multiplex

16 Articles

Electronic Etch-A-Sketch, No Microcontroller Required

June 20, 2024 by 8 Comments

In a lot of ways, Etch-A-Sketch is the perfect toy; simple, easy to use, creative, endlessly engaging, and as a bonus, it’s completely mechanical. We find that last attribute to be a big part of its charm, but that’s not to say an electronic version of the classic toy can’t be pretty cool, especially when it’s done without the aid of a microcontroller.

This is one of those “because I can” projects that we always find so interesting, and more so because it wasn’t entirely clear to [BigZaphod] that he had the skills to pull it off. While his initial design centered around a bunch of 8×8 LED matrix displays and a 256×4-bit RAM chip, the rest of it was a lot of hand-waving. After a few experiments with addressing the LEDs, [Zaphod] started filling in the blanks with a refresh circuit using a 555 — naturally — and a pair of counters. Properly debounced encoders for the horizontal and vertical controls came next, along with more counters to track the cursor and a host of other circuits that ended up looking like a “one of each” selection from the 7400-series catalog.

While we do wish for a schematic on this one, it’s still a pretty enjoyable video, and the end product seems to work really well. The electronic version has a few features the original lacks, such as wrapping the cursor to the other side of the screen. We’d imagine that the buttons on the encoders could be put to work, too; perhaps a click could make it so you can move the cursor without leaving a trail behind. That might be a challenge to execute in logic, but then again, that was the point of the whole thing.

Still jonesing for that mechanical Etch-A-Sketch experience? Not a problem.

Continue reading “Electronic Etch-A-Sketch, No Microcontroller Required” →

Generating Stereo FM Signals, Thanks To Python

October 19, 2022 by 7 Comments

A casual understanding of how AM radio works is pretty easy to come by, and standard FM is only a little more complicated. Things can go off the tracks a bit with stereo FM, though — figuring out how they squeeze two separate audio tracks onto one radio signal is a bit of a head-scratcher. In that case, wrapping your head around the concept might be helped by mocking up a stereo FM signal with an arbitrary waveform generator and a little bit of Python.

Not that [Sebastian] of Baltic Lab was unfamiliar with multiplex FM theory, mind you. As he explains it, his goal was to generate a valid stereo FM signal with a different pure tone on each channel, 700 Hz on the left and 2,200 Hz on the right. Luckily, [Sebastian] has a nice AWG, the Siglent SDG1032X, which has an Ethernet connection that can be used to control it remotely along with PyVISA, a Python package for controlling instruments using the Virtual Instrument Software Architecture protocol.

The meat of this project, and what really helps drive home the concept of putting multiple audio signals onto an FM signal, lies in the Python code that generates the component parts. [Sebastian] does a great job explaining how he programatically generates the sum and difference signals along with the 19 kHz pilot tone, and puts them all together into one waveform. The output of the program is used to generate a series of values that are sent to the arbitrary waveform generator, which outputs the desired FM signal. Looking at the output on a spectrum analyzer, the two audio tones are clearly visible, as are the attenuated pilot tone and some other spikes a little further up.

Just add an antenna to the setup and you’d have the world’s dullest FM radio station — but at least it’d be in stereo. Or if you want to check out the origin story for FM radio, we’ve got something for that too.

(Getting Rid Of) The Ghost In The LED

December 8, 2021 by 38 Comments

Multiplexing is a very old technology in which control signals are intermixed for the sake of being able to control more devices than there are control signals. For [mihai.cuciuc], the problems started when he multiplexed some very efficient LEDs.

The problem? In two banks of six LEDs each, both LEDs connected to a single Arduino pin would light, even when only one bank was turned on at the ground side. The LED In the bank that was switched on lit brightly, and its corresponding LED in the bank that was off would also be very dimly lit. [mihai] was able to determine that the problem was not due to a leaky transistor, but rather due to a quality of the LEDs themselves.

What is an LED but a diode, and it’s well known that diodes also have capacitance. In fact, this quality is exploited in varactor diodes, a specialty diode whose capacitance can be changed by varying the voltage on the cathode. [mihai] deduced that this capacitance was causing current to flow in the bank that was off. Where was the current going? From the Arduino pin that was on, through its attached LED, and then into the rest of the bank of LEDs, charging them like capacitors. [mihai] hasn’t seen this before, but theorizes that for the latest batch of high efficiency LEDs, this minute current is enough to light the LED through which the current is flowing.

[mihai]’s solution is an elegant hack which he’s made available for your perusal. You might also enjoy this introduction to diode basics by W2AEW. If you have any great diode or LED hacks of your own, be sure to drop us a line!

144 7-Segment Displays Combine To Form A Mighty Clock

March 5, 2020 by 25 Comments

What do you do with 144 7-segment displays? If you’re [Frugha] you put them all together to create an epic clock. Each display has 8 individual LEDs — 7 segments, and a decimal point. Put that all together, and you’ve got 1152 individual LEDs to control. This presented a problem, as [Frugha] wanted to control the clock with a single Arduino Nano. Even charlieplexing won’t get you that many I/O lines.

The solution was a nifty little chip called the MAX7219. The ‘7219 speaks SPI and can control 64 individual LEDs. [Frugha] used 18 of them in the clock, giving him full control over all his LEDs. That’s pretty impressive, considering the last matrix 7-segment display we saw required 48 Arduinos!

Another problem is memory – 1152 “pixels” would quickly overrun the 2KB RAM in the ATmega328. This is a clock though — which means only digits 0-9 and a colon. [Frugha] picked a nice font and hand-coded lookup tables for each digit. The lookup tables are stored in ROM, saving precious RAM on the Arduino.

A clock wouldn’t be any good if it wasn’t accurate. A Tiny RTC supplies battery-backed time data. [Frugha] wrapped everything up with a neat layout on a custom PCB. Sure, you could put it in a case, but we think a clock this crazy deserves to be left open – so you can see it in all its glory.