Nand circuit, placement of resistor.

AnalogKid

Joined Aug 1, 2013
10,177
The collector of the first transistor is directly connected to the 9V.
That will not work. If you think of the transistors as switches, and both inputs are high so both transistors are on, then they are trying to create a dead short across the battery. Depending on the condition of the battery, this may or may not destroy one or both of the transistors, but the circuit will not function correctly.

ak
 

Thread Starter

gilleslag

Joined Dec 23, 2020
14
AND gate if you watch the LED, but like a NAND gate if you look at the output voltage.
See circuit below.
If the swithes are off only the LED on the right lights up.
if push both switches the 2 LED's are on, only the LED on the right dims a little. How come that the LED on the right isn't going off completely?
circuit.jpg
 

MrChips

Joined Oct 2, 2009
27,711
Because the circuit with R4 and LED on the right is not a valid way to test the voltage on the collector. Use a volt meter instead.

Or use another NPN transistor as an LED driver feeding off the NAND gate collector.
 

Thread Starter

gilleslag

Joined Dec 23, 2020
14
Because the circuit with R4 and LED on the right is not a valid way to test the voltage on the collector. Use a volt meter instead.

Or use another NPN transistor as an LED driver feeding off the NAND gate collector.
why is it not a valid way to test it? and how should the schematic look like if i use another transistor?
 

ericgibbs

Joined Jan 29, 2010
16,819
hi gill,
I would suggest you don't get into the bad habit of leaving transistor Bases 'floating' ie: disconnected as shown in some of the posted diagrams.
A resistor from Base to 0V is recommended, say around 47k .

You should also consider what 'load' the Outputs are driving.
eg: if the NAND output is connected to another Gate, its possible that above D1 LED may be dimly lit due to the current thru R5 and Q2 Base to 0V.

E
 

MrChips

Joined Oct 2, 2009
27,711
Also, if i remove R4 the LED on the right goes off if i push the buttons.
I think TS is confused between AND gate output voltage and LED being turned on when the output voltage is low.

The LED on the collector side of the output transistor indicates the opposite of the output voltage because the output sinks current when the output voltage is low.
 

MrChips

Joined Oct 2, 2009
27,711
why is it not a valid way to test it? and how should the schematic look like if i use another transistor?
Transistor NAND gate output.jpg

Analyze the currents in the LEDs in the two situations above.

In A, current flows in R1 and not in R4. Only the LED fed by R1 is lit.
In B, a lower current flows in both R1 and R4. Both LEDs will be dim.
 
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