Archive for the Electronics Category

Car Stereo and Speaker Installation

Posted in Auto Maintenance, Electronics on April 21, 2011 by themidnightengineer

This is a continuation of where I last left off regarding my battery drainage problem. The factory amplifier unit had a short circuit so I had to remove it to solve the battery problem. But without the amp, I cannot listen to any music in my car. To replace it, I could either order it from the dealership or try to find a used one in good, working condition from a junkyard.The first option would cost too much (the dealership quoted over $700) and the second option would take too much time and hassle.

Given these choices I decided to replace the factory installed stereo unit and speakers with aftermarket equipment. I’ve been wanting to do this anyway since my car is 15 years old and the existing stereo unit was obsolete.

I purchased a Kenwood KDC-MP245 CD Receiver from Crutchfield. It has four pre-amp channels at 22 W RMS/50 peak of power each.

I also ordered 2 pairs of Polk Audio db651s 6 1/2″ speakers. These speakers are rated for 6 – 55 W of RMS/165 peak power, frequency response of 50 – 22,000 Hz. and a sensitivity of 92 dB. Sensitivity is another term for efficiency. It is a measure of how well the speaker converts audio signal power into sound as opposed to being wasted as heat. The higher the better.

I also got a 50′ spool of 16 gauge blue speaker cable. When replacing factory default speakers with new ones, there are generally two options for wiring the speakers to the stereo unit: use existing wiring or route new wiring. The first option is less work. The second option means you have to take things apart but the sound quality is better. Specific to the case of my car, the factory component amp was a big factor. The existing stereo fed audio input to the amp. In turn, the amp was driving two tweeters, four speakers, and a sub-woofer. The existing wiring configuration was useless for a four speaker only setup. I went with the second option.

The most difficult part of installing aftermarket car audio equipment is routing the new cabling.

The front two speakers were located in the doors which meant that I had to take the door panels off.

There is a wire harness that goes from the door speaker to a junction box in the interior of the car.

The plan was to thread the new cable through the wire harness. But I had to clip and pull out existing speaker wiring to create slack first.

After that, I threaded through the new cabling. The positive and ground leads were fastened to the speaker terminals with crimping nuts.

Making the back end connections at the stereo unit was pretty straightforward.

Installing the rear speakers involved taking the rear passenger seating off.

Overall, it was a big project that took three days to complete. Because it involves access to electrical schematics/wiring layout, knowing where things are located in the car, some skill in electrical work, troubleshooting, and taking apart many different sections of the car interior, it is not recommended for the faint of heart.

The following videos demonstrate the results.

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Kicad Schematic for Minty Boost

Posted in Electronics, Minty Boost on March 31, 2011 by themidnightengineer

Minty Boost is an open-source hardware design developed by ladyada. It is a battery powered USB charger for mobile devices such as cell phones, including the iPhone.

ladyada used Eagle to design the PCB and had it manufactured by a PCB shop. I don’t like Eagle because the freeware version has limited capability and I don’t want to pay for the full version. My plan is to use KiCad, which is open-source, to recreate the latest schematic done by ladyada, and make the PCBs myself using homebrew techniques. 

My KiCad schematic is shown below.

The design is centered around the LT1302 fixed 5V DC/DC converter with high current output. Next, I have to create footprint modules for a few of the components.

Mysterious Car Battery Drainage Problem Solved

Posted in Auto Maintenance, Electronics on November 10, 2010 by themidnightengineer

As mentioned here, I had a problem where something was draining the juice from the battery in my car. I was able to find out, by troubleshooting the fuse boxes, that it had to do with the stereo system.

Removing the radio unit, however, did not make the problem go away. There was something else that was causing the drain.

After consulting the schematic from my Haynes repair manual and much searching, it turned out to be the stereo component amplifier, a separate unit that was located below the front passenger seat.

The green outline boxes in the photo above shows signs of water stains on the unit.

Opening the unit and inspecting the circuit board inside reveals water damage causing short circuits.

AC/Heater Control Unit LCD Backlight Repair

Posted in Auto Maintenance, Electronics on September 18, 2010 by themidnightengineer

Recently the backlight for the liquid crystal display for my dash-mounted AC/Heater Control Unit stopped working. I disassembled the unit to inspect the innards and discovered that the backlight had been provided by three incandescent lamps soldered on a PCB whose filaments had expired.

These tiny light bulbs were sheathed by translucent green rubber “condoms”. Before I disconnected the PCB from the unit, I measured the open-circuit voltage (~9.7 VDC) across the leads of the bulbs.

I also noted the polarity of the leads using a marker on the PCB. This is because I was planning to replace the bulbs with LEDs. LED is a polarized device, with cathode and anode leads, unlike an incandescent bulb. The polarity of the supply voltage across an LED matters, just like with any diode. I do not know why Toyota never designed this backlight using LED lights in the first place, but being that it is a solid-state device, it is much more reliable and efficient than a filament bulb.

I purchased four 5mm white LEDs from Radio Shack that were selling for $1.99 for a pack of two. These LEDs were rated for a luminosity of 7000mcd. I also bought 270 Ω resistors to be connected in series with the LEDs as current limiters to provide a forward current of 25 mA and a forward voltage of 3.3 VDC.   This link has a useful current limiting resistor calculator for simple LED applications.

The following photo shows one of the bulbs (bottom) extracted from the PCB next to the LED (top).

I soldered the LEDs and the resistors onto the PCB. One lead of the resistor was soldered into the via marked positive. The anode of the LED was soldered into the via marked negative. The cathode lead was fed through the lamp base mount hole and soldered onto the other lead of the resistor for a series connection. I covered the  LED/resistor pair on the right with heat shrink tube to prevent possible short circuiting with an adjacent IC.

This view shows the LEDs on the other side of the PCB after the soldering was completed.

I took the PCB to the car to connect it back to the AC/Heater Control Unit. I turned the ignition half-way to provide power. The LEDs turned on. I could tell they were much brighter than the previous bulbs.

I reassembled the unit and covered the LEDs with the green condoms. This is how the display appears now.

Total cost of this repair job: $5.32