Audio filtering, experiment 29 of Make: Electronics part 4 (finished)

I finally reached the second stage of Experiment 29 of Make: Electronics. In the first stage I used an audio amplifier (TEA2025B) and a coil to create a crossover network. During this first stage the source of the audio was an iPod (read here). In the second stage the iPod was replaced by a 555 timer as described by figure 5-51 of the book. In the astable mode the 555 timer generates a square wave. This square wave is led to the audio amplifier. A 100K trimmer between pin 6 and 7 is used to change the frequency of the square wave. The output of the 555 chip (pin 3) has, besides a 680K resistor, a trimmer connected to it. This trimmer is to be used to control the volume.

Circuit used for this experiment. The cardboard box proved to be a good box for this experiment. It has a 4-inch speaker built-in. Just in front of the soldering iron is the coil creating a low pass filter.

Contrary to the book I used a 220 ohms resistor on pin 3 instead of the 500 ohms (which I didn’t have). Because of the problems with the high temperature of the TEA2025B (read here) I used a voltage of 7V instead of 9V. The circuit is functioning great and with the 100K trimmer I was able to change the pitch. The square wave that is generated makes a harsh somewhat unpleasant sound.

The final part of this experiment the low pass filter and the high pass filter will be used (again). I will describe that in my next post but that won’t be until the end of July. Today my summer holiday has started and I will be gone for a couple of weeks.

UPDATE: I used the low and high pass filter. Not surprisingly the low pitched sound generated by the 555 chip is passed on much better by the the low pass filter while the high pass filter generates hardly any sound at all. Reversely the high pitched sound is passed on by the high pass filter while it is hardly audible through the low pass filter.

This concludes experiment 29. In experiment 30 this circuit is used once more to demonstrate and audio distortion.

A closer look at the breadboard with on the far left the 100K trimmer. Next to it the 555 timer in astable mode. In the middle the TEA2025B amplifier and on the right the blue 220 ohm trimmer.

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