Here’s something that might help my Grundig G3. Errant AM broadcast band reception on a shortwave radio is often the result of powerful local AM stations overloading the receiver’s front end, resulting in intermodulation products that are demodulated into cheesy AM programming material the discerning shortwave listener has no desire to hear.

Joseph J. Carr has written a number of useful books for radio experimenters, as I’ve said before. An article featuring the circuit can be found here. With just seven parts, this high-pass filter promises to reduce the strength of frequencies starting somewhere around the top of the AM broadcast band, preventing the receiver from being swamped by strong local transmitters. Headroom should be improved, allowing reception of genuine shortwave broadcasts instead of spurii.

Two toroidal inductors wound with 29 turns on T-37-2 cores, and five 102 capacitors. Need toroids? Check out Amidon and Associates. They serve the Ham Radio community. You can also check out the catalog of Philmore. I got my stuff from Philcap Electronics here in Akron, Ohio, where I recently plunked down a few dollars for a UHF transistor.

With an eye toward proper RF construction techniques, I kept lead lengths short and the components in a line from input to output. The SparkFun board I used doesn’t have huge traces (which at RF tend to perform like little antennas) and I’m happy with the clean build. The best way to construct this would have been on a nice copper ground plane, but I wanted to pop this together in an hour and see if it worked. Next I’ll put the filter in a nice metal box with overlapping edges. The finished product will sit between a long-wire antenna and the G3′s external antenna input.

Hopefully this will improve my shortwave listening experience. Nothing is more disturbing than browsing international broadcasts and suddenly being blasted by some blowhard selling gold coins and wrinkle cream.

 TEST RESULTS:

Here the filter passes 3.5MHz, the beginning of the shortwave band, for the sake of argument. Function generator > filter > oscilloscope. This output signal level is the same all the way to the top end of my signal generator, a Hewlett-Packard 3314A, which tops out at 20MHz.

Watch what happens at the top end of the AM broadcast band:

1.5 MHz, the top end of commercial AM. Wimpy. The filter is attenuating frequencies below about 2MHz by about half, or 3dB. And finally, at 500kHz:

Wee, wee wee!!! Stick a fork in the AM BCB as far as the receiver’s front end is concerned. And all this with a passive filter. Who would have thought that a couple coils of wire and some tiny capacitors would have such a dramatic impact on the passing of frequencies? To be honest, the red iron-powder toroids were a little tough to come by, but using a toroidal inductance calculator I found on the web, the creation of precise little parts was not only possible, but immensely gratifying.

Exploration of filters and op-amps is among the most boring of electronics endeavors when it is done from book learning. Being told that a particular combination of components will subtly affect incoming signals in particular ways is a sure route to a stack of books one will never read again. But building filters and working with op-amps is very rewarding when results seen match formulae and predictable results. If you work with Arduinos or other microcontrollers, the payoff in this field is fast. Within a development environment, microelectronics can be made to behave in predictable ways. But really getting into the details of analog electronics and creating circuits whose parameters are measured with extreme difficulty (or not at all in the case of RF) and finally having results stick is the key to understanding that digital electronics are fundamentally analog.

I do have some concerns that the energy from the function generator is not terminated into a 50-Ohm load, that the readout of the oscilloscope is not normalized for a rounded mV level, and that there isn’t an accompanying graph of mV-frequency response. Sometimes throwing down a circuit and having it behave as advertised on the tin outweighs the science; learning all about how it does what is does is the next obvious step.

Next I’ll detail the construction of an active preamplifier from one of Mr. Carr’s books. I built it on a breadboard, now it’s time to finish it and stick it in an Altoids box. Stay tuned, and free from spurii.

