j.c.f.'s schematics.

Last Updated 11/27/00.
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      In the same way that I prepare sheet music, periodically I want something that can't be had or is too easy to build, so I build it.    Sometimes what I build is actually worth telling other people about...

The Gong Clock.

The Gong Clock

      I don't like being violently torn from sleep's dear bosom in the morning and years ago I was fond of snoozing for arbitrary periods before actually getting up, so I formulated a desire for an alarm clock that you could decide to snooze through, one that wouldn't require physical action to get it to stop trying to get one up (the concomitant state of alertness that goes with physical action being exclusive to snoozing) and that also didn't pierce the sleeping haze with the subtlety of a nuclear weapon.    It would have to have a brief and mellow way of signaling that maybe it would be a good idea to get up instead of the outright and persistent demands made by all alarm clocks I had ever experienced.
      So the plans for the Gong Clock were laid, it would strike as large a gong as I could get my hands on, it would strike it at a specifyable strength and at a specifyable period as well, anywhere from once a second to once an hour.

      First an alarm clock had to be found and seeing as I had no idea whether the finished product would be worth keeping I skimped on the thing, big mistake.    Of course once the Gong Clock proved to be an indispensable part of my sleeping I was left with these incredibly irritating chintzy buttons, soo, out with hot meld glue and some better buttons and we have a much nicer clock.    I'll probably replace those buttons one day too, with something like a keyboard key is my guess:

close up

      As I said, this device has evolved over time, the big white tube you see on the mallet was added when a larger gong was obtained and the whole thing needed to be lengthened (in a later shot you can see plywood screwed to the base to extend the upright as well).    The white blob on the end of the mallet is a small circle of industrial felt stuck over another small circle of leather, this strikes the gong in a much more mellow fashion than a plain wood mallet does.    Return springs for the mallet (I've used two with a washer between them) are musical instrument parts I bought from Ferree's Tools one day when buying parts for my flutes, you need some pretty soft springs:

close up

      From the side you can vaguely make out the solenoid placement details, it's embedded in the upright in the rear.    The pull arm is 1/16" stainless steel welding rod, where it passes through the front block of wood I have a nylon bushing (actually a PC board standoff) lining the hole:

side view

      You can see the solenoid mounted in the upright clearly from the back:

back view

      And to cap the whole thing off, it's solid enough to support my right bedroom speaker and the dozen flower vases on top that.    Number 1 two by fours, I'm no carpenter:


Electronic Construction.

      To build the thing you'll have to assemble the following schematic:


      I bought most of the parts from the Newark Electronics catalog, a great way to buy things but it's not cheap, I prefer the convenience myself.    Notable Newark parts include:       To build it I melted a hole into the top of the Bud box and the bottom of the clock, attached some extra line cord to the 110V with the twist nuts already connecting the line voltage to the clock's transformer, ripped out the piezo electric buzzer and routed it's wires and the extra line cord into the Bud box and then glued the two together with some, guess what, hot meld glue -- you can tell I love the stuff.    I show the coarse period selector on the schematic as a series of DIP switches, in fact I used a hexadecimal encoder (50F7509) but that's overkill, assuming you use the values I use for the timing components a value of 7 (A, B, and C closed, D open) will give a half hour maximum striking period.

      There are several considerations in the design of that schematic that aren't exactly obvious:       The "Hard" and "Soft" adjustments are to adjust the strength with which the gong is struck when the Strength setting is at maximum and minimum.    To set them set the Strength potentiometer to minimum and adjust the "Soft" adjustment until the mallet is just hitting the gong, once that's done, set the Strength to maximum and adjust the "Hard" adjustment so the solenoid doesn't drive the gong up against the upright.

The Battery Charging Indicator.

      I got worried that the battery charger for my ancient Brite Lite head lamp on the recumbent bicycle I ride had no indicator that it was in fact charging the battery, a definite concern given that when a lead acid cell is fully discharged it is more than likely damaged beyond repair.   If a wire fatigued or a connector broke (as has happened once already) without me noticing the cell would get flattened in rather short order and I'd have to find a replacement battery.    So I rigged this circuit up:

charging circuit

      The Brite Lite is a 7V system, if you cared to use this on a 13V system you could change the 1k ohm resistors to 2k.    You could also probably use this on NiCad chargers as well.    I use half of an LM339 because that comparator is good at running off a single power supply rail and it allows inputs to be at ground, something most op amps and comparators don't allow (it also happened to be what I had in the drawer).    The inverting stage is necessary because the 339's output is open collector, grr, but hey, the rest of the package wasn't being used anyway.    The circuit is also sensitive enough that when the battery reaches full charge the LED indicator goes out, which is really handy.
      I just hot meld glued the circuit card to the front of the wall wart and then covered the exposed components with more glue ;)    My camera's lousy for detail stuff, this is a good as I can get out of it:


Should you really want to communicate you can eMail to: jforster@someplace.spam.dont.go

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