Electronics Under construction

[sbelle]

I've been consulting GrandPrix Race Central just about every day while my construction of the electronics described on this site is in progress. I'm having a great time building this. The biggest problem so far is that my local Radio Shacks don't have much inventory of components. I've had to hit three different stores to gather up the needed LEDs, terminals, and phototransistors (I'm still short one of those!).

I've also discovered that Radio Shack has discontinued the 270 ohm resistors (catalog 271-1314) needed on board 2. They do have 220 ohm available, which I believe will do the job, but there is a bit of an unknown there. The specs for the green LEDs on the RadioShack web page (2.2V, 10mA) are different than the specs for the green LEDs given on the package itself (2.1V, 25mA). I'm tending to believe the packaging, which should mean that the 220 ohm resistors will be enough. I'll find out for sure in the next day or two...

[gpraceman]

I've been consulting GrandPrix Race Central just about every day while my construction of the electronics described on this site is in progress. I'm having a great time building this. The biggest problem so far is that my local Radio Shacks don't have much inventory of components. I've had to hit three different stores to gather up the needed LEDs, terminals, and phototransistors (I'm still short one of those!).

I've also discovered that Radio Shack has discontinued the 270 ohm resistors (catalog 271-1314) needed on board 2. They do have 220 ohm available, which I believe will do the job, but there is a bit of an unknown there. The specs for the green LEDs on the RadioShack web page (2.2V, 10mA) are different than the specs for the green LEDs given on the package itself (2.1V, 25mA). I'm tending to believe the packaging, which should mean that the 220 ohm resistors will be enough. I'll find out for sure in the next day or two...

Radio Shack used to be a good place to get components and get help with building electronics, but not any more. You can also try looking in the phone book for a store that actually specializes in electronic components.

If the specs are different on the packaging, then you need to use this formula to calculate the correct resistance to use:

R = (V - VLED) / ILED

VLED and ILED are the voltage and current requirements of the LED, respectively and V is the voltage of your system (probably 5V). Current is in Amps, so you will need to convert the current value from milliamps (multiply by 0.001 to get Amps).

Use the resistor value closest to the calculated value, without going under. In some cases, you may need to combine two or more resistors in series to add up to the resistance value that you need.

[sbelle]

Radio Shack used to be a good place to get components and get help with building electronics, but not any more. You can also try looking in the phone book for a store that actually specializes in electronic components.

If the specs are different on the packaging, then you need to use this formula to calculate the correct resistance to use:

R = (V - VLED) / ILED

VLED and ILED are the voltage and current requirements of the LED, respectively and V is the voltage of your system (probably 5V). Current is in Amps, so you will need to convert the current value from milliamps (multiply by 0.001 to get Amps).

Use the resistor value closest to the calculated value, without going under. In some cases, you may need to combine two or more resistors in series to add up to the resistance value that you need.

I finished up board #2, using the 220 ohm resistors. I did the calculation on the resistance before buying the 220s, knowing that they would be enough IF the specs on the green LED package were correct. I wired in one red and one green LED 20 minutes ago and everything works correctly -- nothing blew up!

[gpraceman]

I finished up board #2, using the 220 ohm resistors. I did the calculation on the resistance before buying the 220s, knowing that they would be enough IF the specs on the green LED package were correct. I wired in one red and one green LED 20 minutes ago and everything works correctly -- nothing blew up!

I'm glad it worked out. I would trust the specs on the packaging, not their website. They may change vendors on a part or use multiple vendors, and each vendor may make the part with slightly different electrical characteristics, so the specs on the packaging should be more trust worthy.

[Da Graphite Kid]

Sbelle,

You can overdrive (run at a higher voltage) an LED just like you can a standard bulb. Just like a standard bulb they will be brighter but burn out quicker. But... (and you just knew there had to be one!), with the life of an LED being 150,000 hours, so what if you used 1.5X or 2X the current and only got 100,000 to 50,000 hours out of one! So I wouldn't worry about too much voltage on the LED - just too little (it would be dimmer).

By the way, I've had the same experiences with Radio Shack - no stock and the parts specs differ. So I've started buying on-line. Try BG Micro or DigiKey for your small quantity electronic needs.


Da Graphite Kid

[John Shreffler]

If anyone is building their own electronics for PWD, and having trouble finding LEDs, resistors, ICs, caps, etc, please drop me an email. I have a lifetime inventory that would make Radio Shack wince. If I have it, it is yours, all I need is the postage.

I also dispense solid design tips on circuitry for free.

I spent 40 years developing circuits that work, and am semi-retired. These days, I make and sell The Judge.

John
shreff@pressroom.com
www.newdirections.ws