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Monday, June 29, 2015

Audio filtering, experiment 29 of Make: Electronics part 2

Finally, after earlier setback, I've got the amplifier and 4-inch speaker working properly. I tried multiple setups today and managed to get rid of the distortion. The book dictates 9V, 33K resistance on audio input. This results in my case in a temperature of 90 degrees (194 fahrenheit). Which is uncomfortably high. I therefore decreased the voltage to 6V DC, increased the audio input resistance to 100K and added 10 ohm resistance to audio output. This gave a workable temperature of 60 degrees (140 fahrenheit). Because of these high temperature it is likely that the TEA2025B amplifiers that I bought for this experiment are part of a rejected batch.

After having solved this problem I added the coil, as a low pass filter, and the 11 uF bipolar capacitor as a high pass filter. Both filters work great with low frequencies on the low pass filter and high frequencies on the high pass filter. The coil is the same that I used in experiment 28 and the bipolar capacitor was made of two 22uF electrolytic capacitors.

Next I'll fit the speaker in a shoebox and create a waveform with a 555 chip as an input for the audio amplifier (instead of the ipod).

Overview over the experiment with on the left the power supply, in the middle the ipod and on the right the breadboard.

A top-down view of the 4-inch speaker (top), breadboard and coil (bottom)

Close-up of the breadboard with on the left the TEA2025B amplifier. 

Friday, June 26, 2015

Audio filtering, experiment 29 of Make: Electronics part 1

This experiment of Make: Electronics demonstrates the use of self-inductance and capacitance in audio filtering. The low and high frequencies of the audio signal can be separated and send to different speakers (woofer and tweeter). The audio chip used for this experiment is the TEA2025B, a somewhat older chip that was used in portable radio cassette players (remember the Sony Walkman). For this experiment I needed a 5-inch speaker. I was able to find an old woofer that came from an sound system that's no longer in use. This particular woofer is 4 inch , 6 ohm (according to the book a minimum 5-inch and 8 ohm speaker is required) and maximum of 40W. I made the two nonpolarized capacitors out of two 220uF capacitors as indicated in figure 5-38 of the book (pg. 249). The experiment requires two .15uF electrolytic capacitors which I do not have. I use two 100nF ceramic capacitors instead.

I have build the circuit temporarily without filtering for two reasons. The first reason is that I want to use this circuit to test the TEA2025B's that I bought from Aliexpress which were in a very bad shape when they arrived. Secondly I want to rule out problems in the non-filtering part of the circuit.

Overview of the experiment. With on the left side the 4-inch speaker. The audio is supplied by a Nexus 7 inch tablet.

My first try didn't go so well. After powering up the circuit the first half minute or so the sound is sort of ok but than a terrible distortion kicks in and makes listening unbearable. Furthermore the TEA2025B becomes extremely hot. I unplugged and checked for errors but couldn't find none. I figured that I'm overdriving the amplifier so I increased the 33K and 10K resistor to 100K and 33K respectively without result. I then changed the TEA2025B for another one, again no result. Finally I decreased the voltage to 4,5V (instead of 9V). Now the distortion is gone but I'm still not happy with the sound quality. The sound is very muffled and undefined (sorry I don't have any words to describe it). Not sure how to go forward from here.

Close-up of the breadboard. The oddly shaped capacitors are the non-polarized capacitors that I made from 220uF capacitors.

Monday, June 22, 2015

Making a Darth Vader voice changer, part 3

I finally finished the Darth Vader voice changer this weekend. I took a belt with two snap hooks from an old bag. Next I fitted the box with the voice changer with two black screw eyes that were large enough for the snap hooks. The enclosure is now comfortable around the neck of a child. In a local electronics shop I bought a case for a 9V battery that fitted nicely in the box. Finally I taped the mic into the Darth Vader mask and the fun could begin.

My two boys both volunteered and got dressed as Darth Vader.  We still have a dark cloak that goes well with the mask. Unfortunately we sold the lightsabers a couple of years ago. The Darth Vader voice is surprisingly convincing but only if you play around with the settings. Even funnier, you can make Darth Vader sound like a robot or as someone that has just inhaled helium (a very high pitched voice). I can imagine children having a lot of fun with the voice changer at a party or at Halloween.

Is there something left to be desired? Yes, the voice changer has four buttons for robot voice, vibrato, higher or lower pitch. As mentioned before I somehow couldn't get the external red push buttons functioning that were placed on the enclosure. So I used the pushbuttons on the MK171 board instead. To access the four pushbuttons on the MK171 board I need to open the enclosure. This is far from ideal and something that needs to be fixed in my next version of the voice changer.
Darth Vader complete with mask and cloak. Unfortunately the lightsaber is missing.

