Nov 24, 2014, 01:40 PM Registered User United States, CA, Santa Monica Joined Jul 2011 498 Posts I am trying to both study how this module works and make a TX. Pretty much I have a RC car remote (FS-GT2B) and what I want to do is miniaturize the TX. I am a little new to radio theory but am very willing to learn!
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 Nov 24, 2014, 01:48 PM Registered User Joined Apr 2012 3,898 Posts I miniaturized myself the TX put everything inside a nunchuck wii. see here. http://www.rcgroups.com/forums/showthread.php?t=2118378 Latest blog entry: DIY Multiprotocol TX Module
 Nov 26, 2014, 06:41 PM Registered User Joined Aug 2010 2 Posts LED1 and R3 I have a question about the PCB/schematics Midelic RX REV F. You use a blue LED with a 1k resistor. Due to my knowledge a typical blue LED has a Forward Voltage of about 3 - 4 V. If you calculate a current of 10mA, the output voltage of the MEGA8 is much to low (should be U = I*R + U_LED = 10mA*1kOhm + 3V = 13V) When using a red LED (1.6V / 10mA) the R1 should be about 150 - 200 Ohm. Am I wrong with my calculation or is it impossible to use this LED / resistor combination?
 Nov 27, 2014, 01:39 AM Registered User Joined Apr 2012 3,898 Posts The color of the led is not important ....I used green, red ,yellow. with 1K..The led is lit and working fine. Latest blog entry: DIY Multiprotocol TX Module
Nov 27, 2014, 08:38 AM
Registered User
Lithuania, Vilnius County, Vilnius
Joined Dec 2006
57 Posts
Quote:
 Originally Posted by snorre.snorre I have a question about the PCB/schematics Midelic RX REV F. You use a blue LED with a 1k resistor. Due to my knowledge a typical blue LED has a Forward Voltage of about 3 - 4 V. If you calculate a current of 10mA, the output voltage of the MEGA8 is much to low (should be U = I*R + U_LED = 10mA*1kOhm + 3V = 13V) When using a red LED (1.6V / 10mA) the R1 should be about 150 - 200 Ohm. Am I wrong with my calculation or is it impossible to use this LED / resistor combination?
Very interesting post. According to the normal specs, you are almost right, but I personally really like the fact that LED turns on much much lower current. Remember that LED is current sensitive, not voltage.
I have noticed that 1 W led (nominal voltage 3-3.3V and nominal current 300-330mA) is shining bright enough using only 100mA ! Actually it shines at about 80% of the max brightness. You can notice that lowering current also lowers power which is dissipated as heat. It means less energy wasted!
I' will stick to the 1 kOm According to the Main DIY law ( U = I*R ) led will draw only I =U/R -> 3V/1kOm= 3mA. Your lipo will say thanks
Nov 27, 2014, 01:43 PM
Registered User
Joined Aug 2010
2 Posts
Quote:
 Originally Posted by montis Very interesting post. According to the normal specs, you are almost right, but I personally really like the fact that LED turns on much much lower current. Remember that LED is current sensitive, not voltage. I have noticed that 1 W led (nominal voltage 3-3.3V and nominal current 300-330mA) is shining bright enough using only 100mA ! Actually it shines at about 80% of the max brightness. You can notice that lowering current also lowers power which is dissipated as heat. It means less energy wasted! I' will stick to the 1 kOm According to the Main DIY law ( U = I*R ) led will draw only I =U/R -> 3V/1kOm= 3mA. Your lipo will say thanks
You are right according the DIY law, but dont forget that the LED and the resistor are in series. That means when the I=3mA, then 3V go to the resistor. The rest (assuming 3.3V output of the ATMEGA8) is 0.3 Volt which goes to the LED. Theoretically this will not work because the usual Forward Voltage of a (red) SMD LED is 2.5V.

Where is here the error in reasoning?
 Nov 27, 2014, 02:07 PM Registered User Joined Apr 2012 3,898 Posts Light intensity percepted by your eye and current flowing trough the LED is not linear.....any current flowing trough the led will produce photons ...........LED intensity at 20mA and one at 1.2mA is not much different.That 20mA mentioned is close to max current..but there is not minimum current. The led will lit if you measure with ohmmeter ...........which is so low current. Voltage drop on LED is around 1.8V...the current through the 1K res is around 1.2mA..is more than enough light ..even at 0.5mA is enough. Your confusion is the relation between light intensity and current passing trough the led. Latest blog entry: DIY Multiprotocol TX Module

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