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| How to wire Illuminated buttons so that they light up when pressed ? |
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| DaOld Man:
--- Quote from: BlueScreenJunky on October 24, 2010, 01:46:09 am --- Thank you ! That's more like what I was thinking about, and thank you Andy for confirming since my main concern was to fry my brand new mini PAC. I will probably try that today and tell you how it went. And yes I think USB from a computer is 500mA max, but the controller in the video is actually using USB to light the 7 buttons and the LEDs from the turntable so I guess it could work ? I have no idea how much the LEDs need. --- End quote --- You are quite welcome my friend. Have you tried it yet? If so, how did it go? Leds will draw around 20 ma each (0.020 amp) The current is decided by what resistor you use, but there is a minimum and a maximum current limit on the LED. Simple (and pretty accurate) formula to use to find resistor needed for LED: RESISTOR SIZE (in ohms) = SUPPLY VOLTAGE - LED DROPPING VOLTAGE divided by LED CURRENT An example using a LED that has a dropping voltage of 2 volts, with Current of 20 ma, and supply voltage of 5 VDC SUPPLY VOLTAGE - LED DROP VOLTAGE= 5-2 = 3 RESISTOR SIZE= 3 / 0.020 RESISTOR SIZE = 150 ohms To find what wattage the resistor needs to be, multiply current by voltage, or in this example 3 X 0.020= 0.06 watts. Just use next common higher wattage, in this case 1/8 watt or higher. (0.125 watts) To find the dropping voltage of your led, either look up the data sheet on the web, or use a digital multimeter with a diode check function. The meter will dimly light the led, and the voltage drop will be displayed on the meter. (may have to reverse meter leads if you dont get a reading). To find current draw, look up the data sheet on the web, consult manufacturer, or a good rule of thumb is 20 ma. Using the formula above, lowering the LED current will make the LED glow dimmer, raising the current will make the LED glow brighter, but remember that the led may burn out when it crosses over its maximum current limit. And running the current higher tends to shorten the life span of the led. Good luck with your project and keep us informed! |
| MonMotha:
--- Quote from: DaOld Man on October 26, 2010, 05:47:08 pm ---To find what wattage the resistor needs to be, multiply current by voltage, or in this example 3 X 0.020= 0.06 watts. Just use next common higher wattage, in this case 1/8 watt or higher. (0.125 watts) --- End quote --- To be clear, it's voltage across the resistor, not total voltage (supply) or LED. You can also take the current, square it, and multiply by the resistor value, if that's easier. Good practice is to make sure your resistor power rating is at least double that of what you're actually asking it to handle. The reason is that they get VERY hot if you run them right at the rating (so hot you'll burn yourself if you touch them) and fail more quickly, but doubling the rating provides so much margin that you don't have to worry. 1/4W and 1/2W resistors are readily available at retail such as Fry's or Radio Shack. 1/8W and 1W+ less so, but they're out there. |
| DeLuSioNal29:
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| DaOld Man:
--- Quote from: MonMotha on October 26, 2010, 07:07:52 pm --- To be clear, it's voltage across the resistor, not total voltage (supply) or LED. You can also take the current, square it, and multiply by the resistor value, if that's easier. --- End quote --- MonMotha is absolutely correct, it is the voltage across the resistor (3 in my example), you use to figure resistor wattage. Thanks MonMotha, sometimes I get in a hurry and leave out the small details. (You should see my life LOL) |
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