Simple ad hoc design continues to be useful

I have two large Rubbermaid tubs under my bench containing various testers, filters, RF gizmos, laser pulsers, and stuff like that.

The oldest is a six-pole 88-108 MHz bandpass filter for a quartz-synthesize d FM receiver that I built as a new grad in 1981. It had a key pad, multipl exed 7-segment LED display, fuzzy dice, and everything--all done in 74F and LS TTL, with a dual-modulus ECL prescaler. (just kidding about the fuzzy d ice.) I chucked the rest of it out when I went to grad school in 1983.

Another early one is an 80-MHz VCXO PLL multiplier that was part of my prot otype atomic force microscope in 1987-88.

The one that gets used the most is the beam alignment gizmo I talked about the other day, built in 1990. It's very simple--a germanium quadrant photod iode running open-circuit, an LT1012 dual low-bias op amp, and two edge-rea ding, zero-centre galvanometers arranged perpendicularly to read X and Y.

The sensitivity goes like 1/beam diameter but is independent of laser power on accoun to the logarithmic characteristic of the diodes.

Cheers

Phil Hobbs

Absolutely. And we document experiments and breadboards, whether they seem to be useful or not. Sometimes they turn out to be useful years later. We have 303 folders now in our PROTOS collection.

But my ancient flea-market HP6212 power supply is a fabulous instrument. It goes to 120 volts and both the voltage and current settings have coarse and fine knobs. That's great for testing LEDs and zeners and, yesterday, VCOs.

Oops, the meter is offscale!

Yes, that looks positively ancient and the lighting enhances the vintage look.

One early unforeseen application of my current supply was in reviving old rechargeable batteries. NiCd batteries were still widely used then, particularly 3.6V packs in toys, cordless phones and the like. But where I live, getting replacements was always a big problem and gadgets that were otherwise OK were often discarded due to lack of a battery.

One day I decided to try charging a discarded battery with the current source. At first, the voltage went up to the full 50V and the current was just a tiny trickle, indicating a very high internal resistance. But then the voltage started falling - slowly, taking hours to drop to normal. With more charging, many batteries became quite usable again.

I haven't tried to investigate the electrochemical action that went on in the process of rejuvenation. Perhaps someone here could enlighten us.

Current limit at max. Output shorted. Meter switch set to read current.

Nothing really wrong about that. Of course I use a DVM when accuracy matters.

Max current is 132 mA. The meter is very accurate.

I used to recharge zinc-carbon batteries for my transistor radio when I was a kid. It sort of worked.

(The original Regency 5-germanium-transisor AM thing. It cost $39, a common weeks' pay in those days.)

Oh, yeah. For finding interference sources in a sensitive motion-feedback gizmo, I wound a search coil and soldered it to an oscilloscope-friendly connector. The first one vanished right after I demo'ed it to the guy in the lab down the hall. I think two or three others followed.

If there's not one in my junk drawer now, it's OK; all the parts of one are definitely in my junk drawer. Yours, too, I guess.

An unshielded drum-core inductor is a great mag field pickup.

Radiator, too.

Yes, and at a PPOE I wound another coil and packaged it so it could be clamped to a multiple coax cable. It injected enough test signal to do a bench-verification of an ultrasound head that had a few hundred separate signal channels. Had to arrange a good mechanical clamp for the tests to get consistent results, though.