kevtris.org

…and now for something completely different! I got interested in super-regenerative receivers a few weeks ago, investigating ways to do simple, cheap radio links. Turns out it’s REALLY easy to do a few hundred feet with only a few parts. The secret words are “Super Regenerative”. A single transistor acts as an oscillator and detector and everything else, all in one shot. It’s an incredibly clever circuit, but it’s also old- older than even the super heterodyne. I won’t go over how it works- I will leave that up to Wikipedia.

My design isn’t 100% original, I poked around on the net for superregen circuits and then cribbed some ideas and came up with this circuit. It uses a second oscillator to quench the RF oscillator, which performs the detection of the signal. I used a schmitt trigger for this, setting it up as a simple RC oscillator, then used the approximate sawtooth produced on the capacitor to push the RF oscillator in and out of oscillation. This quench frequency is around 50-70KHz depending on the setting of the quench frequency. (This is adjusted so that it doesn’t cause a beat note with the 19KHz stereo pilot)

So, that’s what it is. Everything ended up fitting very neatly into an altoids tin. I soldered a piece of metal from the top of an RF shield to act as a battery holder. There’s a few wire loops soldered to the sides to hold wires, and I used some molex connectors to make a neat “install”. Unfortunately, the pictures aren’t very great.

Oh yeah, almost forgot. The coil is 4 turns on a 1/4″ form of #22 or so wire, tapped at 1 turn. Tap is near the ground end. Other than that, building it shouldn’t be difficult. If you cannot locate a suitable varactor diode, a variable capacitor will work… around 60pf max would be good. It’s hard to see, but I built it on perfboard, with the solder pad side up. The blank side of the perf is flat against the bottom of the tin for good shielding of the circuit. I used some resistor ends to attach the PCB to the bottom of the tin by soldering it. (Altoids tins are super nice to solder to, it takes the solder readily and easily due to the thinness of the metal). I used a mix of SMD and through hole depending on what I had to jump. The 74C14 is soldered in SIP fashion which ended up working very well.

View of the inside.

Another inside view.

Aaand the schematic. I forgot to make the part designations a darker colour. Sorry ’bout that. I will eventually get a better version.

Welll, I worked some more and got the power supply designed, built, and assembled. Everything seems to work on it so far. The 5V and 12V outputs, both 2.8V linear LDOs, 5-12V converter (for USB in) and the battery charger seems to charge. The PCB was designed to drop in on top of the first PCB and it fits around the connectors and things perfectly. There’s some openings to allow for placement of the JTAG connector and stuff.

The supply board’s all plugged in and ready to go. It mates with a 10 pin power connector, a 2 pin signal connector and a 2 pin USB power connector. I will use standoffs and screws to join the boards eventually so they don’t come unplugged.

Top view semi closeup

Bottom view semi closeup.

The supply on this synthesizer had to fairly complicated. It had to perform the following functions: run off of 4 recharagable AA cells, run off of a 9V DC line lump, and run off of USB power. On top of that it has to charge the batteries from USB or the line lump. This is a fairly difficult thing to achieve for several reasons. The biggest problem is the output voltages (5V and 12V). The input voltage to the 5 and 12V regulators could vary between 3V (discharged AA rechargables) all the way up to potentially a bit more than 12V. So this means that the input voltage could dive UNDER the 5V output or over the 12V output… causing one supply to go out of regulation.

The solution was twin SEPIC converters which can handle this condition with ease. Running the two SEPIC converters are the two major inputs (USB power and line lump power “OR’d” together using diodes, and battery power). Both of these major inputs pass through P-channel FETs which can turn one source on at a time while isolating the two sources completely. This is done with two P channel FETs per input, wired back to back so the body diodes cannot both forward bias at the same time. I used two dual FETs for this function.

The battery charger simply connects between the power input (line lump / USB) and the battery input.

A PIC micro controls all functionality. It runs on 2.8V so it is quite low power. The PIC will run all the time, waiting for the user to press the power button if it’s totally “off”, and will control battery charging (it can control charging rate to accomodate various operational modes), control powering up of the unit, and it can take temperature of the batteries and internal environment to accomplish quick charging of the battery pack.

I have a “one wire” type interface between the PIC and the FPGA for communications. This will send USB operational modes up, and key presses and battery state, etc. down to the FPGA. There’s a second “one wire” interface between the power board and the to-be-designed capacitive touch panel board which will be the final piece of hardware for this unit.

