Tuesday, December 25, 2012

Stop Casting Porosity!

Volpin makes it look so easy.

The difference is between being able to stumble through all the steps, and being able to do them with confidence and control.  I made a better box mold and a much better slush-cast for the Medkit body, and the lid fit first try.  But there is a bit of alignment still needed and I did have to Dremel out the lid a bit to fit the electronics.  Which means I'm far below the level where I would be offering a kit to other people!  Heck -- my panel lines are still barely acceptable.

 Next project I have to mold I want to try a jacket mold.  I'm tired of going through so many bucks worth of silicone, and the mold boxes I've been doing are barely good enough to keep the molds from folding over, turning my boxes into parallelograms. 

I'm finally seeing the end of this.  Still waiting on custom-printed knobs from Shapeways, have another couple lines of code to make the capacitance-sensor triggered "injector" look right, and of course solder some minimal components into the Medkit.  Oh, and print out the data plates and so some touch-up paint.

Which sorta of segues into an attempt at a schematic for the CBR kit:

I can't vouch for the accuracy of this.  I didn't exactly draw up a schematic before I started soldering, nor did I document that well during the project.  Mostly I opened up the appropriate datasheets and made the appropriate connections on the spot.

In the upper left is a basic 7805-based regulator.  I left the power switch off the diagram.  Below it is the 6-pin programming header that works with my in-system programmer.  Left of center is a minimal Arduino, essentially; 22 kHz resonator, power and ground.  There's actually a .1 uf filter cap across the power leads I didn't bother to draw in.  And some people hold RESET down to ground with a 1 meg resistor but this didn't seem to be necessary.

The important part is the Supertex HV5812, which I found at Mouser; a 20-channel high-voltage driver designed for use with vacuum fluorescent displays.  Like the Soviet-made, old-stock ILC1-9/8 eight-digit, seven-segment display I am using.  I found those on eBay and they were shipped from the Ukraine (cheap, too!)

Above the HV5812 is a Recom DC-DC converter also picked up at Mouser.  It works, but just barely; 24 volts is low for the VFD, which really wants 30-60V.  It also wants a voltage-controlled AC filament voltage, but a small resistor works well enough for this application.  To the right of the Recom a power Darlington (the venerable TIP120) is operating as a switch to (over) drive the poor speaker with nasty square waves.  No attempt at a DAC or waveform generator of any kind here.

Two of the AVR pins are being used for a capacitance sensor using the Arduino library (many AVRs will do capacitance sensing natively as well, which is a more efficient way to go if you have lots of pins and want a fast reaction time).  The indicator lights (20 ma LEDs) are all running off PWM channels, and the two control inputs (button and rotary switch) thus are on the otherwise unused analog inputs.  The rotary switch is using a resistor ladder, thus allowing a single pin to detect a larger number of different switch states.

As circuits go, its actually pretty simple.  Or, rather, no part is complex; it is just a melange of different simple circuits.  The HV5812, for instance; high voltage is supplied to Vpp and the regulated 5v to Vdd, but the four control lines (clock, blanking, strobe and data) are just fed any four arbitrary Arduino digital I/O lines.  Wire up which ever is convenient and then assign them the right pin numbers in code.  The outputs are the same; I pretty much wired them one-to-one to whichever VFD pin was nearest, and then solved the assignments in the lookup table.

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