Here's some stuff I've built. Email me if you're especially interested in some of them, if you need more detailed information, or if you build a similar device for yourself.
I built a magnetometer using a Honeywell HMC1002 dual-axis magnetoresistive sensor. An LM324 quad op-amp was used in a magnetic feedback circuit (feeding into the offset-pins of the HMC1002) to amplify the signal, which was digitized by an Analog Devices AD974 A/D-converter (a 4-channel, 16-bit, 220 kSPS direct-conversion device, total overkill for this application), which interfaced with the parallel port of an old 486 laptop. Data was read as fast as the parallel port could serve it, and 1-minute averages were calculated and stored for later use by Gnuplot.
This circuit is very linear with respect to the magnetic field, and extremely sensitive. It is also extremely sensitive to temperature fluctuations, so just opening my front door in winter time looks like a tremendous geomagnetic storm. Also no set/reset circuit is implemented, which really should be done to maintain the sensitivity of the magnetoresistive sensors over time. I'll try to find time to do that, and to rebuild the analog section to make it less temperature sensitive.
For the time being, here are some sample magnetograms measured with the above circuit.
|I built this simple, stupid, ugly and extremely heavy equatorial platform for Pollux's 200 mm Skyliner telescope. It is made of particle board, stands on four adjustable feet (hockey pucks stuck onto lengths of threaded rod) and uses a stepper motor to slowly turn the telescope as the Earth rotates. The stepper itself turns a threaded rod held in place with ball bearings, and it is coupled to a strip of polystyrene with matching hot-embossed teeth, bolted to the side of the round particle board disc.|
|The hand control unit is housed in the chassis of an old AUI–10base2 Ethernet transceiver, and it has an on/off switch and pushbutton switches for slewing forward or backward. Here is the schematic and here is the firmware for the PIC16F84 microcontroller, which does all the heavy lifting.|
I built a dedicated "Transmit Request" switch into the VersaTuner,
which interfaces to the ACC port on the back panel of the Yaesu
The ACC port is a standard 3.5 mm stereo jack, and it accepts a
negative-going ALC (automatic level control) signal on its tip
connection to allow an external device to limit the output power.
There is also a transmit-request signal on the ring connection,
which when grounded causes the radio to transmit a carrier for tuning purposes.
The sleeve is, of course, ground. I built a circuit which produces
an adjustable negative voltage (to set the transmit power) and activates
the transmit request on the radio. The switch also has a bright LED so
I can't leave it transmitting by mistake.
The circuit is built ugly-style directly onto the switch itself.
Here is the
switch with its internal
LED (the round green pushbutton in the above photo) is
mounted on the VersaTuner's front panel, and a 3.5 mm stereo jack
is mounted on the back. A 3.5 mm male-male stereo audio cable is used to
connect to the radio's ACC port. Since the ALC voltage is negative, the
station power supply cannot be used to power this circuit. A 9 V
battery is used instead. The trimmer potentiometer is adjusted so that
the radio's transmit power is suitable for tuning. A zener diode is used
to keep that power constant even as the 9 V battery dies. A
half-assed attempt is made to avoid RF on the wires (thus the ferrites
and bypass caps).
With this circuit, tuning is easy: Turn on the TX-Request, tune the antenna, and turn off the TX-Request. Construction and adjustment of the circuit were trivial, only drilling the mounting holes was somewhat tedious, as I had to cover up the switches, air variable caps and other stuff to keep the metal shavings out. Also, the vibration seems to have broken the backlight bulb of the meter. I replaced it with a white LED.
|I use an LDG Z-100Plus Autotuner in portable operations. To tune, the tuner can request the radio to transmit a continuous carrier, but this is transmitted at full power, causing needless QRM and wear on the tuner's relay contacts. This modification provides a negative-going ALC voltage to the radio's ACC port to limit the carrier power while tuning.|
I love operating portable with the FT-897. With internal batteries it
is also very convenient. But I never was too happy with Yaesu's FNB-78
NiMH-batteries. Especially since the batteries often saw long periods
of disuse, their internal resistance was often high. I never could use
more than 10 watts power on SSB. So when the batteries finally died,
I had to decide what to do: replace them with new Yaesu batteries (or
cheaper clones), rebuild the old battery packs
with new cells, or something else? I finally settled on using
lithium-polymer (LiPo) batteries,
the same kind that are commonly used in radio controlled model airplanes
Read more here about the batteries, safety
! ! ! UPDATE ! ! !
|After the bad experience with LiPo, I went back to NiMH chemistry, and built an external battery pack out of low self-discharge NiMH D-cells. The pack contains two independent sets of 11 cells each, so it's like the original Yaesu internal NiMH packs but with higher capacity.|
Here is the schematic. (Sorry there are no component values. Figure them out yourself, or email me.) The circuit uses a 12 V relay which powers its own coil until the battery voltage falls below a certain threshold, set by VR1. At that point, the relay switches off, disconnecting all circuitry from the battery. While the relay is energized, regulator U2 draws a constant current from the battery, which is simply lost as heat (the chassis of this device, taken from an old AUI–10base2 Ethernet transceiver, is used as a heat sink). A suitable power resistor would work just as well, but I didn't have any on hand, so I used a regulator instead. A LED flashes on the discharge device as well as on the radio, as the battery is being discharged.
