Carlitos’ Contraptions

I own a few Rev. C Bare Bones Boards witch came with ATmega168 chips burned with the old Arduino bootloader (NG). I wanted to upgrade them to the newer (Diecimilia) bootloader so I built a parallel programmer following the instructions in the Arduino website and using parts I found in the garbage (as usual).

Materials

• Two 470 Ohms resistors
• One 220 Ohms resistor
• Some wire
• A female header at least 3×2
• A ferrite core (not required but looks cool)
• Some heat shrink (it could be replaced by some electrical tape)
• A male parallel port connector (DB-25)

Programmer Schematics
(this is a vectorized version of the original schematics on the Arduino site)

Construction

I got a DB-25 connector from an extension parallel port of a PC I found in the garbage. First, I removed the original cable that came with it and I soldered the programmer’s resistors to the back of the DB-25 and connected them to new wires. Then, I covered the resistors and cable connections with a piece of heat shrink tubing.

The new wires where terminated in a 4×2 female header (even though there are only 5 wires). The wires and female headers come from an old computer (they connected the power buttons and the case LEDs to the motherboard). I cut the header to size from a larger header (form the same old computer) and I used an extra row to mark it with a white pin cap so I could easily remember where the #1 pin is located.

Finally, I added a ferrite core and twisted the cables in order to reduce noise and cross-talk. I don’t think this is really required but it looks nice.

Project

I’m always very annoyed by shoelaces tips breaking and thus making them very difficult to insert in shoe eyelets.

Being acquainted with electronics supplies in general, I decided to try using heat-shrink tubing to repair my shoelaces tips.

The result? Marvelous. It works like a charm and is really very easy to do and costs nothing (if you already own some tubing).

In order to do it, simply cut four pieces (~2 cm long) of heat-shrink tubing, put them at the shoelaces tip, apply heat, and it’s done. Note that the tubing should be of about the same diameter as the shoelaces.

Besides being very easy to do and looking great, the heat-shrink tips are very sturdy and flexible, so they won’t break again. I wonder why they still use hard (breakable) plastic for manufacturing shoelaces tips.

Also, the fact that I used synthetic shoelaces had no effect at all (I was afraid the shoelaces would melt with the heat). Besides the tubing seemed to slightly adhere to the shoelaces when heated, thus resulting in an even stronger structure.

Finally, note that I should have used slightly shorter tubes and removed the old plastic from the tips (it makes small bumps).

Project

Many devices require batteries but do not absolutely require to be absolutely portable. Since batteries are expensive and don’t last for very long, it could be a would idea to add a wall power adapter connector to such devices. This would allow you to use the electricity from the mains to power a usually battery powered device.

I did precisely that to my new Air Hog helicopter I got for Christmas (I now, I’m a bit old for toys, but although I did not ask for it, I’m got I got it ). This small helicopter is lots of fun, but batteries (at least the rechargeable alkaline I use) last for about two flights (the helicopter must be charges by the remote in order to be able to fly for about 20 minutes).

Adding a power adapter connection to some device doesn’t require many materials. You only need: a power adapter (duh), its corresponding connector, and some wire.

Once you have all materials, simply locate the ground and V+ nodes on the circuit (usually the black and red wires respectively) and solder the connector to them. Then make a nice hole on the device to make it stick out if necessary and you’re ready to go.

Make sure your power adapter matches the voltage of the device and always verify all voltages with a voltmeter. If you exceed the expected voltage levels for the device you can be pretty sure to fry it.

(note the cool paper counterweight on the helicopter’s nose)

Project

What to do when a young child listens to very loud music on his headphones?

1. Educate him: by explaining that he must reduce the time he is exposed to loud sounds depending on how loud they are, and how an over exposure to loud sounds could lead to permanent hearing loss. This is the best approach.
2. Build a headphones volume limiter (i.e. the geeky solution).

I decided to build a volume reducer when my girlfriend told me she bought an inexpensive mp3 player (with no software volume limiter) for her little cousins, and that she learned he always sets the volume as loud as possible when listening to music (which worries his parents).

I had the idea of building a device that would interface with the headphones to the mp3 player (or any other audio device) and that would reduce the volume so it would become impossible to set it too loud.

I see basically two ways of doing this:

1. To build a device that would clip (easiest) or reduce (trickier) the headphone’s signal as soon ans it goes beyond a preset amplitude. I think this can be done with a couple of transistors and would be a good solution.
2. To build a device that always decreases the volume by reducing the headphone’s signal amplitude. This can be easily done with resistors but would waste energy and the volume reduction would vary depending on the headphones impedance.

Since I am a bit short on time (it must be done for Christmas) and since I think educating the child would be a much better solution to this problem, I went with the second design choice: inserting a resistor in series with the headphones in order to decrease the signal amplitude by creating a simple voltage divider.

Materials
This little project requires very few materials:

• A couple of resistors. Their value depends on the headphone’s impedance.
• An old lipstick for the casing
• A piece of perf board. Not really required but helps to make it more robust (it’s going to be used by a child, so it must be sturdy)
• A jack plug and a jack socket.
• A piece of wire.

Construction
Simply solder the jack socket to the perf board (which should be cut to fit inside the gutted lipstick ) and put a resistor in series for each channel (left and right). Then connect the jack plug to the resistors and to ground by using the wire.

I drilled a hole on one end of the lipstick so the jack socket would fit tightly and stick out from it. The wire with the jack plug goes out of a hole on the other end of the lipstick.

At first, I chose to use a 10 Ohms resistor for each channel since I figured that the headphone impedance would be around 16 Ohms at DC. Later, I found out that 10 Ohms wasn’t enough , so I added another 10 Ohms resistor in series on each channel, making the total resistance per channel 20 Ohms.

This means that with 16 Ohms headphones, the resulting volume would be 16/(16+20) = 44% of the original volume.

I know this volume reduction technique is very wasteful in terms of power and not very elegant, but it works fine and makes the very loud sounds just a bit more tolerable.

Finally, I used some isolating tape to make the construction a bit more resilient to children.

Project

Besides my POV display, I’m trying to do a robot that detects obstacles using IR light. In order to make it easy to work with IR light, I built a very simple IR detector based on this article.

General Idea
An IR signal is “seen” by a reverse biased IR diode which produces an electrical signal according to the light intensity. This signal is then amplified trough a transistor which powers an ordinary LED. This means that the LED will light up when the IR diodes receives some IR light (from a TV remote for instance).

Materials

• An IR Diode (I got mine from an old TV set)
• An LED (a yellow one in my case)
• A 1k Ohms resistor
• A 50 Ohms resistor (two 100 Ohms resistors in parallel)
• A BJT (I used the 2N3904, but any common BJT should work with the right biasing)
• An old 9V battery (for the casing)
• A new 9V battery (for power)

The Circuit

Construction Notes
I built the circuit by soldering the components together without using a PCB and enclosed it in an old 9V battery.

I used a battery as the casing because it is free, it looks cool, and it includes a battery clip.
The battery’s cap is actually a battery clip that can be soldered to the circuit in order to connect it to another 9V battery.
When using a battery as a case, keep in mind that:

• the inside of the battery should be insulated (with blue masking take in my case)
• it is a good idea keep one or two of the AAAA batteries (that can be found inside the 9V battery) and use them to keep the top and bottom caps in place (make sure they are insulated two).

Project