I should be working on the Sparkfun AVC, but this is fun.
I scaled up the LED driver so that it runs a Luxeon Rebel at 400 mA! I don’t even have to point the Rx at the LED any more: I can pick up a signal from the scatter. I’ll definitely have to try this system over a long range.
 |
| I modified the circuit to run at 12V, 400 mA with a TIP31. That power resistor is overkill, but it’s what was in the junk box. |
 |
| Too bright to look at! |
As promised, here are the schematics for the simple LED voice transmitter.
 |
| Simple two-transistor circuit fun |
Heres a photo of the Tx and Rx system. The Tx is on the left and runs off of a 9V battery. The Rx on the right runs on 3AA cells. The 9V power worked great for about two weeks until the photodiode decided to give off a big puff a magic smoke.
Schematics coming soon!
 |
| Here’s a Tx and Rx pair. The first prototype of the voice modulator is installed on the Tx. |
 |
| It’s not supposed to be black inside. Very stinky when they go boom. |
Here’s a quick demo of the microphone input lightwave transmitter. Circuits details are coming next!
Here’s a short video showing you how to make a optical tube assembly out of standard hardware store PVC pipe fittings. It works great for the surplus 54 mm diameter lenses that I got.
I’ll be writing up the whole system over the next few days. A single transistor current modulates a LED and I receive the signal with a PVC pipe collimator and a single-chip photo-diode and transimpeadance amplifier. Audio amplification is done with a Radio Shack amplified speaker.
In previous testing, I achieved a great link over 600 feet with both the Tx and Rx being handheld instead of tripod mounted, but testing time was limited due to the fact that I had a pizza cooling off in the truck and I had to get home. I think I can set up a >1km path from the deck to the road on the other side of the lake.
I built a copy of the MIT/LL coffee can radar last year, but I’m just now getting serious about documenting my results. The coffee can antennas work fine if you build them exactly as described in the course notes, but I wanted to explore a bit more. Although I do have an antenna analyzer, it’s only for the lower ham bands and won’t help me tune up a 2.4 GHz antenna, so I built a simple reflectometer to look at the reflected power coming from the coffee can antennas. Here’s the video.
I got this HP 33C at a garage sale (plus another one for five bucks total!) and there was a lot of corrosion damage on the inside.
 |
| They don’t make them like they used to. |
 |
| All of the flex traces were hosed and quite a few flex-to-PCB connections were bad too. |
 |
| Never throw away tools. Fix them. |
– Update: Digilent got back to me and they are sending out a replacement board. Very quick and positive response. Way to go Digilent! –
I was planning on reviewing and the new chipKIT Max32 board and comparing performance to the Arduino Mega 2560, so I downloaded the modified Arduino environment and downloaded the blink demo to the board. Everything seemed to be OK, but there was no blinking light. This is pretty much the simplest program on the Arduino and all it does is toggle digital pin 13 high and low with a two second period. Most Arduino boards have a LED on pin 13.
 |
| The chipKIT LED driver for pin 13. They don’t load down the I/O pin. Nice. |
 |
| Q2! Where are you? |
What a bummer. Q2 is missing! I sent a note off to Digilent’s tech support and got a response in just a few minutes. I’m hoping that this is a one-off mistake and that they aren’t saddled with hundreds of bad boards. Did somebody forget to load a reel? I wonder who is making the boards for Digilent.
Anyhow, how about some benchmark action?
Let’s look at the time required to invert a 5×5 single-precision floating point matrix. The time for an Arduino Mega 2560 is 3.2 ms while the chipKIT only takes 260 us. That’s a speed up factor of 12.3. Not too shabby.
My intention for the chipKIT board is to upgrade Tobor’s brain for next year’s Sparkfun AVC with minimal effort. I can drop a Tobor shield onto the chipKIT, keep all of the Tobor code unmodified, and add a ton of processing headroom. That’s the theory, at least. Not all of the Arduino libraries are working on the chipKIT. Currently, (I think) I2C, interrupts, and servo need work.
I’m a big fan of lock-in amplifiers; I’m always scanning eBay for a good deal. Recently I got a pair of EG&G 5207 lock-ins. Both were busted. One seemed to have ended up in the auction chain due to some bad buttons and the other one booted up with error messages. I bought both cheap and was able to Frankenstein a working unit of of the two of them. The main issue was the sticky buttons on the good unit causing the control computer to be unresponsive. That was the unit I saved. All I had to do to get it working was to swap in the good switch array board from the unit with the bad computer. Lots of screws and standoffs, but it wasn’t too bad.
Here’s the organ donor:
 |
| I might be able to get this one working too. I’ll have to fiddle with the CPU card on the far left. |
The organ donor had cards in it for two frequency ranges: 0-20 kHz and 0-100kHz. I pulled the broadband card and put it into the working unit. Even better, the local reference oscillator card from the donor was moved over to the new unit too. So now I have nice single phase lock-in with two front ends optimized for audio and 0-100kHz inputs. I’ve got almost all of the available options in the good unit now.
How well does it work? I’m feeding a 90 kHz 1V peak-to-peak sine wave into the lock-in reference and signal inputs. The correct RMS voltage is 0.354 V. It’s not too far off for a 24 year old instrument that I saved from from the junk pile. I haven’t tested the GPIB or RS232 interfaces, but I bet they work too. I’m not sure how to calibrate the thing, and I’m not even sure I want to try. A few percent absolute accuracy is fine and I’ll mostly be doing relative measurements anyhow.
 |
| All that’s left to do is clean the front panel. |
Plans? Well, I’m going to try building a phase-switched radio interferometer so I can do some radio astronomy.