While I pour a toast, here are a few highlights of the past year:
PID Controlled Solder Paste Fridge
The first project I documented on the site, my solder paste fridge was the end result of a weekend effort to turn an old beer chest into a PID-controlled Peltier cooler for storing tubes of solder paste. A year later, the cooler has a permanent home under my workbench and is still going strong, keeping its contents at a chilly 36 degrees F. Besides solder paste, I keep my POR-15 rust proofing epoxy paint and a few tubes of superglue in the fridge (they never dry out!).
Space Invaders! Making RGB video with the PIC
I needed an excuse to learn assembly language programming on the PIC, and this project fit the bill perfectly. Instead of slogging through yet another PIC tutorial I decided to “just do it” and the video above shows the result. One of my favorite projects of last year, I have plans to build more of these and make some electronic artwork for the lab.
Bluetooth Handset Hack
One aging bluetooth headset plus one obsolete telephone handset equals one retro-fabulous hack that I still use today. The best part: Look for this one in Make: volume 20!
DIY PID-Controlled Soldering Hotplate
I’m a big fan of the hotplate (aka reflow skillet) method of surface mount soldering. Over the course of a few months I designed, machined, and assembled this PID-controlled soldering hotplate to help build the first few prototypes of my AVR HV Rescue Shield kit. Hacking around in the garage is always fun, but creating a new tool is one of the most rewarding things I have can think of.
Here’s a video of the hotplate in action, reflowing the step-up converter on the Rescue Shield:
The AVR HV Rescue Shield
What started as a simple hack to save a crippled AVR microcontroller eventually became a kit that I’ve sold to AVR enthusiasts around the world. The AVR HV Rescue Shield includes a cool custom PCB, integrated 5V-12V step-up power supply, and is completely open source. I only made one batch of these, and when they’re gone, they’re gone, so head over to the AVR HV Rescue Shield product page to order one today!
The vintage telephone speaker I used for my Bluetooth Handset project has a mysterious component wired across it’s terminals, as shown above (it’s the black cylinder with two leads on the upper left). The component measures as an open circuit on my DMM, but obviously it has some hidden function.
So far my guesses are:
A lightning arrestor/spark gap or back to back diodes that protect the person using the phone from strikes or other high voltage on the line
Some sort of lowpass filter to keep you from hearing otherwise audible tones used for signaling
A device that keeps the high voltage ring signal from damaging the speaker
Kylie recently gave me an old, broken Motorola HS820 bluetooth headset. The headset suffered from a defective microphone that resulted in extremely low volume on outgoing audio, even though everything else worked fine.
Upon receiving it, I proceeded to rip the headset apart, interested to see what was inside. I found a fairly simple PCB with a discrete bluetooth module in the center. The PCB is not labeled, but given that this is an older headset (3-4 years old) most of the connections are large enough to attack with a pencil iron and solder wires to.
This headset was begging for a project.
On my last trip to Weird Stuff, I came across the beauty shown below. The instant I saw it (and three other boxes full of others like it) I knew exactly what I was going to do with the broken headset: make it into a Bluetooth Handset instead.
Shown below is a vintage International Telephone & Telegraph telephone handset. ITT manufactured phones in the 60s and 70s; later they spun off that part of the business to Alcatel and then got into some trouble with the government in 2007.
The handset has some heft to it and feels great to use. It is amazing how accustomed we have become to using extremely ergonomically poor cellphones. Just compare the shape and size of a modern cellphone to a handset like this and you can understand why it is so refreshing to pick up and use one of these.
They don’t make them like this anymore. Unscrewing the faceplates reveals a speaker and a microphone which falls out onto the floor if you’re not careful. I remember phones like this when I was very young but hadn’t seen one in years.
Sticking the guts of the HS820 into the handset was not that difficult or time consuming. The speaker works as-is. It turns out the impedance of the speaker that came with the bluetooth headset is around 30 ohms, while the vintage handset speaker is 42 ohms, close enough. The audio quality is excellent!
The original microphone on the HS820 was an electret, which is not the same as the carbon style on the handset. Despite this, I was able to get the microphone on the handset to work by adding a 1k series resistor and wiring it to the same terminals on the headset as the original. Without the resistor, my voice was too loud and distorted. The value took some experimentation and I may continue to play with it, or eventually give up and install a modern electret style mic instead (but the original is just too cool).
I added a pushbutton to replace the multifunction button on the headset. Holding down this button turns the handset on and off. Pushing it answers calls and probably does other stuff that I haven’t played with. The headset has volume buttons too, but the volume can be controlled via software so I didn’t wire them up to anything.
I also added a charging jack where the cord originally was. The washer is needed to fill the relatively large opening for the cord.
A ridiculously bright blue LED is wired in place of the status LED on the headset. I found the LED holder in my junk box.
This shows the connections for the pushbutton (yellow and green wires in the center) and the LED (red and black in the bottom center). I removed the original pusbutton with my hot air rework station and soldered the new wires in place, then put a dab of hot glue on top to keep them in place. The big blue thing in the middle is the bluetooth module. I imagine that modern headsets do not have a discrete PCB for this. The HS820 PCB tucks inside the center of the handset when installed and stays in place without any special mounting.
The other side of the HS820 PCB shows the 3.7V lithium battery and the connections for the microphone (lower left), speaker (lower right) and charger (upper right). The battery is glued to the PCB.
I modified the Motorola charger that came with the headset by cutting off the original 3 pin plug and replacing it with a 4.7mm power connector to match the jack on the phone. The third pin of the original connector wasn’t being used anyway.
To charge the handset, you just plug in the charger. The LED lights up to show that the handset is charging and goes out when it’s done. It flashes during normal use, blinding spectators.
Here’s a video of the handset in action (thanks Kylie!).
I’m planning to use this for Skype on my desktop computer, but I may just have to carry it around for a few days to see what other people think of it…
Update: As mentioned in the comments, these are for sale at ThinkGeek.com, search for “retro handset”.
Update 2 (11/20/09): Welcome, Make: readers! Questions about this project? Head over to the forums for help!