So I spent another week or so, and finally got all the power supplies mounted, power sequencer done, and everything connected up on the board so far. I made a power control/sequencer dealie so that I can turn the supplies on 2 at a time. This will prevent me popping the circuit breaker if I tried to turn all 16 on at once. The caps charging would suck a dip in the mains big enough to cause the lights to dim in the city I think. Other than that the pics are fairly self explainatory I think. Everything seems to be working and tomorrow I will mount the remaining 6 LED boards, and try it out on a full array. Note that the bottom LED boards show the data flipped. I thought I had set the bits in the controller but I guess I didn’t. There’s bits on the display signal streams that let my mirror the displayed data along X and Y, but I obviously didn’t set them, so the lower half is upside down. whoops.

Well even though I haven’t posted in awhile, I have been busy! I designed, ordered, and received custom machined brass parts that solder to the LED boards so I can mount them. These brackets allow me to mount the display boards without having anything that passes through the board or anything like that- this is important of course if you want a matrix that looks good.

I spent about a week looking for brass U channel stock to make the brackets, but I came up empty. The “legs” have to 1/16″ thick and spaced 1/2″ apart. This seems to be unobtainum. While I found plenty of U channel, it had 1/8″ thick legs, which wouldn’t work. It *has* to be 1/16th of an inch or else they’d hit chips and routes and stuff which is definitely Bad(tm).
(click ‘em and watch most of the pics grow)

After soldering 5 brackets to all 13 boards I have (12 good, 1 for parts and testing), it was time to mount them. I went to Lowes and bought all the materials needed. plexi, 1 by 6, and two kinds of plywood. The board that everything mounts to is 1/2″ thick plywood. It’s 48*32 inches which is in the “friggin’ huge” category! The entire unit will be about 34*54 inches when done, and under 5 inches thick. Inside will be 12 LED boards, 12 power supplies and the controller along with a blower I salvaged from a microwave oven. I designed the unit so I could mount 16 boards, if 4 more ever show up (anyone got extras? please? but I will only stuff 12 for now. I have one more board than this, but it’s missing alot of LEDs and has tons of problems. I used it for fixing the other 12, and for testing stuff so I didn’t screw up the good boards.

Anyways, enough of that. here’s the pics that everyone wants to see I’m sure:

Well it’s been awhile since the last update. I did alot of work on the FPGA synthesizer thingus. No pics or video yet unfortunately; most of it has been internal work so not much to show. I added FM synthesis and pokey.

Anyways, today’s hotness is LEDs. Lots of LEDs. Bucketloads of LEDs. I obtained a 72*24 RGB LED matrix board with drivers and stuff on it. I spent a few hours and traced it out and got it going. Here’s some pics of the fun.

(click some for bigness)

This is the “first light”. I have some patterns and a frame buffer going, but one of the drivers was hooked up wrong. Probing little tiny FPGA pins with a multimeter can be a pain.

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Not much to say here, just a test.

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Aaand, the other test screen (which is actually a continuation of the above)

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Thought I’d try a cheap “diffuser” (piece of paper)

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Finally, a bit overexposed.

My poor bench supply didn’t have the balls to run this very well, and the output was drooping by almost 2 volts! Next step is to try a small part of the array with 12 bit colour to see what happens.

Weeell, here’s the update. I have been working on this project for exactly 2 weeks now… The software end of things anyways.

Since last time, the PCB has been fully assembled, all the hardware has been debugged/tested/working except the USB part (been lazy on that). Been a whirlwind week of SD cards and FAT file systems and LCD displays!

As it stands, it will boot any SD card I stuff into it, load the firmware off the card into RAM, then run it. The firmware right now can load/display the directory of the card, lets you step through it including subdirs, etc and then load/play SID tunes.

You’d think sorting a directory list would be fairly easy- I spent more time than I want to admit on that piece of code. The unit can load up to 1023 directory entries off the card and then sort/display then in just a few seconds. Most directories are pretty much instantanious. I figured 1000+ entries would be more than good enough for this project. Who will wanna hit “page down” 200 times to see all 1000 files? hehe.

I made a custom font last night that allows me to cram 5 lines of text on the LCD, with kearning so that the text is packed in nice and close without wasting pixels. It can show about 30-40 characters on a line this way.

Also I bought one of those $30 CVS “one time use” camcorders and made it a bit more than one time use by making up a cable and then unlocking it. For more info on the CVS camera thing, check out HERE.

I will include a pic or two some time about my cable I made. Anyways, I mention the videocamera because I used it to make a small video of me looking and sounding like an idiot describing the device You can watch me make a fool of myself on Youtube:

And here’s a few pics of the thinger (clicky for bigger)

That’s it for now, enjoy