling an analog multiplexer to switch the voltages. The second 555 is the actual VCO that generates the tones. When an axis on the accelerometer outputs a voltage that is equal to a reference voltage set, the two tones being generated will exactly match and no difference in frequency is heard. To detect a 45 degree angle, you can take the z axis and either the x or y axis outputs and put them both into the multiplexer instead of the reference voltage.
I left the circuit on a breadboard as I didn't see any rule stating that the design must be a finished product, hopefully that holds true. The 555s are basic NE555 timers that I have had forever. The analog multiplexer is a 508A that came off of some old wire wrapped boards that I believe to be used in old radar hardware. The 508A is a very cool chip, it adds to the vintage of the 555s. The accelerometer is a ADXL-330, 3 axis plus or minus 3G which I happened to have on hand. Another idea originally was to cycle between both x and y axis of it along with the reference voltage in order allowing you to be able to level something in two dimensions. This proved to be more difficult to listen to so I stuck with the single axis idea to measure.
I tested the accuracy of this level to see if it was actually practical to use. I mounted this circuit to a small board and lined it up on a piece of wood. I then 'listened' for 45 degrees and drew a line on the board. Using a regular level and protractor I found that I was at 46 degrees which was pretty close. My accuracy became better in subsequent tests resulting in most of my lines being right at 45 degrees. It is cool to see that this circuit has an actual practical application.