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I need a simple diagram showing me how to switch 12V LEDs with a 2N7000 using an Arduino. My experience is moderate, but a simple diagram should get me started. To summarize I want to switch 12V with 3V using 2N7000 MOSFET, a LED strip, and an Arduino Uno. Also when I hook up the 12V LED strip's ground through the D and S of the 2N7000, it ligths up without anything connected to the Gate. That means it is open until it recieves what signal on the Gate?

I've tried this:

schematic

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    \$\begingroup\$ Looks like "I want you to do my work". What have you done yourself already? What are you trying to achieve in the end? A link to the relevant datasheet would be helpful too. \$\endgroup\$ – Keelan Feb 21 '13 at 9:46
  • \$\begingroup\$ I have googled and tried making the same setup myself, but have not got the same result, so i need a diagram for this exact scenario. also i have found setups that sometimes are opposite of eachother. What i am trying to achive is switching 12v LED strips (ground for R G and B), using and arduino. with this: fairchildsemi.com/ds/2N/2N7000.pdf \$\endgroup\$ – Tom Sand Feb 21 '13 at 9:54
  • \$\begingroup\$ Post your schematic. \$\endgroup\$ – Leon Heller Feb 21 '13 at 10:23
  • \$\begingroup\$ Alright, first drawing ever but here is the current setup: dl.dropbox.com/u/11643892/2N7000-1.png and my question is what do i have to supply to turn the LED's off ? \$\endgroup\$ – Tom Sand Feb 21 '13 at 11:40
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Using a 2N7000 is, as a first approximation, just like using a BJT to switch a load. The major differences are:

  1. the gate is (very) high impedance, so no resistor is required to limit the current, as a resistor would be needed in series with the base of a BJT

  2. the gate threshold of MOSFETS is typically higher than the 0.6V it takes to get a BJT to turn on

The datasheet will specify a gate threshold voltage \$V_{GS(th)}\$, which is the voltage at which the transistor begins to turn on. You want to apply more than this if you intend to operate the transistor as a switch. You also want to apply safely less than the specified absolute maximum gate-source voltage, which if exceeded, will destroy the device. I happen to know off the top of my head that 5V is a good gate voltage for 2N7000, which is convienent because you can connect it directly to your Arduino.

What you are missing, it seems, is how to use a transistor as a switch. Searching for "transistor switch" turns up a few million tutorials, but here's the brief version:

schematic

Applying 0V to the input makes the transistors appear like an open switch, and your load (represented by resistors) is effectively disconnected from the power supply. Applying a higher voltage (whatever voltage is used for the logic in your circuit, usually 5V in the case of Arduino) turns the transistors on, and they look like a closed switch, effectively connecting your load between Vcc and ground.

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  • \$\begingroup\$ What i dont understand is why the 2N7000 is acting like a closed switch whn i just connect the Drain snd Source, and dont put anything on the gate. should it not then act like and open switch ? \$\endgroup\$ – Tom Sand Feb 21 '13 at 12:19
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    \$\begingroup\$ @TomSand because the gate is so high impedance, even just the noise from your finger can be enough to turn it on. Essentially, the gate and drain make a capacitor (if you look at the schematic symbol, you can see the resemblance), and even if you disconnect it, it can maintain its charge and keep the transistor on. This is, interestingly, the basis for flash memory. It's common to put a highish value resistor (10k, perhaps) from the gate to ground in the circuit to be sure any residual gate voltage can go to ground. \$\endgroup\$ – Phil Frost Feb 21 '13 at 12:24
  • \$\begingroup\$ @TomSand I'd suggest building this circuit on a breadboard, then try turning the transistor on by jumpering the gate to +5V. Then remove the jumper. Then jumper it to ground. Then remove the jumper. Then try licking your fingers and touching the gate. \$\endgroup\$ – Phil Frost Feb 21 '13 at 12:27
  • \$\begingroup\$ Alrigth then i now managed to turn them off ! =) (dl.dropbox.com/u/11643892/2N7000-2.png) using this setup. also fading, as i hooked this all up on a breadboard connected to a variable powersupply. I think i was expecting the 2N7000 to act opposite. What is a good transistor for that ? (keeping the gate open until i gets +5V(to turn on)). Next step is hooking this up to the arduino wich should be no problom now thanks ! \$\endgroup\$ – Tom Sand Feb 21 '13 at 12:56
  • \$\begingroup\$ @TomSand sounds like you should ask a new question. \$\endgroup\$ – Phil Frost Feb 21 '13 at 12:57

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