Here’s my schematic for a Sony remote-controlled audio volume using a pair of AD5220s. As they don’t like to be fed voltages outside of their operating range (0-VCC) I added a 100K resistor to each input so that the audio can float between the 100K fixed resistor and the 100K digital potentiometer.
After figuring out what the remote’s heretofore useless volume controls were sending, I had the PIC look for one of two button codes. Both digital pots then increment or decrement, giving across-the-room controllability to my speakers.
At this point, it’s an in-line design, but it’s easy enough to replace an existing volume control knob. I just wanted the flexibility of still being able to use said knob when I wanted to. Otherwise, leaving it in the middle of its range gives fine results.
This shouldn’t be used in a critical “performance” audio situation. When the buttons are pressed, sweeping the digital pots through their range, there is audible zipper noise during the changes. Brrrripppp…. Apparently there are some zero-crossing detection schemes that can eliminate this, but I’m happy enough to control audio levels from my couch now.

The basic circuit is credited to Dr. Jan Kazula, and the amazing Melanie at the MELabs PicBasic forums explains it well here.
About a year ago I constructed a three-button remote for a solid-state relay project. Press a button, and a code is sent via RF to a receiver inside an outlet box that switches whatever is plugged into the corresponding outlet on. Neato. But all the while the remote is sitting on the coffee table, the microcontroller and the voltage regulator are drawing power. How could I reduce power consumption without putting a dumb on/off switch on the remote? Who ever heard of a remote that you had to switch on before using it? I thought of including a mercury switch, or a capacitive touch-sensing chip, or letting the microcontroller nap every few microseconds, but none of these would do what I wanted the circuit to do, which was to make as little use of the battery as possible.
As soon as I saw the basic circuit, I knew there had to be a way to build it out for three buttons. Of course, this scheme is used inside all sorts of equipment that has to draw nothing while it’s sitting, but the process of figuring it out for myself was an incredible way to learn. Now I have a solid understanding of the circuit, and can explain how it works to other people.
So it works like this: When any of the three buttons is pressed, Q1 turns on. This powers the microcontroller while the button is still held down. The microcontroller’s first job is to pull the base of Q2 HIGH, providing a path through Q2 to keep Q1 turned on. Then the microcontroller polls the buttons to see which one is low, performs a task (like sending a radio transmission), and then pulls the base of Q2 LOW, turning everything off. The microcontroller senses a HIGH at the other button inputs because they’re pulled up with 12K resistors, but a pressed button provides a path from sensing inputs 1, 2, or 3 to ground.
I was surprised to find that an ordinary 7805 regulator attached to a battery with no load is sitting there sucking power. What on earth for? The answer is that (at least in the ON Semiconductor version) there are 22 active transistors inside, as well as a whole bunch of passives like resistors and Zener diodes. Just attaching it to power, there’s a basic bias for the device, and the IQ, or quiescent current of the thing is around 5mA. This makes an ordinary regulator a very poor choice for a battery-operated thing, especially if it doesn’t have an on/off switch.
So once this circuit worked, I tried out a new regulator I just got from Mouser, the 2950CZ-5.0 low-dropout regulator. Current consumption on my meter went to nil, but I knew it was drawing something. That something turned out to be around 75 uA, or 75 millionths of an amp. Much better. In looking at other power source schemes for the project, I found that the NCP-1400 has a quiescent current draw of 1.5uA to 15uA, and this step-up regulator is probably going to be the final choice for something that has to sit a long time between uses.

HAHAHAHA!! Look what I got. Two dollars well spent for a part I’ve spent five years looking for. Yeah, it’s sort of stupid. The LM3909 used to be widely available and was used to flash an LED or oscillate in the audio range, driving a speaker. Please don’t go looking for this at Radio Shack. This is what is known as N.O.S. – New Old Stock. Somebody bought it and sat on it like an egg for twenty years, and since nothing hatched, they sold it to me.

