Behind The Scenes Of The Theremin Design

How I design my electronic circuits and prepare the videos to show them to you.

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Did you ever ask yourself where I get the schematics of the Theremin circuits and other gadgets that I present on my YouTube videos? The answer is simple: I do some research on books, on specialized magazines and on the Internet. I see solutions created by other people, if any, and then I think about what would better work for my case. Sometimes it ends up to be a modification of something that somebody else did, maybe for a totally different purpose. Sometimes, I just use the general idea to create something different, new, my own design that is more appropriate for my needs.

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Either way, I usually build a number of prototypes of what I need, then I take some measurements in lab, then I start making further modifications to my original design, until I obtain exactly what I am looking for.

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Also, more often than not, I figure that the circuit I am testing is too sensitive to certain parameters of the circuit itself. Maybe is a capacitor which value needs to be adjusted a little bit, or a connection between two or more components that causes issues because of capacitive or inductive coupling with other components. That is when I try to change my design to reduce such sensitivities, so that the circuit can be assembled by anyone with the exact same results as mine. And this is what is called engineerization, or adjusting the design for mass production.

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And sometimes, to do so, it is not enough to test the single circuit. Instead, I need to connect the circuit with other pieces that have to work together with it, and see if further unwanted interactions happen, so that I can eliminate them or, at least, reduce them so that they become negligible.

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Sometimes this process goes fast, sometimes takes a long time. And that’s why my videos are not published at fixed intervals. Unfortunately, since this is done only as a hobby, I don’t always have enough time to dedicate to my project, so days go by until, finally, I am done. Then I finalize my schematics, I build the last prototype and the final product and, in the process, I also record all these activities so I can end up making a video out of them.

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Then the video editing process starts and, once the video is finally ready, I release it on YouTube for you to watch it.

One day I will be able to do this full time. Who knows, maybe when I retire. Or, maybe, if you all give me a hand, this could become my new full time job (donations, donations, donations). We’ll see.

Thank you for reading this article. And, as usual, happy experiments!

An LED Bar Graph VU Meter

VUmeter

Bar graphs VU meters can be easily made with a simple integrated circuit. There are several of them with different characteristics, but they all present the same basic functionality: one or more LED in a row are used to visualize, more or less precisely, the voltage amplitude presented to the input. That voltage can be either a direct current or an alternate current and, in particular, it could be the output of an audio amplifier.

But, what is inside these integrated circuits? How do they make possible this kind of behavior?

Here is the design of a simple gadget that shows how a bar graph VU meter works. Building and using this device is certainly a fun way to learn the principles used to design the bar graph integrated circuits.

bar_graph_vu_meter

The basic element of the circuit is the one made with the components Q1, R1, R10, D1, and D10.

This block of components then replicates several times to increase the number of LEDs used in the bar graph. In this particular case, the same circuit is replicated 8 more times, for a total of 9 LEDs in the bar.

You can also see that each block, or stage, receives as the input the output from the previous stage.

Diodes form D10 to D17 are used to provide a different threshold to each stage. In fact, let’s say that the first stage is triggered when the voltage on the anode of D10 reaches about 3V. In order to trigger the second stage we will need 3V on that stage, but that means that the voltage at the first stage has to go up to 3+0.6V, or 3.6V. The extra 0.6V is the forward voltage of diode D10.

Similarly, to reach each further stage, we will need an input voltage 0.6V higher for each stage we want to light up.

In the end, to light up the last stage we will need an input voltage of at least 3 + 8 times 0.6V, or 7.8 volts.

Once the threshold is reached in a stage, the corresponding transistor switches on and starts conducting a current that is only limited by the LED and the series resistor which, in this case, is 330 ohms. With the values in the circuit, the LED current will be about 20 mA.

So, when we apply a voltage to the input terminals, depending how high the voltage is we will see a number of consecutive LEDs lighting up, while the remaining will stay off because their respective stages have not been triggered yet or, in other words, the voltage at those stages hasn’t yet reached the threshold imposed by the diodes.

Note also that resistors R10 to R18 are not all of the same value. This is because by the time the voltage reaches the threshold in the last stage with transistor Q9, the voltage on the previous transistors is higher and higher while we move to the left of the circuit, since we need to add back the 0.6V that the diodes are dropping. Therefore, to avoid damage to the transistors on the left, we need to increase the base resistor when moving from the right to the left of the circuit.

