Retrotechtacular: Fundamentals of AM and FM Radio Communication

How radios send and receive information can seem magical to the uninformed. For some people, this week’s Retrotechtacular video, “Frequency Modulation – Part 1 Basic Principles”, from the US Army Department of Defense 1964 will be a great refresher, and for others it will be their first introduction into the wonderful world of radio communications.

The stated objective is to teach why FM radio communication reduces interference which normally afflicts AM radio communications. Fundamentals of AM and FM is a better description, however, because the first part of the video nicely teaches the principles of AM and FM radio communications. It isn’t until later in the clip that it delves into interference, advantages of FM modulation, and detailed functioning of FM radio. The delivery is slow at times and admittedly long, yet the pace is perfect for a young ham to follow along with plenty of time to soak in the knowledge. If you’re still on the fence about becoming a ham here’s some words or encouragement.

Though the video isn’t aimed at ham radio users it does address core knowledge needed by amateur radio hobbyists. Amateur radio is full of many exciting communication technologies and you should have a clear understanding of AM and FM communication methodologies before getting on Grandpa’s information super highway. Once you have your ham license (aka ticket) you have privileges to create and test amazing ham related hacks, like [Lior] implementing full programmable control of a Baofeng UV5R ham radio using an Arduino.

Join us after the break to watch the video.

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The folly of adding an auxiliary audio input to a Hyundai Sonata

Why auxiliary audio inputs haven’t been standard on automotive head units for decades is beyond us. But you can bet that if you’re looking at a low-priced sedan you’ll need to buy an entire upgrade package just to get an audio jack on the dash. [Jon W’s] Hyundai Sonata didn’t have that bells-and-whistles upgrade so he decided to pop his stereo out and add his own aux port.

A big portion of this hack is just getting the head unit out of the dash. This is made difficult on purpose as an anti-theft feature, but [Jon’s] judicious use of a butter knife seemed to do the trick. He lost some small bits along the way which were recovered with a Shish Kebab skewer with double-stick tape on the end.

With the head unit out, he opened the case and plied his professional Electrical Engineering skills to adding the input. Well, he meant to, but it turns out there’s no magic bullet here. The setup inside the unit offered no easy way to solder up an input that would work. Having done all of the disassembly he wasn’t going to let it go to waste. [Jon] grabbed a nice FM transmitter setup. He wired it up inside the dash and mounted the interface parts in the glove box as seen here.

It’s nice to know we’re not the only ones who sometimes fail at achieving our seemingly simple hacking goals. At least [Jon] was able to rally and end up with the functionality he was looking for.

Building a simple FM transmitter bug

simple_fim_transmitter_hack_a_week

[Dino] got his hands on an FM transmitter “bug” kit via a friend, and thought it would make for an easy and fun Hack a Week project. The kit is simple two transistor half-wave FM transmitter, which the manufacturer suggests could be used to bug a room, hence the name. After poking a bit of fun at the instructions, [Dino] gets to work building the transmitter, wrapping things up in a little less than an hour.

Once he finished soldering everything together, he takes a few moments to test out the bug and to explain how various parts of the board work together in order to transmit the FM signal. He mentions that adding a dipole antenna would make it easy to extend the range of the transmitter, and briefly teases next week’s episode, where he plans on constructing a similar dual-stage transmitter.

This sort of FM circuit is one of the first few simple projects you would see in a beginner’s electronics class, so if you know anyone that is just starting to get their feet wet, be sure to pass this Hack a Week episode along.

Continue reading to see [Dino] explain the ins and outs of his FM bug transmitter.

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Simple FM transmitter that shows off its own circuitry

[Sean Michael Ragan] built this FM transmitter which shows off its circuitry via a clear plastic dome. The device is electrically identical to one we looked at in September. That version championed a construction method that used small squares of copper clad as solder points which were each super-glued to a large copper-clad platform serving as a ground plane. [Sean] is using a printed circuit board that was laid out by Sonodrome. You can check out their own glass-jar transmitter build where the board artwork is available for download.

One of the tips we enjoyed from [Sean’s] step-by-step build is the coil wrapping. He used the threads of a 1/4-20 bolt to guide copper wire as he wrapped a total of four turns. Once the bending is done, just unthread the bolt to separate it from the coil and gently stretch the wire for a 12mm distance between the two leads. Not only is this visually pleasing, but it will help with transmission clarity.

Minty FM Transmitter

[Chis] wanted an FM transmitter bug device packed in a mint tin, and that is just what he made. Featuring only 17 discrete parts, running off of a 3volt coin cell battery, and small enough to fit inside of a plastic mint box and still have plenty of room for the mints.

Using a simple design the audio enters the device through a electret microphone and hits a transistor for amplification, the signal is then passed to the oscillator section of the circuit which features an LC tank type design which generates the carrier frequency and mixes that with the signal for a range of about 100 feet indoors.

Each section is broken down into steps where it is thoroughly explained with animations, theory, and simulation, if you are wondering how a transistor, wire, and capacitors make an FM transmitter, or if you would like to just make the final project, schematics, pcb files, and assembly instructions are provided as well.

Join us after the break for a short video and be sure to check out the other radio transmitters we have featured as well.

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All-band receiver hides in plain sight

This handheld radio has a little secret. You’re not going to be able to listen to Limbaugh since the original FM tuning circuit has been removed and replaced by a diode detector. Now [Miguel A. Vallejo] a discreet way to look for interesting radio signals in public.

The first step that he took was to remove the circuit board from the case and depopulate the tuning circuit while leaving the audio amplifier hardware. Next he referenced a proven design and built the diode detector circuit on a piece of protoboard. Finally he patched the new circuit into the original audio amplifier (seen in the image above) and put everything back in the case. Now he can listen in on data burst from a keypress on a computer keyboard, RF data communications, and slew of other noise sources.

This would be really handy for tracking down the electrical noise that’s screwing up your project.

[Thanks Superlopez]

FM bug using salvaged SMD parts

If you’re a soldering ninja this FM transmitter bug is for you. It’s quite similar to the one we looked at yesterday, but this uses 100% salvaged parts. Two phones donated components; a Nokia 3210 for its voltage-controlled oscillator and a Nokia 1611 for the rest of the parts. The bad news is that mobile technology like cellphones use some of the smallest surface mount packages known to man. That’s where the soldering skill come into play. The good news is that if you’ve been scavenging for discarded phones in order to reuse their LCD screens you already have these parts on hand.

[Thanks George]