The back of the package is better than many RS parts today and has a pinout as well as the following text:

Features

- Operates over a year on one “C” battery

- Bright, high-current LED pulse

- Requires only a capacitor and LED for 1.5VDC flasher

- Low-voltage operation (1.2 to 5V)

- Low current drain, less than 0.5mA

- Powerful; as an oscillator will drive 8-ohm speaker

Absolute Maximum Ratings

Power dissipation: 500mW

Power Source: 6.0V

Operating Temperature: -25′C to +70′C

So what, you ask, am I going to do with it? Granted, one could make a blinky or audio oscillator so many different ways. The coolest thing about owning this chip is the ability to construct some of Forrest Mims’ projects. After all, it’s immensely encouraging to throw down on the breadboard when more complicated projects are not going well. I also have a circuits cookbook with page after page of recipes for this chip. There’s a 5-transistor discrete-component circuit floating about on the web, but this chip is a cool part to have in the collection.

Okay, this is good magic. Visit www.musicfromouterspace.com and build yourself an electronic noisemaker. You will need seven 1-Meg pots and some other odd components, but the end result is a lot of fun. Sorry about the cell-phone photos on the site; sometimes I just have to get posting, quality-be-darned.

Mr. Ray Wilson gives the schematics for a number of cool sound projects on the site, and I was only able to build a good version of this when I had the proper components. The WSG is meant to be built with larger panel-mounted pots, but I wanted to see the thing through without going to the trouble to make a permanent version. I dunno. This is pretty cool, I just may get around to finishing one.

Here’s just a quick example: wsg-test. You can hear that it does all sorts of deep and dirty phasing stuff, with one oscillator modulating another. There’s also a nifty filter at the output stage that can alter the cutoff frequency and resonance of the generated sound. This will be cool to play with further. It’s especially good at juicy bass tones.

This has got to be useful for somebody. Here’s an Eagle template that I’ve started using for my PIC 16F876A projects. I put it together because I couldn’t find one that was done already. This file is NOT a PDF. Once you download it, change the extension so that the filename is “16F876A.sch” and it will open in Eagle. Right-click on the following file and save it to your computer, modify the extension, and then have at it in Eagle. 16f876a

The 5-pin header is correct for the MELabs U2 programmer, and the schematic was taken from the manual. The diode is present to protect VDD from the high programming voltage present at the /MCLR pin when the device is programmed in-circuit.

I can’t offer any sort of warranty for this template; I just know it works for me. Consider it open-source, and feel free to modify it for your purposes.

Here again, a circuit answers a specific need. A tall, skinny, dark hall closet was in need of some illumination – and apparently a semiconductor. I had purchased a $9 garden light set that had four powerful LED pairs and a beautiful solar panel. Since four deep shelves needed light, I figured I would mount the LED fixtures and install a roller-type switch so that when the door shut, the lights would go out. I put the solar panel aside for some other cool project in the future and harvested some other neat components that comprised this set: long wires that terminated in DC barrel jacks, double-ended DC plugs on long wires, plastic covers for the jacks, a battery case, an SMD charging circuit. This was truly a lot of stuff for what I paid for it. I did end up throwing away the cast spikes that were meant to be stuck in the ground outside. There truly was no place I could set them without being poked.
At 12V with suitable resistors for each LED leg, the circuit drew 80mA, and I could have called it a day. What disturbed me was that the resulting presentation was that of a refrigerator light appearing to be on all the time. What I wanted was an indication to the user that Magic was taking place, so I enlisted an IRF510 power MOSFET and a small electrolytic capacitor.
When the switch is engaged, the .47uF begins to charge through the 4.7 Megohm resistor. A small amount of current is withdrawn by the 4.7 Meg leading to ground, but that part has its place later. The capacitor is still able to charge up to the triggering voltage of the MOSFET, and the drain-to-gate channel is opened, producing a nice fadeup for all the LEDs. When the closet door is closed, the capacitor still has charge, so the light stays on momentarily, until that charge is drained away by the second 4.7 Meg resistor to ground.
These values can easily be monkeyed with, and I did a fair bit of monkeying. A larger value for either the charging/discharge resistor or the electrolytic will produce a longer fade time, but in this case the time was determined by “feel.” The most important thing was the user interface, and so I decided to work with the values until I had achieved the Magic. Of course, the time could have been calculated by R times C, with the goal being the triggering voltage of the MOSFET, but I enjoy plugging components into breadboards more than I do scratching pencil on paper.
The result is a circuit that has been all but forgotten, drawing no current while dormant, waiting to play back its simple light show when the next person opens the door.

