555 Contest Entry - Audible Level

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.

Triode preamp : part 1

My recent purchase of my Harmon Kardon A300 amp has created a serious fascination with vacuum tubes. The sound of this amp is so good and the electronics behind it are so simple. I was able to find the schematic to my amp online, which was very important when I began replacing the caps in it... the labeling on them wasn't the best and it was hard to make out the values on some of them. Looking at the schematic, this amp was so simple! If you took out the electronics necessary for the turntable preamp, and removed unnecessary features such as presence, stereo reverse, and the bass and treble controls (which I always leave flat anyway), there are only a couple dozen components remaining per channel.

I decided I had to start designing my own amp. A few years ago I was fed up with the cheap 1-bit D/A converters found in most CD players and wanted to make my own with nice Burr-Brown 16BIT DACs. I would finish the output stage with a nice tube preamp. Of course I never built it, so I was due. I decided to make a simple preamp with a basic triode.

I first needed some tubes. eBay of course would be my source. The most common triodes such as ones in the 12**7 family are expensive! 12ax7s' go for some serious money, mostly because almost every vintage amp out there uses them and the sources of new old stock 12ax7s' is disappearing. The Chinese are making tubes again, but apparently they sound like ass. My solution is to buy large lots of tubes on eBay for cheap and hope for the best. Besides, there are tons of other triode tubes out there, others will be suited just fine for my purposes.

So several boxes of tubes showed up at my door about a week later showing why drinking a bottle of wine while eBaying always leads to suprises. The good news is everything I bought was only $20 and I had about 150 tubes to work with. In this lot I found a bunch of NOS 5963 triodes. Perfect.

My first design is crude, quickly made, only a single mono channel, but it works!

Yes, it's built on a piece of pine (clear pine that is!). Remember, I already admitted it was crude...

It's totally simple, there are only about 15 passive components. Ill have to upload the schematic once I have a legible version of it that is finalized. I made some component value changes during testing for level adjustments. Here is what the input(top trace) and output(bottom trace) looks like on my scope:


There is some noise in the waveforms, but ignore this as it is from my signal generator which is garbage. The left traces show a 1v peak to peak input and the output of the preamp, which looks very clean... and inverted. The right traces show what happens when the input is increased to 2 volts peak to peak, which drives the tube a little harder. The bottom trace shows the output result which is clipped, or in the case of tubes, more compressed or squashed. This is one of the most interesting properties of tubes. When their headroom is reached, they don't have a hard 'clip' like a transistor, it is a more 'squashed' effect on the waveform, which creates a more pleasing distortion.

Here is another view showing the effect on my scope with an X-Y plot:


I had to turn the scope illumination on to give my camera something to focus on, but you can see the squashed waveform on the bottom right which has the soft curves instead of a hard flat break if this was a transistor clipping.

I still have a lot to play with the circuit, I want to figure out why the bottom of the waveform is squashed a little more than the top. I'm only running the tube plates at about 60V, instead of the usual 120v. (This was a limitation of the transformer I was using for this circuit.) Overall i'm pretty impressed with the sound of it, especially with the complete design and build only taking about 5 hours. Listening to music through it definitely sounded good, and playing my Roland synth through it while overdriving the tube for distortion had some sweet results.