In 2008 I started a project, and I never finished it for those who visit this site. I had a working project, but never did good documentation. I have finally pulled through with the final product! In this post you should be able to find: both schematics, both programs, and some pictures. What you won’t find is the circuit boards. I made them using the old fashioned way with Ferric Chloride and some Radidio* Shack press on etching strips. (*intentional misspelling)
Sept. 22, 2009 - Waiting on parts to come in can be boring so I went on a dig through my parts bin and found this LED still in its original packaging. While the packaging leaves a lot to be desired, I actually found the datasheet on RS’ website. I played with this LED and some 1k resistors on a breadboard, and now I’m going to make a night light that changes colors. The original program that I wrote lasts about 15 minutes, and it cycles through all of the colors [edit – at least I thought it would). It uses a rough PWM method that I wrote to fade in and out.
After a little math work I decided on using 20mA for all of my colors. They are rated for 30 to 50 mA depending on the color, but with this I can use the output pins on my PIC12F675. This saves a lot of transistors for another project. Continue reading →
This is a quick post to update any followers of the site. I spent the beginning of the week installing laminate floors in my house. I neglected most of my college course work, and now I’m trying to catch up.
I’m working on a 4 speaker Cigar Box Amp. It’ll make it to eBay sometime next week. It’s a working prototype. The electronics are basically the same except that I’m adding an external power jack, a hard battery clip, and a front cover that may or may not look good. I covered most of the front with fabric to cover the speaker holes. I’ll have pictures up next week in the Cigar Box Photo album. I’m also removing the power switch and using a stereo input jack as the switch, and I’ve added an LED to signify that the power is on.
The suitcase amp is still under construction. I do not like version 1.1, and I’m probably going to scrap the 2 watt version for now. It has a horrible “hummmm” when used with wall-warts, and I probably need to add some smoothing capabilities to the incoming power. I am going to test it out with the LM386 circuit and see how that sounds.
That’s about it for Tech-tut.com’s latest updates. I haven’t said this in a while but remember that “it’s more than just a project.”
This is a quick demonstration of how to mod a 1k ohm potentiometer to be able to work in a region between 0 and 25-30 ohms. The Make schematic for the cigar box amp calls for a 25 ohm rheostat, which is either hard to find or much more costly than it needs to be.
The amplifier circuit from the pages of Make is a very simple one, but I had some issues finding the 25 ohm rheostat that the schematic calls for. This is the only modification to the circuit that I made. I used a 1k ohm pot for the volume, but I modified it with a 33 ohm resistor to get it as close to the schematic as I wanted to. The volume doesn’t completely cut out, but who wants to use an amp at zero volume anyways?
modified amp schematic
The trick to this mod was easy. I just used the parallel resistance formula where total resistance equals the inverse of the sum of the inverse resistances (or the inverse of the sum of conductances). Either way, I know that when the volume pot is at 0 ohms, most of the current will flow through the pot as if it were just a wire. If the pot were to go to infinite resistance, the current path would be through the shunt resistor.
I used my graphing calculator to graph y = 1\((1/1000)+(1/x)). Then I traced the line to find the Y-value (my shunt resistor). I chose the point where x = 25. Y was so close to 25 that I just rounded up. Well, I didn’t have a pile of 25 ohm resistors so I used a 33 ohm resistor.
The volume pot now works a little better. It’s not perfect, but it provides a better range than just a 1k pot.