BJT as switch...

ScottWang

Joined Aug 23, 2012
7,409
You should change the conception of the circuit, because you want to using +5V to be the power source, so
+5V → xxΩ → 2VLED → 2VLED → Vce(0.2V) → GND
You can calculate the value of resistor.
 

Thread Starter

RRITESH KAKKAR

Joined Jun 29, 2010
2,829
OK, i was just checking and testing the whole experiment after taht i have some result it was good to study..
i have used series combination of two red led which has Vf of 2V and combined several parallel led with them near about 15+15=30 it was working fine as i increases the led the currents increasing very slowly in very few mA near 2mA. and the voltage across 110ohm was 1V, after that i have added total 12 parallel 110ohm one by one the voltage become across R was 0.78V and current also increases and there was not so much power diss across R which was achievement then earlier....
so, my red led project is over and i am thinking about white LED which has Vf of ~3V so, how to combine with 5V 2V across resistance so, should i use 9V for it making 3 or 2 in series??/
Don't drive the LEDs from the 5V regulator. Use the 9V source directly.
I have to controll from PIC and 9V will make 4 series of red led
 

MrChips

Joined Oct 2, 2009
30,824
Use 5V logic from PIC to drive the gate of a MOSFET (or base of BJT).
Use the 9V supply to provide current to 1-4 LEDs with current limiting resistor.
 
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