How about a flashlight that lights up when you hold it and turns off when you put it down!!!
No hassle of switch or anything, just touch the the case and see the LED turn on. I was going through this site when I found an instructable to do the same thing using Arduino using capacitive touch. Well I am the guy who believes in cost cutting and why waste an entire Arduino when same thing can be achieved using just two transistors.
Step 1: Things You Need
Things you should have before proceeding:
1) 2 TIP 31 transistors
2) 9V battery for powering up the LED
3) two pins for establishing contact
4) A white LED (I used the upper part of LED flashlight)
Step 2: Proceeding With the Circuit
Now, lets take a look at the circuit diagram. I have used two TIP 31. Here we are making use of transistor as a switch. As soon as you touch the base and positive terminal of battery with your skin a very small current flows through your skin to the base of the transistor owing to the high resistance of your skin. This very small base current puts the transistor in "ON" mode and LED lights up.
As you know the base current is very small and thus the collector current is also less and not able to light up the white LED to appreciable brightness. Here to overcome this problem, we will use another transistor (TIP 31) to amplify the current coming to the LED. The emitter current from transistor 1 (see fig) which in previous case was used to drive the LED is now fed into the base of transistor 2. The effect is the collector current of transistor 2 is amplified further by beta and thus now the LED lights up brightly.
Step 3: Putting Together Everything
For the LED, I opened an old LED flashlight lying around in the house. You can use a standalone LED but here the advantage is the lens is already attached. For the case I used an old plastic box and cut out a hole at its back using a knife heated at gas stove. The hole isn't accurate as no laser cutter or a professional hot knife could be afforded.
Make the circuit board and use your creative imagination to fit it in the box with the 9V battery. Take due care so that battery compartment can be opened anytime without disrupting the whole circuit.
When everythings done, you need to find a way so that when you touch the case a contact is made to the inner circuit board (between the base and +ve terminal of battery). In my case, I have attached two pins that pierce through the plastic body making contact with the inner terminals With two pins you can also attach aluminium foils and stick it to the plastic body and thus when yo will hold the case in natural position LED will light up.
When done enjoy the touch.