Category Archives: Electronics

Low-pass filter, not so much…

The low-pass filter didn’t do the job. There must be a coupling issue between the CD player and the car audio. With the current CD player, I can crank the volume all the way up and just barely hear it on the car audio, if the signal from the RF box is cut. This definitely points to a coupling problem.

One thought I had was to build a Cmoy style amp, using the car +12volts as the power or this Simple Car Preamp. This should provide plenty of power, amplification and the coupling needed. There isn’t much to the circuit, so it could be built inside the car radio or into a car lighter adapter.

Car Stereo AUX In – Low Pass Filter

Low pass filter from WikipediaThe AUX in Jack I added to my car stereo works great with one exception, when the CD player isn’t playing you can hear static, or whatever the radio is tuned to.

I’m not the only one with this problem, but I’m having trouble finding the other sites…
What I want is to allow the voltage from the RF module to the AMP, without the Audio signal making it to the AMP. I want a low pass filter.

I started with an article on Low-pass filters on Wikipedia. The human hearing range is about 20Hz to about 20,000Hz. I want signals lower than 20Hz to pass and higher frequencies blocked. The Wikipedia article gives a formula, but not the units for the calculation.

I found this website, which I used to calculate values of 150 Ohms and 47,000 nano Farads.

I grew up working with micro Farads, not nano Farads, so I did a little bit of searching and turned up this webpage, which indicates that a nano Farad is 1/1000th of a micro Farad. So 47,000 nano Farads (nF) is 47 micro Farads (uF).

150 Ohms and 47 uF are pretty standard values and should be easy to find. I should only need one resistor and one capacitor, per channel, making this an extremely easy project. Once the filter is made, I’ll post the results

AC versus DC

s_fireworks10.jpgAn article on AC vs DC on sparked some interesting comments…

This comment especially caught my attention. In it, the commenter mdpd looks at some basic electrical calculations to show why we use high voltage AC to distribute current around the nation.

The short answer is this: We transmit electricity at high-voltages because it can travel longer distances, over thinner wires. We use AC because it can be converted to a lower voltage on-the-spot, with nothing more than a transformer.

The calculations provided in the comment will be very useful when I start looking at solar-power and/or wind power. I believe there will be some limits to what I can do with 12 volts DC.


In-Car CD Player Holder

180px-cd-ph-in-car.JPGI put together plans for a new project, an In-Car CD Player Holder. It was put together using what I had on hand. Most of the dimensions are not critical. The holder rests between the passenger seat and the center console. There is a tight enough fit that the seat holds it in place. The top could be made a different shape to accommodate an MP3 player, Pocket PC or just about any small device.

The Gadgeteer

the-gadgeteer.pngThe Gadgeteer is becomming my favorite website for new gadgets and gear. Julie takes the time to write most of her own reviews and she looks at current products and accessories.

I found this site when I was looking at the OQO Model 02 and found a review on The Gadgeteer.

I don’t know why I haven’t discovered this site before. The archives go back to October 1997. Well, I’ve added it to my RSS reader so I won’t miss anything.

Cool bicycle Persistance of Vision project

As an introduction to computers, the kids and I have been building the SpokePOV. The kids have been into bicycle projects this summer, so this is a great crossover project.

The Spoke POV is a project I first saw on There is a kit available from

Photo of completed project from

We are using green LEDs and hope ours will be this cool!

We ordered the kit with the extra ROM so we animate the images. We picked up some 96/4 tin/silver solder from Radio Shack. It is lead free, so I don’t mind the kids using it.

The SpokePOV uses an ATMEL ATtiny2313 microcontroller and and has two rows of 30 LEDs. The LEDs blink out a pattern as the wheel spins that the eye perceives as a complete picture (follow the above links for action photos). There is a hall-effect sensor, so the processor can sense the wheel rotation and calculate how fast it is spinning, to create the proper pattern.

So far, we have a completed circuit board. There is a Windows program to create the images and download to the Spoke POV. We’re having a bit of trouble with that.

AUX in Jack for my Car

I have a cool little CD player that I picked up at Goodwill for $3.00. I’ve been using it with a small FM transmitter, to listen to CDs in my car. There was always a bit of static, the CDs sounded flat and recently, a station in my area began transmitting on multiple frequencies, making the FM transmitter almost unusable.

I wanted to be able to plug my CD player directly into my car radio so I stopped into RadioShack and picked up a 1/8inch stereo plug to 1/8inch stereo plug patch cable and a 1/8 inch stereo jack.

I thought the hard part was going to be getting the radio out. Once I popped the trim off there were three screws to remove and the radio slid right out. I had to work the radio around the trim, but was able to get it out enough to remove the top and bottom covers.

Next, I had to find the signal from the radio frequency module to the amplifier portion (I actually did this step in he car and later took this photo on the work bench).

I plugged the cable that I purchased into the headphone jack of the CD player, connected a ground wire from the plug on the cable to the ground of the car and began trying the tip on various connections in the radio. I eventually found the left and right signals. I used my Black & Decker “Wizard” (like a Dremel) to cut the traces.

Next, I cut the circuit board on the face plate of the radio and drilled a hole for the jack. The jack was small enough to fit under the circuit board, but there weren’t any traces on the circuit board where the jack was going so I just got it out of the way.

I soldered wires so that when the patch cable was removed from the jack, the traces were reconnected. When the plug was inserted, the signal from the CD player was connected to the amp side.

When I put it back together and tested it, it didn’t work. The radio played when the patch cord was removed, but the CD wouldn’t play. After some trial and error I found out that radio cut out when the traces were cut. I had to short them together.

When the CD plays, it cuts out the radio, when it’s off, the radio plays and it sounds great. I almost can’t wait to go to work tomorrow, so I can listen to CDs on the way in.

This project was on a 99 Alero with the stock Delco radio.