# LED switching circuit

Discussion in 'Homework Help' started by sylarhero, Nov 1, 2017.

1. ### sylarhero Thread Starter New Member

Nov 1, 2017
2
0
Hi!
I am trying to switch an LED with an NMOS and I made a simulation for it in TINA. I uploaded a picture about the circuit, to the NMOS gate I put 5Vpp 2.5V offset square wave, and I do not understand why am I having 2.5V on the LED when my square wave is at the LOW state (0V) and only around 4V when it is at HIGH state (5V). Like this the LED is not switched off totally, I want to switch it off when I do not have signal, and switch on when there is. What is the problem? Thanks a lot.

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Last edited: Nov 1, 2017
2. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Try looking at the voltage across the resistor in series with the LED. That way you can tell what current is actually flowing through the LED in each state of the input.

3. ### crutschow Expert

Mar 14, 2008
21,745
6,270
The 2N6755 has a threshold voltage of 2.5V where it is barely on, and requires a Vgs of 10V to fully turn on as shown below.

You need a MOSFET with a threshold of a volt or so if you want to run it fully on with a Vgs signal of 2.5V.

4. ### sylarhero Thread Starter New Member

Nov 1, 2017
2
0
In reality i am using an IRLL014N which has 1...2V threshold voltage but it is the same problem. I want to run from an arduino which has 0-5V on the IO pins so thats why I tried to analyze it with this square wave. The LED is not turning off totally, however its lighting weakly but still.....

Apr 26, 2005
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6. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Make sure that the voltage on the MOSFET gate does actually go to 0V.

7. ### dl324 AAC Fanatic!

Mar 30, 2015
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I have some leaky 2N7000 that do that when they're supposed to be off. I suspect they got zapped by ESD; though I always tried to handle them correctly.