The base-emitter of a transistor is a silicon diode. its voltage is about 0.7V when it is turned on. The datasheet for a 2N3904 transistor shows that it is 0.8V when a saturated transistor has 20mA collector current so I was a little off. I am attaching the graph from the datasheet.Where does 0.7V Come from? Voltage drop after the resistor or is this related to the resistor?
20mA is 0.02 Amps. Then the calculation is 1.7V/0.02= 85 ohms. Use 82 ohms. You calculated 0.09k ohms which is 90 ohms.Calculating R2, it says that the current-limiting resistor has 1.7V across it. How do we calculate that with ohms law? I'm 1.8V/20mA and getting .09 so I don't see how that translates... Maybe I have a problem with conversions.
The transistor base-emitter diode sets its base voltage at 0.7V to 0.8V.For voltage after R1, I would do I = 5V/2.5K Ohms = 2V ?
Great, I see .8V now! Also realize I should have been using the voltage difference calculated from the voltage coming into the resistor and the desired voltage after the resistor. We already know 2mA is whats needed. Perfect.The base-emitter of a transistor is a silicon diode. its voltage is about 0.7V when it is turned on. The datasheet for a 2N3904 transistor shows that it is 0.8V when a saturated transistor has 20mA collector current so I was a little off. I am attaching the graph from the datasheet.
The base resistor has 5V on one end and 0.8V on the other end so it has 4.2V across it. Then its value should be 4.2V/2mA= 2.1k but 2.2k is fine.
20mA is 0.02 Amps. Then the calculation is 1.7V/0.02= 85 ohms. Use 82 ohms. You calculated 0.09k ohms which is 90 ohms.
You used 100 ohms so your collector and LED current is 1.7V/100 ohms= 17mA which looks like the same brightness as 20mA.
The transistor base-emitter diode sets its base voltage at 0.7V to 0.8V.
The 2.5k base resistor does not have 5V across it, instead it has 5V - 0.8V= 4.2V across it. You were calculating its current which is 4.2V/2.5k= 1.68mA.
2.5k is not a standard resistor value but 2.2k is a standard value and will produce a base current of 4.2V/2.2k= 1.9mA which is fine.
Thanks! I went mostly off of a how to I found on instructables. You can check it out here...Awesome!
Post it in a "HOWTO" format, with schematics, source code, and pictures in the completed projects area if you wish, it would fit in nicely.
I do like the frosted clear LEDs best.
THank you!Well done! I 'm interested in that project.
Do I see a serial interface with a computer there? Can you elaborate a bit on that interconnection protocol?
Did you use a Serial port or a USB-to-Serial converter? Is that a MAX232 IC on the board input? How do you load your data from the PC?
A very cool project overall with great development potential.