A Belt with two snap hooks, screw eyes and battery holder for a 9V battery complete the Darth Vader voice changer.

Enclosure, now with screw eyes.

Inside of the enclosure with battery holder.

Wednesday, June 17, 2015

Components from Aliexpress

I received 50 TEA2025B amplifiers over the mail today. I ordered these IC's about two weeks ago at Aliexpress.com. Aliexpress is an affiliate of Alibaba. I have never ordered components from Aliexpress before and I was curious to see how they would perform. Eager to get my hands on these OpAmps I quickly unpacked but was horrified with what I saw. All 50 IC's were just thrown in a plastic bag and put in an envelop. The only thing protecting the IC was a small piece of bubble plastic wrapped around the plastic bag. No wonder that half of the IC damaged. Multiple pins on an IC were bended severely. I just wonder why the seller didn't take the trouble to put the IC in a conductive foam which is common practice. I'm a very disappointed customer.

50 TEA2025B's with plastic bag envelop. Why send it like this?

Close-up of the IC's with multiple bended pins. No wonder if you send IC's like this

Straighten the pins and put the IC's in conductive foam. I'll will test the IC's later to asses the damage.

Tuesday, June 16, 2015

Self-inductance, Experiment 28 of Make: Electronics

After making an improvised coil (experiment 25) and cutting a coil out of a speaker (experiment 27) in this experiment the self-inductance of a coil is demonstrated. As a coil in this experiment I used 44 meters of coated copper wire on a spool.  In parallel with the coil two low current LED's are connected. The polarity of one LED is opposite to the other. In the Make: Electronics book a 220 ohm (rated 0.25 watt or higher) resistor is used. Instead I used to two 470 ohm resistors in parallel (this totals to 235 ohm resistance) to protect to resistors and more important my breadboard.

After pressing the button (without releasing it) one LED lights up briefly. Releasing the button prompts the other LED to lights up briefly. This experiment demonstrates that the coil opposes any change in current first by blocking the current and forcing the current to pass through the first LED and then, when the button is released, by releasing it's stored energy over the second LED.

All components on the breadboard. Two 470 ohm resistors in parallel were used to counter for the 1/8 watt resistors that I use.

Experiment in progress. On the right the 44mm copper coated coil.

Saturday, June 13, 2015

How a speaker works, Experiment 27 of Make: Electronics

I couldn't get the cylindrical neodymium magnet of the required size so I skipped experiment 26. Next experiment 27: Loudspeaker Destruction of Make: Electronics is a very simple and very short experiment but worthwhile nevertheless. It demonstrates how a loudspeaker works by literally cut it to pieces. Luckily I had an old 2 inch speaker lying around from a broken alarm clock. Something keeps me back in destructing brand new components.

First the black paper (the diaphragm) at the front facing side of the speaker is removed by cutting a wide circular piece out of it. A flexible yellow fabric (the spider) becomes visible. Now this yellow fabric is cut out in a wide cirkel. When the yellow fabric is pulled out of the speaker it contains a coil on a plastic cilinder (the coil former). The coil on the plastic cilinder is placed between a static inner and -outer magnet. When an (varying) electrical current is going through the coil a (varying) magnetic field is created. This varying magnetic field interacts with the magnetic field of the static magnets forcing the coil and the fabric and the black paper to move up and down creating sound waves.

There are many images and videos on the internet to illustrate how a speaker works but I discovered that none of them is as instructive as taking one apart yourself.

The black paper (diaphragm) of a 2 inch speaker cut out with an X-Acto blade. 
The flexible yellow fabric (spider) cut out of the speaker. A left over of the diaphragm is visible on the spider.
The spider pulled out of the speaker. On the inside of the spider a coil becomes visible. The coil fits right between the two magnets of the speaker.
In the centre of this image the yellow fabric with the coil, a very fine copper wire wound around a plastic cilinder.

Wednesday, June 10, 2015

Making a Darth Vader voice changer, part 2

About a week ago I wrote about a Darth Vader voice changer that I am making. I finally have finished a prototype but not without problem. I soldered all the external components to the Velleman MK171 kit. When I tested the circuit I discovered that three red pushbuttons (see images below) weren't functioning. I disconnected all four red pushbuttons and tested them on a breadboard with an LED. They all worked fine. I reconnected the buttons to the kit and again three of them didn't work. Running out of ideas and time I decided to use the regular mini pushbuttons instead that came with the kit. The voice changer works excellent and the sound is loud (due to the 2 inch speaker and the box) and resembles Darth Vaders however changing the voice requires opening the enclosure which is far from ideal.

