AM Transmitter Test

I put the transmitter from the previous post to the test. I changed the circuitry a bit though. I kept the oscillator (50 MHz) is now continuously powered. I programmed the ATTiny 2313 microcontroller (using PWM output) to send an oscillating signal to the base of a transistor (NPN). In this way the microcontroller PWM output didn’t supply power to the oscillator, but rather grounded it. I got a big boost in range this way. Yesterday I couldn’t even hear the signal in the parking lot of my apartment, whereas today I heard it loud and clear. I decided to take a drive with my scanner, laptop, and Argo to see how far away I could get and still detect the signal. With this bare bones transmitter setup (using a 2M J-pole antenna) I was able to detect it over 4,000 ft away. The receiving antenna was a 2m ~1ft high antenna magnet-mounted on top of my car.

__In retrospect, I should have run Argo at my apartment and drove the transmitter farther and farther away. I presume that my transmitter is functioning decently, and that if I attached it to a proper antenna (and had a better receiving antenna) I might be able to get some cross-town distance? I’m still learning - this is the point though, right?

This is where I was when the signal died. The red marker (upper right) is my apartment where the transmitter was, and the signal began to die right as I traveled south on Chickasaw past Lake Underhill (~4000 ft away). This immediate loss may be due to the fact that I passed under power lines which parallel Lake Underhill which interrupted the line-of-sight path between my 3rd story apartment balcony and me. If this were the case, supposedly if I kept driving south the signal may have improved.

A Foot in the QRSS Door

I’ve been very busy over the past couple weeks. Last Thursday my boss approached me and asked if I could work over the weekend. He wanted to complete and submit a grant by the deadline (Monday at 5 pm). To make a long story short I worked really hard (really long days) on Friday, Saturday, Sunday, and Monday to accomplish this. Monday afternoon when it was done (at about 4 pm), after which I went home and collapsed from exhaustion. I don’t know how my boss does it! He worked on it far more than I did, and over that weekend he didn’t sleep much. Anyway, in exchange for my over-weekend work I got Tuesday and Wednesday off.

I knew in advance that I’d have two days off to do whatever I wanted. I prepared ahead of time by ordering a small handful (I think 4?) of ATMEL AVR type ATTiny2313 chips from Digi-Key at $2.26 per chip. They arrived in the mail on Monday. Unlike the simple PICAXE chips which can be programmed a form of BASIC cod from 2 wires of a serial port, the AVR series of chips are usually programmed from assembly-level code. Thankfully, C code can be converted to assembly (thanks to AVR-GCC) and loaded onto these chips. The result is a much faster and more powerful coding platform than the PICAXE chips can delivery. PICAXE seems useful for rapid development (especially if you already know BASIC) but I feel that I’m ready to tackle something new.

I built a straight-through parallel programmer for my ATTiny2313 chips. It was based upon the dapa configuration and connects to the appropriate pins. To be safe I recommend that you protect your parallel port and microcontrollers by installing the proper resisters (~1k?) between the devices, but I didn’t do this.

I decided to dive right in to the world of digital RF transmission and should probably go to jail for it. I blatantly violated FCC regulations and simply wired my microcontroller to change the power level given to a 3.579545 MHz oscillator. The antenna is the copper wire sticking vertically out of the breadboard.

These crystals release wide bands of RF not only near the primary frequency (F), but also on the harmonic frequencies (F*n where n=1,2,3…). I was able to pick up the signal on my scanner at its 9th harmonic (32.215905 MHz). I think the harmonic output power is inversely proportional to n. Therefore the frequency I’m listening to represents only a fraction of the RF power the crystal is putting out at its primary frequency. Unfortunately the only listening device I have (currently) is the old scanner, which can only listen above 30 MHz.

