[Jeri] uses light bulbs in an oscillator

Way back when [Ms Ellsworth] was a kid, she kept seeing the same circuit over and over again in her various op-amp books. It was a Wien bridge oscillator, a small circuit that outputs a sine wave with the help of a light bulb. Now that [Jeri] is much wiser, she decided to play around with this strange oscillator and found it’s actually pretty impressive for, you know, a light bulb.

The interesting portion of the Wien bridge is the gain portion of the circuit. It’s just a simple resistor divider, with a light bulb thrown in on one of its legs. When the current increases, this causes the light bulb to warm up (not enough to glow, though). When the temperature increases, the resistance in the light bulb increases, making the oscillator reach an equilibrium.

It’s a clever setup, but what about swapping out a resistor in place of the light bulb? In the video, [Jeri] tries just that, and it’s a mess. Where the light bulb circuit is amazingly stable with very, very low distortion, the resistor circuit looks like a disaster on the scope with harmonics everywhere.

A very cool build that would be perfect for an audio synth, but as [Jeri] says in her YouTube comments, “This doesn’t have enough distortion for indie bands.”

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Quick and easy Arduino-powered theremin

[Martin] sent in a great guide to a simple Arduino based theremin. It’s a very small build – just a single common IC and some passive components – and easy enough to build in an afternoon.

The theremin is based on a simple LC oscillator built around a 7400 quad NAND gate IC, a wire antenna, and a few caps and resistors. When a hand moves closer to the antenna, the frequency of the oscillator increases; when a hand moves away, the frequency decreases. On the software side, the oscillator is connected to the internal hardware counter of the Arduino. Every time there’s a change in the voltage output by the oscillator (all the time, varying slightly with the distance from a hand to the antenna), the counter increases by one. This counter is tallied up over 1/10th of a second, and the distance from the instrumentalist to the theremin can be determined. From there, it’s just outputting a frequency to a speaker.

All the code, schematics, and board layouts are available on [Martin]’s guide, and most of our readers probably have the parts to build this lying around their workbench. You can check out a video of [Martin]’s theremin in action on his guide.

Remove your Arduino’s external oscillator to gain a free pair of IO pins


[Mark] from SpikenzieLabs was wrapping up a project using an Arduino the other day and found himself in need of a few more I/O pins. He could have added extra circuitry to the project, but he decided to see if he could gain a few pins by removing a few components instead.

He put together one of his Minuino boards, but rather than installing the crystal and its associated capacitors, he added a couple of pin headers in their place. It’s well known that the internal clock on the chip is not as precise as a crystal, but [Mark’s] project was not that time sensitive, so he had no problem sacrificing the oscillator for a few extra pins.

With his new I/O pins in place, he merely needed to tell the ATmega chip which clock it should be using, and he was well on his way. While this might not be the best solution for all projects out there, if you are building something that values pincount over precision, this hack is for you.

Check out the video below to see [Mark’s] hack in action.

[via HackedGadgets]

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Multivibrator in theory and practice

We must admit that we’ve been guilty of using a microcontroller to make two LEDs blink alternately in the past. It’s not the worst transgression, but it stems from our discomfort with analog circuits. Luckily, [Ray] published an illustrated guide on building multivibrator circuits. This is a simple method of assembling a two-output oscillator. All it takes is a pair of NPN transistors, which are then switched by on and off based on a resistor-capacitor (RC) timer.

[Ray] does a good job of walking us through how the circuit works at each stage of one complete cycle. You’ll need to read carefully, but the supplementary schematics he uses to water down snap shots of the various electrical states really helped us understand.

Of course, blinking LEDs isn’t the sole purpose of a multivibrator. It is a method of producing a clean square wave which can be used as a clock signal for TTL logic chips. Oh, who are we kidding, see the blinky goodness for yourself in the video after the break.

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Gain wizardly knowledge about crystals

Make sure your test equipment is handy, then give this video series about crystal oscillators a spin. [Shahriar] of the Signal Path Blog put together a four-part video blog post totaling about an hour. In the discussion he covers the ins and outs of crystal oscillators and ring oscillators. His focus is on how these parts are used as timekeeping devices for microcontrollers. This isn’t a lecture that skims the surface of the topic, it takes you down the rabbit hole, discussing theory, how the devices are built, how to use them, and the pitfalls of doing so.

Our favorite part is in the fourth segment when [Shahriar] measures the effect that temperature has on crystals by spraying them with an inverted compressed air canister. We always thought we were just screwing around when freezing stuff like that. It didn’t occur to us that we were conducting serious experiments.

We’ve embedded the first segment of the video after the break. Continue reading “Gain wizardly knowledge about crystals”

Experimenting with a permeability tuned oscillator

[Scott Harden] has been working through a design for a variable inductor to use as a PTO, or permeability tuned oscillator. What you see above is the most recent fruit of these efforts. The variable inductor is made up of the green coil of wire with a threaded bolt in the core. Turning that bolt moves the tip in or out of the coil, affecting its inductance.

Traditionally, tuning RF oscillator circuits has been a function of an adjustable capacitor. But capacitance is only part of the circuit, with inductance being the other important portion. Since variable capacitors that are capable of affecting a large change on the frequency of a circuit can be quite expensive he set out to find another way. This is what prompted the development of his first PTO project.

[Scott] produced a demo video of the hardware seen above which we’ve embedded after the break.

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Digital Wavetable Oscillator

The tabulaRasa is a digital wave table oscillator, and features control of frequency, wave table selection, and interpolation. The device is split up into 2 parts. One is a pcb with a healthy amount of resistors, 3 potentiometers, ST TL074 JFET op amp, atmega328 and a SD socket.

The second part is software for your computer that allows you to edit or create your own waveforms. There are 3 different modes of control. Breakpoints, which allows you to set the waveform points and allows up to sixteen. Harmonic allows amplitude control over 16 harmonically-related sine waves, finally, the third mode lets you load in short sound clips.

Once you’re happy, save to a SD card and pop it into the board, and you’re ready to make some noise. The project page states at the end “tabulaRasa is in the last stages of development, and will be available soon.” so you cant get your hands on one just yet, but if you’re interested [Greg] has a kickstarter page setup where you can find out details on pricing.