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.



Building a crystal oven

Radio communications depend on stable oscillator frequencies and with that in mind, [Scott Harden] built a module to regulate temperature of a crystal oscillator. The process is outlined in the video after the break but it goes something like this: A small square of double-sided copper-clad board is used as a base. The body of the crystal oscillator is mounted on one side of this base. On the other side there is a mosfet and a thermister. The resistance of the thermister turns the mosfet on and off in an attempt to maintain a steady temperature.

This is the first iteration of [Scott's] crystal oven. It’s being designed for use outdoors, as his indoor setup uses a styrofoam box to insulate the oscillator from ambient temperatures. He’s already working on a second version, and mentioned the incorporation of a Wheatstone bridge but we’ll have to wait to get more details.

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Using videotape tape as a controller

Behold the Bodystick, an instrument built and demonstrated by [Erich Lesovsky]. It’s a bit like a string bass but instead of strings there is a strip of VHS tape. Apparently not all VHS tape will work, but if you have the right kind you can run voltage through it and then change the resistance with a touch of your finger. It seems that the hand not touching the tape needs to be touching a conductive pad, completing the circuit. The resulting resistance changes the oscillator values on a CD40106 CMOS chip. This project is a bit out there (just like [Erich's] Mega-Tape-O-Phone), and in keeping with its peculiarity is the demo video after the break. Enjoy!

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Pure TTL based clock

We’ll just say, [Kenneth] really likes clocks. His most recent is a pure 7400 series TTL based one, ie no microcontroller as seen in the past, here, here, and here. The signal starts out as a typical 32,768 crystal divided down to the necessary 1Hz, which is then divided again appropriately to provide hours and minutes.

As far as TTL clocks go, this is nothing too original; until it comes to his creative button interface. By using a not as sexy as it sounds multivibrator, he can produce a clean square wave instead of the figity signals produced from buttons to advance and set the time. Like always, he also provides us with a thorough breakdown of his clock, after the jump. Continue reading “Pure TTL based clock”