Another thing to notice is the trimpot located at the connectors for the input signal. The circuit as it is, is capable of handling signals up to the value of the power supply, which is 9V.

However, we can adjust the trimpot to handle higher signals, just by moving the trimpot cursor toward the end that is connected to ground, in order to get only a fraction of the actual input signal.

Conversely, if you had a very small input signal, that could not trigger even the first stage of this circuit. In that case, you could still add a very simple amplifier between the source of the signal and the input of this circuit, which would help increase the level of the signal to the required value.

Finally, you see that the bar graph meter is powered with a 9V power supply. I used such value so you can use a 9V battery, if you like to try the circuit.

However, if you wanted to use this circuit as part of a more complex system having a higher value of the power supply, you could just modify resistors from R1 to R9 and use the power supply of that system.

For example, if you planned to use 12V instead of 9, you would use resistors of 470 ohm rather than 330, and everything would work just fine.

Remember, however, that 9V is the minimum voltage you can use to correctly power the bar graph circuit. You can only increase the power supply voltage to a higher value and increase accordingly the resistors R1 to R9. That is because we need a power supply that exceeds the voltage at the base of the leftmost transistor Q1, which can be as high as 7.8 volts, as we said before.

The circuit can be easily assembled on a per-board, like in the case in the frnt picture of this post. There, I used a 3 prongs connector to provide the power supply and the input for the external signal.

Once the circuit is assembled, set the trimpot with the cursor toward the ground side, then power up the device and put a signal to its input. The signal should be the highest possible with the amplifier to which you are attaching the VU meter. Then, adjust the trimpot until all the LEDs are lighted up. Now the circuit is tuned and you can input to it any signal that changes between 0V and the max you used for the tuning.

For further information, and to see the VU meter in action, take a look at this video that I posted on my YouTube channel.

Happy experiments!!!

Another Theremin Post

Announcing the new video on the design, implementation and testing of the power supply module of the Theremin.

You can find the video at this link.

Also, refer to the link on the right column about the Theremin project to find all the files so far published on the construction of this unusual electronic musical instrument (schematics, 3D designs, work art).

And, finally, don’t forget to look also at the other videos on my YouTube channel.

 

DC Electronic Loads

An electronic load to test DC power supply devices up to 100W.

What do you use when you have to test a new power supply that you just built, or one that you bought and want to know if the declared specs are true?

One thing you’ll need is a passive load that you attach to the power supply output to drain a certain amount of current, both to verify that the power supply is capable of providing that amount of current, and to verify the amount of ripple that was not filtered away by the power supply itself.

rheostatA classic method for doing so is to use a rheostat, which is essentially a potentiometer capable of dissipating the amount of power produced by the power supply. The resistance of the rheostat can be changed and therefore different amount of currents can be used to test the power supply. However, rheostats are big, heavy and cumbersome.

An alternative to rheostats is to have a so-called Electronic Load. These are electronic circuits that are capable to emulate the functionality of a rheostat.

I’m proposing here two simple versions of an Electronic Load, functioning in DC, and capable of dissipating up to 100W, all of this in a very condensed space, and very light in weight.

The first version is a very simplistic one.

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It uses a cascade of three transistors, in a configuration called Darlington. This configuration is effectively equivalent to a single transistor with a gain (hfe) that is the product of the gain of all the transistors in the configuration. This allows for a very little control current flowing into the potentiometer used to regulate the base current, and for a high current available between the collector and the emitter of the transistor Q3, in the above schematics.

This circuit does not need its own power supply, since it gets what it needs directly from the power supply under test. The resistor R1 is calculated based on the highest voltage that the circuit will be able to handle, in this case 50V. It is important to note, however, that using this load at lower voltages will prevent the possibility to use the full excursion of the potentiometer, thus limiting the sensitivity of the circuit for the control of the current.

The next circuit eliminates the sensitivity problem by providing it own power supply to control the base current of the Darlington, thus eliminating the dependency from the external power supply voltage.

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In fact, in this case, the base current can be adjusted with the potentiometer RV1, which is polarized through the resistor R1 and the trimpot TR1, which can be adjusted to maximize the useful range of motion of the potentiometer, regardless of the voltage applied at the input terminals. This way the electronic load can have the same sensitivity for any input voltage. A digital Volt/Am-meter, powered by the same internal battery, provides a visualization of the voltage of the system under test and the current being drained from it.

