FM Signal Detection The Pulse-Counting Way

Compared to the simple diode needed to demodulate AM radio signals, the detector circuits used for FM are slightly more complicated. Wrapping your head around phase detectors, ratio detectors, discriminators, and quadrature detectors can be quite an exercise. There’s another demodulation method that’s not so common, but thankfully it’s also pretty easy to understand: the pulse counting detector.

As [Allan (W2AEW)] notes in the video below, pulse counting is a bit of a misnomer. Pulse counting works by generating a narrow, fixed-width square wave pulse at a set point in the received FM signal’s waveform, usually at the zero-crossing point. Since the frequency of the modulated carrier changes, the duty cycle of the resulting pulse train varies. That means there will be a fixed number of pulses, but by taking the average voltage of the pulse train, we can tease out the original audio frequency signal.

Simple in theory is often more complicated in practice, and [W2AEW] goes into some detail about those complications, such as needing to use a down-converter to make the peak-to-peak frequency deviation in the pulse train more easily detectable. As is his style, he walks us through a test circuit to prove that the theory works in practice. A simple two-transistor circuit generates the pulses at the zero-crossing point, a low-pass filter cleans up the signal, and a cheap audio amplifier reproduces the original audio. It’s a crude circuit to be sure, relying on the stray capacitance of the breadboard to work, but it proves the point and serves as a jumping-off point for further experiments – perhaps using an Arduino to count the pulses?

We always enjoy [W2AEW]’s videos and learn a lot from them. Not long ago we featured another of his videos talking about the mysteries of RF modulation; SSB, anyone?

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Twelve Channels Of LEDs Give RRRRGGGGBBBB Light

If you’ve ever searched Mouser or Digikey for LEDs parametrically, you won’t find just one red in your LEDs. You won’t find one green. There is quite literally an entire rainbow of colors of LEDs, and this rainbow goes into infrared and ultraviolet. You can search LEDs by frequency, and an RGEB LED is right at your fingertips. The ‘E’ stands for Emerald, and it’s better than a Bayer filter.

[ayjaym] over on Instructables realized anyone could buy a dozen frequencies of high-power LEDs, and the obvious application for this is to turn it into a tunable light source. The Angstrom is twelve LEDs, all different colors, and all controlled by PWM and piped down a single optical fiber. It’s an RRRRGGGGBBBB LED, ideal for microscopy, forensics, colorimetry, and seeing octoreen.

The heart of this device are twelve 3W star LEDs, with the following wavelengths: 390, 410, 440, 460, 500, 520, 560, 580, 590, 630, 660, and 780 nm. That’s deep red to almost ultra violet, and everything inbetween. These are powered by a 5 V, 60 W power supply, and controlled via a Raspberry Pi with 12 PWM channels in a circuit that’s basically just a bunch of MOSFETs. Proper heatsinking is required.

The impressive part of this build is the optics. A 3D printed mount holds and connects optical fibers and sends them into an optical combiner that is basically just a square acrylic rod. This is output to another optical fiber that will shine on just about anything. A webpage running on a Raspberry Pi sets the PWM channels of all the LEDs, and the resulting output shows up at the end of an optical fiber. It’s great if you want to look at something in a specific frequency of light. It also looks really cool, so that’s a bonus.

Get Twelve Charlieplexed PWM Outputs From An ATtiny85

Most of us are aware that charlieplexing can drive a large number of LEDs from a relatively small number of I/O pins, but [David Johnson-Davies] demonstrates adding another dimension to that method to create individually controlled PWM outputs as well. His ATtiny85 has twelve LEDs, each with individually-set brightness levels, and uses only four of the five I/O pins on the device.

Each LED can be assigned a brightness between 0 (fully off) and 63 (fully on). The PWM is done by using one of the timers in the ATtiny85 to generate a periodic interrupt, and the ISR for the interrupt takes care of setting the necessary ratios of on and off times for each charlieplexed output. The result? Twelve flicker-free LEDs with individually addressable brightness levels, using an 8-pin microcontroller and just a few passive components on a tiny breadboard. There’s even one I/O pin left on the ATtiny85, for accepting commands or reading a sensor.

[David] really wrings a lot out of the ATtiny series of microcontrollers with his compact projects, like his Tiny Function Generator (which recently got an update.) He also demonstrated that while charlieplexing is usually used with LEDs, charlieplexing can be used with switches just as easily.

Active Strain Relief For 3D-Printer Filament

Buying 3D-printer filament is little like eating potato chips: you can’t stop at just one. You start with basic black PLA, then you need a particular color for a special project, then you start experimenting with different plastics, and before you know it, you’ve got dozens of reels lined up. Trouble is, unless you move the in-use reel right over the printer, the filament can get a bit unruly as the printer sucks it up. What to do?