After finding a Nikon N4004 and two decent Nikkor lenses at a Goodwill store for $22, I dug out my Speedotron studio flash equipment. While everything worked fine with the long cable for triggering the strobes, I wanted to see if I could imitate the Pocket Wizard system using a few dollars’ worth of parts.

This isn’t exactly a simple thing to do. The TX-RX pair I have was discontinued at SparkFun, so I had to dig out the datasheet for the units. One requirement of the system would have to be reliable triggering. Another issue has to do with timing. The strobes need to fire while the shutter is open, not while it’s closing. The third important detail is that the receiver needs to have a contact-closure element that can deal with the 70VDC trigger voltage at 43uA. This element should be faster than a relay, so ultimately a device will have to be chosen.

One would expect that simply sending data out the one radio would appear at the other. Naw. There is far too much random data that appears at the output of the receiver for this to work. So there has to be a microcontroller on both ends. For reliability of asynchronous serial, a crystal oscillator should be employed, or at least a ceramic resonator.

I chose a repeating data stream so that if the first were missed by the receiver unit, there were several more chances for the beastie to fire. Above is the output of my Saleae Logic, a device I like a lot. This transmission was receiver over-the-air and is a faithful replica. The only problem is that the transmission of “ABC1″ takes .017S in this version. Seeing as that’s about the same time the shutter is open, I need to seriously bring that down. I typically use a sync speed of 1/60S and that’s the most generous situation for firing the flash. The TX-RX units have a maximum baud rate of 4800bps, and I’m currently sending 2400. This needs to change. Also, I need to determine the lag time between sending and receiving, and I’m having trouble when the grounds of the devices are connected and I apply the logic analyzer to both units. I’m currently using a 4MHz oscillator, and this could be bumped up to 20MHz.

So there’s a lot of overhead to chop out at the moment. I send a pulse to adjust the AGC on the receiver, and that too is adding time. I think the most reliable solution for what I want would be a Bluetooth or other solid link, but that’s going to cost about as much as the PocketWizard system. I’ll post more as things progress. I need to get out my bit-axe and chop out time so this works. After all, film is rather expensive.

The TDA2003 is a good old amp-on-a-chip. According to the datasheet, it’s meant for car audio, with an output power of 10W. Here it is in bridge configuration, with an output of about 20W. Audio at line level goes in at the 1/8″ jack seen at the bottom, feeds the amp via a 10K pot for a volume control, and the blue screw-terminal thingy at the top connects to the 8-Ohm speaker.

The four diodes to the lower right were meant to drop some of the power from a vehicle battery to subsequent downstream electronics; the TDA2003 can handle a voltage up to 18VDC (40V peak), but other attached stuff didn’t really want to see over 12V.

So for about five dollars in parts, this is a pretty decent utility audio amplifier.

Here’s an amazing three-transistor circuit that broadcasts whispers to an FM radio in the house. This design is awesome. What makes it so special is the buffer transistor before the antenna; the output stage doesn’t load down the preceding electronics. The circuit can be found at www.talkingelectronics.com, run by an incredible Australian named Colin Mitchell.
It’s horrifying how well this circuit works. I was half expecting the spooky van to pull up in front so I really didn’t use it much. In time I’ll post some more circuits. I mostly concentrated on receivers, because it was a magical experience building circuits out of a handful of parts that brought the world inside. You don’t need an expensive commercially-made product to participate in radio communications.

Edit: So what’s the missing part in the middle of the breadboard? That’s where I plug in a tunable inductor. It’s interesting that you can swap these out and tune a large portion of the VHF band. The schematic for this transmitter, “The Wasp” can be found on www.talkingelectronics.com