Remember when I talked about the illegal part? Yeah, I detected harmonic signals being emitted way up into the high 100s of MHz. I don’t think it’s a big deal because it’s low power and I doubt the signal is getting very far, but I’m always concerned about irritating people (Are people trying to use Morse code at one of the frequencies? Am I jamming my neighbors’ TV reception?) so I don’t keep it on long. Once I get some more time, I’ll build the appropriate receiver circuits (I have another matched crystal) and install a low-pass filter (to eliminate harmonics) and maybe even get a more appropriate radio license (I’m still only technician). But for now, this is a proof-of-concept, and it works. Check out the output of the scanner.

Something I struggled with for half an hour was how to produce a tone with a microcontroller and the oscillator. Simply supplying power to the oscillator produces a strong RF signal, but there is no sound to it. It’s just full quieting when it’s on, and static noise when it’s off. To produce an AM tone, I needed amplitude modulation. I activated the oscillator by supplying power from the microcontroller with one pin (to get it oscillating), and fed it extra juice in the form of timer output from another pin. The fluctuation in power to the oscillator (without power-loss) produced a very strong, loud, clear signal (horizontal lines). I wrote code to make it beep. Frequency can be adjusted by modifying the timer output properties. The code in the screenshot is very primitive, and not current (doesn’t use timers to control AM frequency), but it worked. I’m sure I’ll write more about it later.

Thoughts from Future Scott (August 2019, 10 years later):

What a good start! But what a bad design =P

Driving a can oscillator's power pin with two microcontroller pins is not a good idea. Also, you were SO CLOSE to getting frequency shift keying to work! Rather than turning the can oscillator on/off with the microcontroller, just leave it on continuously and send a microcontroller pin to the can oscillator's VCO pin. I'm sure I didn't know what that 4th pin does when did when I originally wrote this (and most diagrams of can oscillators online leave that pin disconnected).

Either way, I'm happy this day happened - this was the start of years of hobby radio frequency circuit design!

Puxing 777 Radio Headset Schematic

I successfully created a speaker/microphone/transmit button circuit for the puxing 777 which probably works for all puxing radios. Instead of simply using circuits found on other websites (always for other radios), I decided to reverse-engineering an earphone/microphone headset that came with the radio to determine how it worked. I can’t claim that I’m an expert in electronics theory, but I can say that I faithfully rebuilt the circuitry within the factory-shipped headset and it worked. The result allows me to leave my handheld radio in its charger while casually listening/transmitting with a button that I made instead of having to reach around and awkwardly squeeze the transmit button on the side of the radio. Once again, I built this circuit and it was successful for me, but there may still be a better way to do it.

The microphone is a 20-cent electret microphone with no special modifications. The speaker I used is a standard 8ohm loudspeaker with no special modifications. The switch is a keyboard-style (push-to-talk) switch, and the capacitor I used is good for 10nF.

If you have any ideas for improvements, let me know! I’ll post some photos once I have my completed little “base station” set up. My ultimate goal is to turn an el-cheapo handheld VHF radio into a decent desktop transceiver by combining it with a nice antenna (located on a balcony at 30ft) and a convenient, easy-to-use switches/buttons/microphone/speaker/etc on a desktop panel.

Code Practice Oscillator and Keyer

Over the last couple weeks whenever I had the time I’d work on creating a little Morse code keyer. After a few different designs I came up with the winner. Basically it just uses a bar of aluminum which rocks on a metal pin. Thumb-screws on each side of the balance point (fulcrum?) can be adjusted to modulate the distance the paddle has to go down to be activated, and how high the paddle goes up when released. A couple springs (one pull-type and one push-type) help give it a good bounce between keys. Two knobs control volume and frequency. I especially like the ability to control the frequency! A capacitor inline with the speaker helps smooth the output a bit too. It’s not professional, but hey - for a couple bucks of parts I made a functional keyer and had fun doing it. Now I guess I should put more time into learning Morse code…

Thoughts from future Scott (August 2019, ten years later)

Wow this is rough! I'm 90% sure this is based on a 555 circuit. lol @ the use of Jenga blocks. It looks like the wire was sourced from cat5 cable. That aluminum slab later became the base and heat sink for an IRF510-based push-pull amplifier.

Scott is Licensed