# transistors

Discussion in 'General Electronics Chat' started by electronics1, Feb 7, 2010.

1. ### electronics1 Thread Starter Active Member

Aug 21, 2009
42
0
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

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
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.

Aug 21, 2009
42
0

4. ### AllVol AAC Fanatic!

Nov 22, 2005
55
2
what more do you need?

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
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.

Last edited: Feb 7, 2010