There is still a little work left. I have to attach the mic to the inside of the Darth Vader mask and I need something to easily lock and unlock the back of the enclosure.

Image of the Darth Vader voice changer with the mic lying on top. The red pushbuttons are not attached to the pcb.
Back of the voice changer with the microphone wire.

Inside the enclosure of the voice changer. The pcb is mounted on the back of the enclosure.


Sunday, June 7, 2015

Make: Electronics Experiment 25

I skipped experiment 24 of Make: Electronics. The reason is that I'm not really interested (at least not right now) in completing the Intrusion Alarm. I do appreciate what the author of the book, Charles Platt, is trying to achieve. He gives a couple of good hints for upgrades of the Alarm without presenting the complete schematics for the upgrades and leaves it to the reader to finish the job. However, I have yet got to find someone who did finish experiment 24. Anyway, I'll move to the last chapter of the book. In chapter 5 the topics branch out. Since I'm mostly interested in audio electronics I'll continue with experiment 25 to 30. These experiments cover magnetism, speakers and audio filtering.

Experiment 29 and 30 require the TEA2025B audio amplifier from STMicroelectronics. This IC is very hard to acquire locally. I finally ordered 50! of them in China (Aliexpress.com) for a very low price (14 cents a piece). Until I receive them I continue with the other experiments.

Experiment 25 demonstrates magnetism as an induced effect of electrical current through a wire. A long piece of wire is winded around the shaft of a screwdriver. The ends of the wire are connected to an AA battery (see image below). Due to the induced magnetism a paperclip or some other lightweight iron is attracted to the screwdriver. Very neat. This experiment in one form or the other can be found numerous times on the web. A nice example is Colin Cunninghams YouTube movie on the subject at https://www.youtube.com/watch?v=STDlCdZnIsw.

Experiment 25: Wire winded around the shaft of a screwdriver.

Tada, magnetism induced. I used eight feet of wire and a 9V block battery.

Thursday, June 4, 2015

Making a Darth Vader voice changer, part 1

To be able to mimic Darth Vader during a party or any other occasion is a lot of fun. Unfortunately commercial solutions are costly. The Hasbro Star Wars Darth Vader Voice Changer costs $145 on Amazon. So why not make one myself. My voice changer will consist of the following parts: a cheap Velleman kit (MK171), a wooden enclosure for this kit and a simple plastic Darth Vader mask that we already have.

The MK171 kit consist of a PCB and all necessary electronic components. The most important component in the kit is the HT8950 chip. This 16 pin chip (from Holtek) is designed for voice modulation. The frequency of the input audio signal can be shifted up or down by the chip. Changing the output can be accomplished by four push buttons for: shifting frequency up, shifting frequency down, vibrato effect and robotic effect. For my purpose I'm interested in the frequency down function since this invokes a lower pitch to the listener, exactly what is needed for this Darth Vader type of voice. This way even a kids voice can be changed to Darth Vader's.

A small microphone is included in the kit to supply the input signal. I intend to integrate the microphone into the simple Darth Vader mask (possibly with velcro), close to the mouth. The sensitivity of the microphone is controlled by a trimmer. The output of the HT8950 is amplified by a LM386 OpAmp chip. With enough amplification and a proper speaker (not included in the kit) the sound effect can be overwhelming even when one speaks softly into the mic. Amplification is controlled by a yet another trimmer that is included in the kit.

The wooden enclosure is made of plywood. Four equal pieces of 15 x 7 x 1.2cm. The front and the back are made of 6mm thick plywood. Besides MK171 kit the enclosure needs to accommodate: control buttons for the  HT8950 chip, speaker, potentiometer to control the volume and an on/off switch.

Next step is to finish the enclosure and fit all external components. Then I need to test the MK171 kit with the external components before fitting it into the enclosure. Lastly complete the set with the mic integrated in the Darth Vader mask.

MK171 kit, wooden enclosure and Darth Vader mask. The three parts needed for the cheap Darth Vader voice changer. 
Work in progress. The enclosure of the voice changer with the external components. The four push buttons at the bottom left to control the HT8950 chip.
Spray painting the wooden enclosure in an improvised card board paint booth.