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Driving the LEDs

A project log for Multiplexed 7-segment display

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pavelPavel 04/27/2017 at 04:420 Comments

Here is planned layout of single digit:

All segments have common current source (coming from transistor emitter) and separate drains (collectors of other transistors).

After having several experiments on breadboard, I came to unexpected solution, where I do not need any resistors for LEDs to light properly.

On the picture below is simplified circuit for driving single segment, with measured voltages and current through LEDs (lowermost resistor is 47 kOhm, the LED at base of bottom transistor is Green).

I do not like this solution, because LEDs are seem unprotected without resistor in series, but somehow it works. Also there seem to be that almost half of all current supplied by top transistor is coming through its base. Is there some more robust solution? Any help will be appreciated, meanwhile, I'll experiment further.

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