amplifier electronics woodworking

Darth Vader voice changer 2.0 part 5: finished

This is the last of a series of five where I design and build a Darth Vader chest box. See the bottom of the this to links to the previous parts. The main features of the Darth Vader chest box are:

  • enclosure laser cut plywood (6mm)
  • easy control with four push buttons
  • DIY voice changer circuit with Holtek HT8950A
  • LM386 amplifier
  • build-in speaker
  • audio-in (3.5mm)

When I started the Darth Vader chest box early December 2015 I didn’t expect it would take me almost two months. Finally this week I finished it and I’m happy to say that it works great. For a couple of weeks it was almost finished but there always seemed to be some work to be done. A major problem was that I couldn’t get the 3D printed container for the audio-jack right. I tried it several times at my local fablab but it just didn’t fit. Finally I ordered the container from 3D Hubs and it had a perfect fit.

Audio-jack container (right) and volume knob (left). The blue parts were printed by me while the black parts were ordered through 3D Hubs. the X,Y-dimensions of the blue parts were off probably due to over extruding. The black parts were fine.
3D modeling amplifier electronics laser cutting

Darth Vader voice changer 2.0 part 4: almost finished


The last month I’ve been working on my Darth Vader chest box. I’ve designed and built my own circuit with the Holtek HT8950A voice modulator. I create a laser cut case and designed 3D printed parts for the chest box. Since I had to learn a lot of new techniques, e.g laser cutting and 3D printing, this is by no means an easy project for me. With the project in its final stages now some design problems turn up that need fixing.

Soldering the board.

This week I soldered the components to the board. First I soldered the voice changer components and made sure this part of the circuit is working. Next I soldered the amplifier. I tested the total circuit and it worked the first time which is always a joyful moment. I find the Adafruit perma-protoboard very easy to work with since I’m able to copy the layout from the breadboard. Next I grouped all the buttons that operate the HT8950A on a board and soldered them to a piece perfboard. The buttons on the perfboard fit nicely into the laser cut side panel that I already made. Operating the chest box is easy with this (a major issue with my previous chest box).

Soldered perma-protoboard and breadboard side-by-side.
Close-up of the soldered board with the HT8950A voice changer chip on the right and the LM386 amplifier on the left.
Operating the voice changer with this panel should be easy.

Design issues

Last week I made 3D printed parts for the front of the chest box. Unfortunately I found that these parts didn’t look good with the laser cut box. The plastic parts just didn’t do justice to the laser cut plywood. I therefore decided to laser cut all the parts that sit on the from of the box with I think is aesthetically more pleasing.

Another problem arose with the female audio jack connector that I need to plug in the microphone. The thread of this 3.5mm connector just isn’t long enough to be fitted onto the 6mm thick plywood. I designed a container to solve this. The audio connector fits into this container and the container is screwed to the case. The .stl file can be downloaded here: Hopefully this container solves the problem.

Container for the audio jack connector created with OpenSCAD. The container, screwed to the chest box, will keep the audio connector tightly to it’s place.

Yet unsolved problems

  • I need to attach a nylon belt to the chest box. I’m thinking about popper snap fasteners attached to the belt to open or close the belt.
  • The HT8950A works fine with a proper audio signal as input but the microphone that I have, a small electret microphone, doesn’t give any audible output (except for noise). I assume that the signal is too weak and therefore needs amplification.

Here are the links to all blog posts I wrote about this chest box:

amplifier electronics laser cutting woodworking

Darth Vader voice changer 2.0 part 3: amplifying and case revisited


The last couple of weeks I worked on a homemade voice modulator that is easy and cheap to built. First I’d like to mimic the Darth Vader voice changer, but with the press of a button it can be changed to robot or helium kind of voice effects. Last week I’ve steadily continued my work. The project has two main parts, the electronics and the case.