How about building an active strain relief system for your filament collection? That what [Daniel Harari] chose to do, and we have to say that it looks pretty slick. The idea is to keep the filament slack before it enters the printer’s extruder no matter where the reel is positioned relative to the printer. The active bit is a little like a low-force extruder, using a couple of pinch rollers from an old 2D-printer to pay out filament when needed. A clever sensor, consisting of a 3D-printed funnel and a copper wire contact loop, detects when the printer has taken up all the slack in the filament and triggers a payout from the feeder. In a nice touch, the feeder motor is controlled by a couple of 555s rather than a microcontroller. The short clip below shows the feeder being triggered and paying out a little more slack.

In the final analysis, this is just another in a long series of filament management projects, from dry-boxes to filament meters to end-of-spool alarms. It may be overkill, but [Daniel] put a lot of thought into it, which we always appreciate.

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A Quartet Of Drills Put The Spurs To This Electric Utility Vehicle

Low-slung body style. Four-wheel drive. All electric drivetrain. Turns on a dime. Neck-snapping acceleration. Leather seating surface. Is it the latest offering from Tesla? Nope; it’s a drill-powered electric utility vehicle, and it looks like a blast to drive.

Surprisingly, this isn’t a just-for-kicks kind of build. There’s actually a practical reason for the low form factor and long range of [Axel Borg]’s little vehicle. We’ll leave the back story to the second video below, but suffice it to say that this will be a smaller version of the crawler NASA used to roll rockets out to the launch pad, used instead to transport his insanely dangerous looking manned-multicopter. The running gear on this vehicle is the interesting bit: four hefty electric drills, one for each of the mobility cart wheels. The drills are powered by a large series-connected battery pack putting out 260V at full charge. The universal motors of the drills are fine with DC, and the speed of each is controlled via the PWM signals from a pair of cordless drills. The first video below shows [Axel] putting it through its paces; he didn’t hold back at all, but the vehicle kept coming back for more.

We know this cart is in service to another project, but we’d have a hard time concentrating on anything if we had the potential for that much fun sitting in the shop. Still, we hope that multirotor gets a good test flight soon, and that all goes well with it.

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Lighting Up A Very Wiry Candle

Entries into the Circuit Sculpture Contest tend to be pretty minimalist by nature, and this LED candle by [Amal Mathew] is a perfect example. The idea here was to recreate the slim and uncomplicated nature of a real candle but with a digital twist, and we think he’s pulled it off nicely with a bare minimum part count and exaggerated wire length that gives it the look of a thin pillar candle.

To give the LED a fading effect, [Amal] uses a ATtiny85 programmed with the Arduino IDE. His code uses the analogWrite() in a loop to gradually increase and then decrease the PWM frequency. With the LED connected directly to one of the pins on the ATtiny85, the simple program achieves the fading effect without needing any additional components.

On the opposite side of the candle, connected by long copper wires, is the single CR2032 which provides power for the circuit. In a nice touch, [Amal] has turned the battery 90 degrees relative to the rest of the circuit, so it can serve as a weighted base. We imagine getting it to stand up might be a little fiddly from the looks of it, but once it’s up and merrily fading in and out, it really helps sell the candle idea.

The finished product might look fairly straight-forward, but in his write-up on Hackaday.io, [Amal] gives detailed instructions on how to build your own version if you’re not a bare microcontroller wizard. This includes direction on how to program the ATtiny85 using an Arduino Uno; a neat trick to know even if you aren’t planning on making any candles in the near future. The next logical step is making it so you can “blow out” the LED, which should only take the addition of a resistor and some updated code.

There’s still plenty of time to enter your own functional piece of art in the Circuit Sculpture Contest. Just write it up on Hackaday.io and submit it before the January 8th, 2019 deadline.

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A Plasma Speaker Using A TL494

We’re used to loudspeakers as circular components with a paper cone and a big magnet inside which is suspended a coil that is connected to our audio amplifier. But moving-coil speakers are not the only way to create sound from electricity, there are one or two other weapons in the audio designer’s arsenal.

One of the more spectacular and entertaining is the plasma speaker, and it’s one [Marcin Wachowiak] has been experimenting with. A continuous plasma in the form of a discharge between two electrodes is modulated with an audio signal, and the resulting rapid changes in the volume of plasma creates a sound. The value of a plasma speaker lies in the exceptionally low size and mass of the element producing the sound, meaning that while it can only effectively reproduce high frequencies it can do so from a much closer approximation to a point source than can other types of tweeter. For this reason it’s beloved of some audiophiles, and you will find a few commercially produced plasma tweeters at the high-end of the audio market.

[Marcin] isn’t in it for the audiophilia, instead he’s interested in the properties of the plasma. His plasma speaker does do the job well though, and in particular he’s put a lot of thought into the design of its drive circuit. At its heart is the ubiquitous TL494 PWM controller that you may be more familiar with in the context of switching power supplies, this one applies the audio drive as PWM to the gate of a MOSFET that switches the primary of a flyback transformer. He’s added refinements such as a gate discharge circuit and a second primary winding with a freewheel diode.

The result is an effective plasma speaker. It’s difficult to judge from his YouTube video below the break whether he’s achieved audiophile purity, but happily that’s not the point. We’ve shown you a few other plasma speakers in our time, if the subject interests you then take a look at this rotating plasma vortex, or a version using a 555 timer.

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