The relay has four switches driven by a single coil. One switches the
discharge circuit, two others switch various sensing lines used by the
battery charger. Thus CONN1 (at the end of the cable) connects to the
battery pack, and CONN2 (on the chassis of the device) is for connecting
the battery charger. Pressing switch S1 begins the discharge cycle, and
when fully discharged, the charger is connected to recharge the pack.
(If the charger is not attached, the battery is only discharged.)
Sorry I don't have the pinout of the Yaesu FNB-78 battery pack on hand, but if you can't find it on the Internet, email me and I'll peek inside the discharge device which pins need to be connected.
|The Butternut HF9V vertical HF antenna is extremely popular among amateur radio operators. I built a copy of this amazing antenna out of aluminum tubes, sheet metal, wires, hose clamps, miscellaneous hardware, and some surplus doorknob capacitors I bought from eBay. The total cost of the project was less than one fifth of the cost of a factory-made HF9V. Details on this project are on a separate page.|
|Quadrifilar Helical Antennas (QHAs or QFHs) are extremely omnidirectional, perfectly circularly polarized antennas that are easy to make and look good. Here is an account of building VHF and UHF versions of this antenna, dualband operation, simulation results of various configurations, and a program to create NEC2 models of QHAs.|
|For portable operation on 2 m or 70 cm, I built this travel Yagi out of a fishing rod. The lower sections of the rod are the mast, and two telescoping sections are the boom. There are five elements on 2 m and eight on 70 cm. The beam is fed by a single coax, with only a choke balun matching it to the 2 m radiator.|
I once acquired a broken spectrum analyzer for free. I still don't know
the condition of its "high band" (2.75 GHz and up), but
the low band of 0–2.9 GHz came to life just by replacing the
front-end mixer with a cheap one from Mini-Circuits. Details are
on a separate page.
Next I must try to build a tracking generator. I'm designing it around a PLL circuit to directly synthesize a signal for mixing with the analyzer's 1st LO signal.
|A current clamp is sometimes more convenient for measuring large currents (like the tens of amps going into an amateur radio transceiver) than an inline current meter. I had used a Fluke i310S current clamp at an earlier job—it had no readout display, just an analog output cable for connecting to an oscilloscope or multimeter. I wanted the same kind of analog output for the cheap Uni-T UT203, so I modified it myself. Now it has both a digital readout, as well as a 10 mV/A analog output.|
The Behringer Eurorack UB502 mixer (apparently identical to the
Behringer Xenyx 502), which I used as a volume control
between my PC and my Genelec 1029A speakers and as a separate headphone amp,
had stupid volume controls: The
headphone volume was affected also by the "Main mix"
fader. So if I turned the "Main mix" fader to zero in order
not to annoy my neighbors with loud music, the headphones would
go silent as well!
I wanted to control the amplifier's volume (via the
"Main mix" fader) and headphone volume
independently, so I did this modification
on the mixer. Now the "Phones" fader controls the headphones, and
the "Main mix" fader controls the stereo amp.
Later I replaced the UB502 with a t.Mix MIX 802 mixer (apparently identical to the Tapco MIX.60) which I also modified the same way, because I was using it for the same purpose—independent volume control of speakers and headphones.
Later I replaced the MIX 802 with a Behringer MX802A mixer, and of course I modified it as well. That modification was a bit more involved, because the component reference designators in the schematic did not match those on the board!
The Behringer Eurorack MX802A mixer has two AUX sends on all
four mono channels and both stereo channels. The AUX1 sends are wired
pre-fader, the AUX2 sends are post-fader. I wanted them all to be
pre-fader, so I can route audio more freely, i.e. route audio
to either AUX (which could go to a soundcard input or a ham radio),
without having it end up in the main mix as well (which would mix
together soundcard or ham radio output instead).
The MX802A does have provisions to make the AUX2 sends of the four mono channels pre-fader. That modification is quite official and it is explained in the user manual. But I wanted the same modification also for the two stereo channels, which was not provided for! Thus I had to devise my own modification.
I did a similar modification on my t.Mix MIX802 mixer, which only has a single AUX send, and it is of course post-fader, whereas I wanted it pre-fader.
|The Behringer Eurorack MX2004A mixer has standard pre-Eq Inserts on its eight mono channels. I wanted to convert half of these into post-Eq Inserts. This enables using four mono channels as independent microphone preamps with equalizers, by taking the signal out via the Insert connector but feeding nothing back. Or they can still be used as Inserts, feeding the audio signal (or some completely different audio signal) back into the mixer. Read more about this modification.|
|The t.Bone freeU Twin HT 823 is a cheap wireless dual microphone system, which is otherwise quite ok, but has ridiculously low volume—low enough that SMPS noise was distinctly noticeable in the background. As there's no AF gain control on the microphone transmitter's outside, I went inside to find it. The audio quality improved quite a bit!|