Building Your Own Glowing Cyber-Balls?

krezel asks: "So I've been drooling over the Ambient Orb, a cool little gadget 'glowing ball' that you changes colors based the 'health' of things you specify. It can do stuff like fade from red to yellow to green as your stock portfolio improves. However, being a poor college student I can't afford its $200 price tag. I've found lots of sources for super bright multi-color LED's. Cast a couple of them in some translucent resin, hook them up to a power source, and you've got yourself a cheap glowing ball. But I've yet to find any good information on how to build hardware that will let me control relays for devices like this through my serial or parallel port. Basically I'm looking for a cheap way to build a board that will let me control 4-8 relays (for each color) over my serial port, and some info on how to write the software for it. This could be a very cool project, and I plan on making the plans available, and the code Open Source, when I'm done with it. Any ideas?"

Comments

Use parallel port or microcontroller

[AaronW]

One easy method of doing what you describe is with some simple microcontrollers. Years ago I worked with Motorola 68HC11s using a serial interface. If you want to control LEDs, you should be able to hook them up directly without requiring relays. You could even adjust brightness by pulsing them quickly in software. Many modern microcontrollers have built-in serial port support as well as embedded flash and RAM.

Of course controlling 8 relays or LEDs with the parallel port is much simpler.

Since the parallel port output is basically just TTL levels, just buffer it through a 74LS244 or something similar and use that to drive the LEDs directly. You can directly control each of the 8 data pins on a parallel port by writing directly to the base I/O port (i.e. port 0x378 is the default for LPT1). It's easiest to use inverting output with TTL driving LEDs.

Something like the following circuit:

D0 ---|>---/\/\/\---| D0 = parallel port data pin 0
|> is a buffer (i.e. 74LS244)
/\/\/\ is resistor
| (+5) is a 5 volt power source separate from the parallel port.

Make sure that the ground pin of the parallel port is connected to the ground of your circuit. For the 5 volts, a 7805 is a simple solution when using a separate DC power supply.

All of the above listed parts should be available at your local Radio Crap.

When D0 is 0 (low) current will flow from the 5 volt supply, through the LED and resistor and from the buffer to ground. When D0 is 1 (high), no current will flow.

When choosing a resistor, take into account the voltage drop across the LED. Blue LED's typically have a higher voltage drop than red or green. Red LEDs are typically around 0.7 volts whereas blue can be upwards of 3v.

Also make sure that whatever buffer you use can sink the appropriate amount of current. Most LEDs typically will take up to 15-20ma of current. It might also make sense to use an inverter instead of a buffer since the above circuit will cause a LED to light when the data bit is 0. a 74LS04 is a cheap easy-to-use inverter chip that is readily available.

With 20ma of current, choose a resistor based on the voltage.

Use the basic equation, V = I*R, where V is voltage in volts, I is current (in amps) and R is resistance in Ohms.

For example, for a red LED with 20 ma with a 5 volt source use:

R = (5 - 0.7) / 0.020 = 215 ohms. Since resistors come in standard values, choose the next highest value, i.e. 220 Ohms.

For blue, with a 3.6 volt drop you would use

R = (5 - 3.6) / 0.020 = 70 ohms. The closest match is 68 ohms, but it's usually best to error on the side of caution so choose the next larger value.

One thing you do not want to do is use the parallel port to drive LEDs or relays directly as you could possibly damage it. TTL outputs typically are not designed to output much current and are typically better at sinking current than sourcing it.

Note that I'm no expert on this and I'm sure you'll see better solutions listed here.

-Aaron