TinyCircuits Forum
TinyCircuits Products => TinyDuino Processors & TinyShields => Topic started by: dtekumse on October 13, 2016, 05:11:39 PM
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Hello TinyWorld :)
Could somebody, please, explain me how LEDs in USB TinyShield (used for TX and RX indication, designated as D1 and D2 in the schematic) were chosen?
Namely, those are amber - which means that the voltage should be around 2 V. The resistor is of 1 kOhm and the supply voltage is 3.3 V. Now, the current through the LED in this case is:
I = (3.3 - 2) / 1000 = 1.3 mA
This seems like quite a low current for a LED... I looked through some LNJ packaged amber LEDs, and almost all of them require 10 mA... so, how does this actually work?
Thank you in advance.
drm
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I believe the leds on the USB shield are using 5v (from the USB cable) not 3.3v
The TinyDuino has a 3.3v regulator on it that steps the 5v down to 3.3v.
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The schematic for the current version is available at https://tinycircuits.com/products/usb-tinyshield , the source for the LEDs is the low power 3.3V regulator built into the FTDI chip. TinyDuino does not have a built in 3.3V regulator, it runs at 5V if 5V is supplied(through a diode). TinyScreen+ and future ARM based boards do have a 3.3V regulator.
As far as the amber LEDs drm, you're right about what you've noticed and caluclated with respect to the forward voltage of the diode and the approximate current passing through it. However there isn't really a 'minimum' current for an LED- usually in a datasheet you'll see a maximum and a test current, but you can pick a lower current that still results in visible output- we found that a standard 1K resistor made the LED visible, so we went with it.
Thanks,
Ben