transistors

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electronics1

Joined Aug 21, 2009
42
I want to understand how this electronic circuit work ?
http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/ietron/nand4.gif
This electronic circuit needs to do NAND gate
In this sketch there used two 2N222 transistor but I build this circuit with two 1815 transistor
My problem is that if I connect to A + 6 volt the led light weak without dependence on B
But how this organize whit the NAND gate
 

mik3

Joined Feb 4, 2008
4,843
A NAND gate outputs a logic 0 when both inputs are high, otherwise the output is logic 1. The led lights weak because of the 4.7K resistor which is too high for a led. Replace the 4.7K with a 450 Ohm resistor and the led will be bright.
 

Audioguru

Joined Dec 20, 2007
11,248
The logic input volages are not shown. If they are only 5V then the base current is only (5V - 0.7V)/10k= 0.43mA. Then the minimum collector current for the transistors is only 4.3mA which produces dim LEDs.
For 20mA in the LEDs the base resistors should be 2k ohms.

EDIT:
The circuit does not show an LED driver.
If a 2V red LED is connected to the output then its current is only (6V - 2V)/ 4.7k ohms= 0.85ma.
If the LED is a 3.5V blue LED then its current is only 0.53mA.
No wonder they are dim.
 
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