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I'm looking at the Spark Fun Big Dome Push button. The spec on the page (and other places selling it) says that the LED is 12VDC. Does this mean that I need 12V as a minimum? Am I not able to just swap out the LED for one of my own (much lower voltage) if I dismantle it? I can't seem to find a datasheet for it.

Ultimately I want to have it connected to my machine via USB - which is why I can't provide it with 12V. My first thought at accomplishing this was to use an ATTiny85 I have connected via a USB to Serial breakout (or one of those USBs with an FTDI chip in them).

Any help would be much appreciated. Or alternately if my method to read the button's input is roundabout please let me know as I don't have an EE background!

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The LED and switch are separate.

The LED says it needs 12 V, this might be because they've connected several LEDs in series, or used a large resistor. No way to find out except to ask them, or buy one and look. If it is multiple LEDs you might be able to cut some tracks and put them in parallel, so they work at 5V

The switch is a 3 contact microswitch, so will be very easy to read from a small microcontroller. Your plan is probably the neatest and quickest way to get a switch to talk to USB.

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  • \$\begingroup\$ Thanks very much! That covers everything. I will buy one and take it apart to see what it holds. \$\endgroup\$ – bean Jan 4 '15 at 21:57
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    \$\begingroup\$ The LED could have a built-in resistor to run directly from 12V. \$\endgroup\$ – Andrew Morton Jan 4 '15 at 22:02
  • \$\begingroup\$ I didn't see this before. Thanks! Yes. This agrees with what Gipsy Danger's said. \$\endgroup\$ – bean Jan 4 '15 at 22:24
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    \$\begingroup\$ More likely it's 4 LEDs in series, to light up the big button, so about 8 V drop, and then a resistor for 12V operation. So it won't glow at all at 5 V, without rewiring. But you can let us know when you've taken it apart. \$\endgroup\$ – tomnexus Jan 4 '15 at 22:30
  • \$\begingroup\$ Ahh. I had not thought about that. I assumed it was one. I'll see when it arrives \$\endgroup\$ – bean Jan 4 '15 at 22:37
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Most LEDs have a similar voltage drop (usually somewhere between 1V8 and 3V3 but it can be out of this range in specialist devices) so this would suggest that the designers of the switch have integrated a series resistor that has been selected for use with a 12V supply.

Looking at the link you included in your post it looks as though the button can be dismantled so if you could get to and replace that resistor, you could make it work with a 5V (USB) supply but changing the LED itself won't make a great deal of difference.

Hope this help.

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  • \$\begingroup\$ Thanks! I see. Do you know what their reason would be for including this resistor for a 12V supply? Spark Fun's a hobbyist site and I didn't think this was a normal voltage for hobbyists! \$\endgroup\$ – bean Jan 4 '15 at 22:14
  • \$\begingroup\$ A lot of dev kits will use a 12V input and then have on board power distribution to generate the various other voltages needed, this is now a lot less common but because it was the standard for so long a lot of people have 12V supplies. \$\endgroup\$ – Gipsy Danger Jan 4 '15 at 22:19
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The LED is has a resistor setting the optimum current at 12V

5V will likely make it glow dimly, 10V will look good and 12V great.

If you want to connect it to a computer over USB consider using a USB to rs232 adaptor these provide about 12V DC under software control on three outputs, (which will be good for blinking the LED) and have 5 input pins. which will respond to 1 to 20V signals, in addition to this serial ports can generate interrupts potentialy removing the need to poll the I/O pin.

Else it should be possible to parallel another resistor with the existing resistor to make the LED operate well at your desired voltage.

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  • \$\begingroup\$ Thanks! I see. I was not aware that I could change the V-out over USB. But I don't think I want to do that as I need a constant voltage for the ATTiny which I'll be getting to talk back at me via USB. \$\endgroup\$ – bean Jan 4 '15 at 22:40

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