Electronic circuit

For the project I choose the HT8950A voice modulator from Holtek as the heart of the circuit. It is cheap, versatile and easy to work with. I previously had the voice modulator working. I only had to amplify the signal which seemed simple enough. Well, that turned out to be a bit more troublesome than expected. After connecting the LM386 to the circuit and powering it up an annoying hiss was introduced. After some experimenting I figured that the breadboard was to blame and decided to copy the circuit to another breadboard. Although on the new breadboard the hiss appeared somewhat reduced but it was still at an unacceptable level. Even when I removed the input signal from the LM386 the hiss continued. I connected the HT8950A with a audio-jack to an external amp. I wanted to make sure that the hiss was coming from the LM386. With the external amp the hiss was gone. The fact that I had hiss without an input signal indicated that the supplied voltage wasn’t clean.  To clean it up I placed a small capacitor (10nF) from pin 6 (V+) to ground and voila the hiss was gone (see schematics below). It took me some time but I’m almost ready to finish this circuit and solder it to perfboard.

Two (almost identical circuits) with the Holtek HT8950A voice modulator and the LM386 amplifier. Both circuits gave a hiss which was unacceptable. Eventually a simple 10uF capacitor from pin 6 to ground did the trick for me. 
Schematics of the Darth Vader voice changer. I was able to eliminate all the hiss and noise from the LM386.

Printing the case.

I decided previously to make a T-slot plywood case for the Voice Changer but I’m not satisfied with the result. First I made a beginners mistake with the tab width. It was chosen poorly resulting in fragile edges of the case. Also I discovered that a T-slot case is not the best choice for this project. The case needs to be sturdy and the T-slot isn’t. I’m afraid it will fall apart when in use. Therefore I’ll made a regular finger joint that will be glued together. This new case was much better. Gone are the fragile edges and I’m confident that glued together it will be very sturdy.

Laser cut case made out of plywood. I first used a T-slot type of case but made a beginners mistake with the tab width and positioning. As a consequence the edges become very fragile. The front of the case has engravings for the positioning of large controls of the voice changer. These controls on the front will we dummies just to mimic the look of a Darth Vader voice changer. The actual controls will be on the side of the box.
Laser cut case with regular finger joint. This attempt is much better.

Here are the links to all blog posts I wrote about this chest box:

laser cutting woodworking

Darth Vader voice changer 2.0 part 2: designing the case

The first voice changer that I made had a handmade 1/2″ thick plywood case. Making it was very time-consuming and it’s a bit heavy. I felt I could do better. This year I got interested in laser cutting but until now I only read about it. The laser cutter isn’t as widespread as the 3D-printer. The laser optics combined with the CNC bring a whole range of possibilities when compared to creating by hand. First of all you can design with a computer and then send your file to the laser cutter greatly increasing the precision of the case. Second I can make casings that I never thought were feasible, e.g. it is possible to curve the wood by cutting in certain patterns, make complex wood joints normally the territory of professional carpenters or create detailed engravings in just minutes, just to name a few.

In the area where I live there is a FabLab that provides me the possibility to use their laser cutters for a modest fee. I’ve already been there to test the equipment.
To create a case I started with an online tool called Makercase. It’s an easy to use tool that, once you’ve entered the parameters on length, width, thickness etc, provides you with an svg-file. The svg-file can be opened and changed in Inkscape, a free vector drawing program. Adobe Illustrator and Corel Draw are fine too but they are expensive and proprietary programs. Inkscape has a learning curve so I have spend the better part of the week to get acquainted with the program. I nevertheless made good progress and hopefully I’ll be able to have the case ready this week.

Image of a part of the generated svg-file. I choose the T-slot design and a thickness 1/4″. This is half the thickness that I used for the earlier version of the Darth Vader voice changer. The black lines are for laser cutting while the red lines are for laser engraving.

The files to laser cut the plywood can be found here. I included a .svg file and an .eps file depending on the laser cutter that you’re using.

Here are the links to all blog posts I wrote about this chest box:


Darth Vader voice changer 2.0 part 1: the HT8950A chip


Last June I wrote about a Darth Vader voice changer that I made. Back then I used the Velleman MK171 kit as the electronic part of the voice changer. Although it worked pretty well I felt that some area’s needed improvement. First, I couldn’t get the push buttons mounted on the housing to work making it hard to switch the voice effect. Second, I didn’t like the limitations that obviously come with such a kit; you’re more or less bound to the design as intended by Velleman.