I will soon publish a video on my YouTube channel that shows the Electronic Load I built for myself. Please watch for that video to come out. And, in the mean time, can you think which one of the above schematics I used? Did I use the simple one, because less expensive? O did I sacrifice a few extra bucks to gain more sensitivity on the regulation of the load?

I strongly suggest to subscribe for free to my YouTube channel, and also to click on the bell icon that appears after the subscription is done. This will allow you to automatically receive an e-mail whenever I publish a new video. This way you won’t have to go periodically to my channel to check for new posts.

Happy Experiments!

Theremin Part 2: Pitch Reference Oscillator and Box Lower Section

Presenting the files needed to reproduced what done so far for the Theremin project.

As promised in the YouTube series on the Theremin, here are the files of the parts already designed that you’ll need to build a Theremin like mine:

  • The Pitch Reference Oscillator Schematics: pitch_reference_oscillator
  • A zip file for the bottom section of the Theremin box, containing the OpenSCAD file, the .stl file and the .gcode file: box_base.
  • A zip file containing the inkscape versions of the front and back panel artworks, along with a png version: theremin_panels

Once you build the Pitch Reference Oscillator, remember to tune it to 172 kHz, by acting on the variable capacitors C6 and C7. You do so by measuring the frequency while changing the capacitance of C6 and C7. The frequency can be measured with an oscilloscope or a multimeter that is capable of measuring frequencies. The tuning should be done while the potentiometer RV1 is in the middle position.

Expect to see more of this in the next posts, once the other pieces of the Theremin will be built one at a time and the corresponding videos will also be posted on YouTube.

If you like to be automatically informed when new videos are posted, please go the YouTube channel, select a video,  and click on both the SUBSCRIBE button and the bell symbol on its right side.

You can also follow this blog as well, to be informed of new posts via e-mail.

See you soon with more of this and, in the mean time, happy experiments!

Capacitors – Part 1

A brief introduction to capacitors: what they are, how they are made, and their basic functionality.

capacitorsA capacitor is an electric device capable of storing energy in the form of electric charges (electric field).

In the most simple form, a capacitor is made of two conductive plates facing each other and an insulator in between, which is normally called a dielectric. The two plates are then attached to wires, that are used to connect the capacitor in an electric circuit.

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The schematic diagram reflects exactly the physical nature of the device:

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When a capacitor is connected to a power supply, like a battery, electrons leave the plate that is connected to the positive side of the battery, while the same amount of electrons is pushed into the plate connected to the negative side of the battery. Once the difference of charges at the plates of the capacitor is enough to establish a voltage on the capacitor that is identical to the battery, electrons stop moving around the circuit and an equilibrium is reached.

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At this point, if the connection with the battery is severed, the capacitor will retain the amount of charges on its plates: extra charges on the negative plate and defect of charges on the positive plate. If we connect a load to the capacitor, for example a resistor, charges will start moving in the circuit pushed by the voltage at the wires, called electrodes, of the capacitor. So, electrons will leave the negative plate moving toward the load, and an equal amount of electrons will move from the load into the positive plate of the capacitor. The movement of the electrons causes the voltage at the plates of the capacitor to lower until, when an equilibrium of charged is reached, the voltage will be zero and the current will stop flowing through the circuit. At this point all the energy that was stored in the capacitor has been used and the capacitor is said to be discharged.

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Both during charge and discharge, the ratio between the amount of charge stored on the capacitor and its voltage remains constant. This can be verified experimentally. We define this constant as the capacitance of the capacitor:

C = Q / V

which is measured in Farad. However, since the Farad is a very big unit, capacitors are normally measured in fractions of Farad, like microFarad (μF, 1 millionth of a Farad)), nanoFarad (nF, one billionth of a Farad), and picoFarad (pF, one trillionth of Farad).

Using the above formula, and calculating the work done to move the charges in and out of the capacitor with the help of some calculus, we can determine the energy stored in a capacitor as:

energy

And, finally, the actual capacitance can also be determined by the physical parameters of the capacitor itself. We can see experimentally that the capacitance is directly proportional to the area of the plates of the capacitor, it is inversely proportional to the distance between the plates, and depends on the type of dielectric in between the plates. The type of dielectric is identified in the formula by the Greek letter ε (epsilon). Each type of dielectric has its own value of ε (permittivity), which is the product of the vacuum permittivity and the relative permittivity of the material.

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For more information on this subject, please look also to the corresponding video on my YouTube channel.