One of the projects I’ve been considering for a while is trying my hand at building some crystal radio receivers. Crystal radios are pretty simple and traditionally use a germanium diode as the detector element because of its low forward voltage drop.
Found several people selling 1N34A germanium diodes on eBay and ordered a batch of 100 a couple of weeks ago. They arrived in the mail yesterday, and today I got around to checking them out. The diodes themselves are unlabeled so there’s really no way to tell what they are by looking at them.
The forward voltage (Vf) drop of 1N34A diodes is supposed to be around 0.25V. According to my DMM, a random sampling of the ones I got showed a Vf of 0.29-0.30V. I figure that’s pretty close.
For comparison, the Vf of some random 1N4148 silicon diodes was around 0.6V.
Now to do some homework and see how to go about building a crystal radio receiver.
My first attempt at building a circuit using the ugly construction technique. It’s supposed to be a simple oscillator circuit using a J310 transistor.
One of the advantages of ugly construction is that if you’re working from a schematic or circuit drawing, building is pretty easy. I found that soldering components to the copper clad required a bit of patience, because it’s essentially a very large heat sink. Put the soldering iron on the copper clad, add solder until you get a good sized pool, leave the soldering iron in place and place the component.
For this particular circuit, Vcc is applied to the big resistor with the free lead and output is off the capacitor with the free lead. I soldered on a piece of wire to make the ground connection easier. Haven’t applied power to test it out yet. Will see if it works later on.
After getting a bit of advice from some locals about soldering, I thought I’d get in a little more practice with the SA602s and the breakout boards. It was also a good excuse to see how my new glasses are for working at the bench.
To get a little more room to work, I switched to a conical tip on the soldering iron and soldered the board onto the header pins first. This makes for a pretty stable platform to work on.
Add a little bit of flux paste, tin one of the pads and then tack on the SA602 to the tinned pad.
Then flip it around and solder a pin on the other side.
Then solder the rest of the pins, making sure not to leave the soldering iron on the board too long, and waiting a few seconds between soldering each leg to let things cool down a bit. A method that I found worked pretty well was to place the soldering iron tip on the pad, apply a touch of solder to the tip, push it towards the pin, and then draw the tip back along the pad.
I soldered a total of 4 SA602s onto the breakout boards. Here’s the result of about an hour’s worth of practicing.
Using the same method, I soldered one of the SA604 chips onto the SOIC-16 breakout board. Although the SA604 is about the same width as the SA602, just longer, the SOIC-16 board is quite a bit larger than the SOIC-8 board. Having the header pins on the same side as the pads gives you a little less room to work with as well. Still, soldering the SA604 was pretty easy.
My attempts at cleaning off some of the residual flux left some cotton fibers behind from the swab I was using. I don’t think it will affect how these work, but I’ll spend some time trying to clean them off.
I think I’m getting the hang of this now. Next, learn how to use these.
These are surface mount ICs, so I ordered up some SOIC-8 and SOIC-16 breakout boards from Adafruit.
The package from Dave arrived earlier this week, and the breakout boards arrived in the mail today.
The breakout boards came bubble-shrink wrapped which was a little unexpected. I thought they’d just come loose in a ziplock bag or something.
The boards are double sided, with SOIC-8/16 spacing on one side and TSSOP-8/16 spacing on the other side.
Interestingly enough, the SOIC-8 boards are half the thickness of the SOIC-16 boards, which are a regular thickness circuit board. Both have sets of holes (standard 0.1″ spacing) to solder header pins to, making them convenient to use in breadboard projects. Or you could just solder wires to them if that’s what the project calls for.
Off to the workbench to do some soldering. For SMD parts, these are actually pretty large, and soldering is relatively easy. First, tin one of the pads and then with tweezers, line up the IC and then heat up the tinned pad. Use the soldering iron to push the solder towards the IC pin and you’re done.
I used a toothpick to apply a little bit of flux paste to the rest of the pads, and then soldered the rest of the pins. That part goes pretty easily with flux.
Pro tip #1: Don’t do this with the board clamped in a vise, or if you do, don’t leave the soldering iron on the board for too long. These SOIC-8 breakout boards are pretty thin and too much heat will make them melty. Oops.
Once you’re done, it’s time to add some header pins. This part is easy. Stick the header pins into a breadboard, put the breakout board on the header pins and solder.
Pro tip #2: In your enthusiasm to solder, don’t forget to pay attention to where Pin 1 of the IC is supposed to go (ignore that bad solder job on Pin 2…easy to fix). That stripe on the SA602 should be where the 1 is printed on the board. Oops.
The panel mount potentiometer I used in the CPO was missing the nut needed to secure it into place. After determining that I needed a 1/4″-32 thread hex nut, I went scouring my Radio Shack and hardware store bins looking for some.
At Radio Shack, I found a bag of assorted hex nuts in their parts bins, with the right thread number, but none of them were smaller than 5/16″. Next stop was a big box hardware store. There I found 1/4″ nuts, but the finest thread they had was 28. The hex nuts they did have with a 32 thread were all #10 or smaller.
Then I went to a couple of the smaller box hardware stores. Much larger selection of nuts and bolts, but still the same problem as with the big box hardware store. Nothing with a finer thread than 1/4″-28, and nothing larger than a #10-32.