For my Tardis tissue box cover, I used a mosfet for the first time. Let's see how I used it !

I bought some 2N7000. Their are cheap, and they can handle 200mA, ore than enough for my day-to-day use.

Mosfet: 2N7000 N-Channel

Unlike bipolar transistors, MOSFETs are voltage controlled. It's easier to use them than BJT (current controlled) : you just apply enough voltage to the gate and the switch operates.

With a pin from an Arduino, you can switch on/off a load off 20mA.
By adding the MOSFET in your circuit, you can switch on/off a load of 200mA !
The pin of the Arduino will only be use to control the state of the MOSFET.

In my Tardis tissue box, I used the MOSFET to turn off / on some leds. I even used the PWM capability of the ATMega to pulse the LEDs (the MOSFET is turned on/off with a variable duty cycle).

Source: Wikipedia

Circuit

• Source to GND
• Gate with 10kΩ pulldown
• Gate to PWM with 1kΩ
• Drain to resistor (22Ω) to LEDs to Vcc

Note: I only put one LED in the schematics as an illustration