The HT8950A chip

For this project I wanted to deviate from Velleman’s MK171 design and experiment with the HT8950A chip, the heart of the kit. The HT8950A chip from Holtek is a voice modulator that provides two special effects, Vibrato and Robot (pin 2 and 5 respectively). It also provides frequency level shifting enabling the user to shift the frequency up or down (pin 3 and 4 respectively). This is perceived as a higher or lower pitched sound resulting in a ‘Darth Vader’ type of sound when the frequency is shifted two steps lower. More detail on this can be found in the datasheet of the HT8950A. An external LED connected to the LAMP pin (pin 11) changes it’s brightness when the input voice signal changes. Pin 12, the AUDIO, provides the resulting sound that can be amplified.

Breadboard with in the middle the HT8950A. For this chip relatively few components are needed to get the voice modulation to work.

The chip can be acquired by buying the MK171 kit from Velleman. This kit also provides you with a LM386 amplifier however it is cheaper to buy the HT8950A without the other (generic) components from eBay or Aliexpress. If you do so make sure that your supplier of your choice has sufficient positive feedback. It’s important that you use the HT8950A (DIP-16) and not the HT8950 (DIP-18) which has a very different pin lay-out!

Building it on the breadboard

On the datasheet of the HT8950A (page 5 onward) schematics can be found how to connect the chip. The amplifier of  the schematic on page 5 or 7 can easily be exchanged for another one of your liking. I’ll probably use the LM386 because it’s cheap and I have several on stock. In order to operate the HT8950A one only needs a couple of generic components (7 resistors, 5 capacitors, an LED and a Zener diode) so building it is not too complicated.

First I built the circuit on the breadboard without an amplifier. The HT8950A actually has it’s own built-in amplifier consequently connecting a small speaker to AUDIO (pin 12) is sufficient to give an audible sound, though barely. Alternatively a headphone can be used. This is especially useful if your roommates start complaining about the weird noise. Later I also connected a homemade amplifier to the circuit (to my roommates despair). The Robot mode sounds good but while testing the frequency level shifting mode a lot of noise became apparent. Placing a capacitor over pin 8 and 9 (AO and AIN respectively) reduces this noise considerably. Optionally a 25K potentiometer can be placed between the 33K resistor and pin 8 to tune the sensitivity of the microphone or any other sound input.

Although I’m rather pleased with the result. I would like to take this experiment a little further. Next I’ll try to further optimize the sound, add the LM386 amplifier and solder the circuit to a perf board. Then I’ll either use the plywood housing that I made for the original version or make a lighter one.

Overview of the circuit. The Audio in is coming from an old FM radio (top-left). I’m using a headphone to listen to the modulated sound while keeping my roommates happy.
Schematics of the experiment. The capacitor between AO and AIN varied between 47nF an 220nF to reduce the noise. I found that the 47nF gave the best results.

Here are the links to all blog posts I wrote about this chest box:

electronics Make:Electronics

Audio filtering, experiment 29 of Make: Electronics part 3

A small update on the filtering experiment 29 of Make: Electronics that I wrote about earlier this week. To improve audibility especially of the lower frequencies I had to built an enclosure. The book proposes a plastic box but I had a shoebox made out of cardboard lying around. I cut a circular hole in it just wide enough to fit the 4-inch speaker and fastened it with four bolts and nuts. I placed the breadboard with the amplifier on the bottom of the shoebox. I connected the audio and power and filled the box with isolating material. Then I played a track unfiltered, with low pass filter and with high pass filter. The box definitely made an big improvement to the sound. Conclusion: a shoebox can make a tolerable (and cheap) enclosure for a speaker.

Cardboard box with speaker built in. Labbench and iPod for power and audio-in respectively. 

Breadboard on the bottom of the shoebox with the coil clearly visible. The coil acts as a low pass filter.

Shoebox stuffed with isolating material in an attempt to improve the sound for this experiment.

electronics Make:Electronics

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. 

electronics Make:Electronics

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.