Oct 022012
 

I picked up this Tektronix 575 Curve tracer at an estate sale this weekend for $50. It seems to basically work, although if you look closely, you’ll see that the out and back I-V sweeps on the 2N2222 IV curve don’t lay on top of each other.

This is the Tek 575 as as found it in a barn. Seventy pounds of awesome!

Restoring this unit will be my introduction to vacuum tube electronics! Like I need a new project!

 

This looks almost OK, but notice how the up and down voltage sweeps aren’t the same.

Here’s the collector I-V of a transistor as the base current is stepped. The fact that the curves are almost horizontal tells you the collector current isn’t much of a function of collector voltage; it depends mostly on the base current, which is stepped up and down to generate the family of curves.

Here’s the collector I-V curve of a 2N2222 transistor as the base current is stepped. Each I-V curve for a given base current should lay on top of each other.

 

And finally, I shorted the collector to ground, turned off the base stepping, and set the x-axis to display base volatge (should be constant). I don’t get a vertical line.

 

Collector shorted to ground via a 1k load resistor. Should be a vertical line. About 2V of collector sweep.

I’ll document my progress getting this unit back into spec. The fact that it basically works makes me hope that it’ll be a relatively straightforward job.

 Posted by at 5:58 PM
Mar 212012
 

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!
 Posted by at 11:29 PM
Mar 202012
 

As promised, here are the schematics for the simple LED voice transmitter.

Simple two-transistor circuit fun

And here is a short video going through the design of the electret mic amplifier.
 Posted by at 6:59 PM
Mar 202012
 

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.
Here’s the burned up amplified receiver. I ran it off of 9V for weeks before it decided to give up. It gave off a strange death squeal that I heard in the speaker before it went dead. 
It’s not supposed to be black inside. Very stinky when they go boom.
 Posted by at 1:03 PM