I'm in the process of building my Gauss meter to measure the strength of magnets and I came up with an awesome idea for the on/off switch. I didn't want any switch visible from the outside so I made a circuit that allows the magnet you are testing to turn the device on and off from the outside. In the video, I go into how the circuit works in a bit more detail as well as show it in action.
Step 1: What You'll Need
This is what you will need to build this circuit:
Reed Switch x2
N Channel MOSFET x1
P Channel MOSFET x1
100k Resistor x2
6.8k Resistor x1 (Not critical, can be a bit above or below)
Step 2: How It Works
The P Channel is off when the circuit is done being assembled. When the switch on the right makes contact, it turns the P Channel MOSFET on which causes the N Channel MOSFET to turn on. This latches the circuit on.
When the switch on the left makes contact, it causes the N Channel MOSFET to turn off which subsequently turns the P Channel MOSFET off with the rest of the circuit.
Here is a link to the simulator I used in the video. You can play around with the switches and change values to see what happened.
Step 3: Applications
You could use this circuit for anything you want to have a hidden switch. You could build a secret compartment that can only open with a magnet held in a secret spot. Let me know in the comments what you could use this circuit for.