Introduction: TOUCH LIGHT
This is a simple but interesting thing I came up with. You might have come across certain videos in Youtube where someone claims they have electric powers and turns on a regular looking led bulb with their bare fingers.
Well the thing the is trick is inside the bulb and obviously there's a battery inside. Anyone build this circuit that turns on the LED upon a touch.
Step 1: SCHEMATIC
The circuit is provided above, it uses minimum components and features the BC548 transistor. You can replace BC548 with almost any other NPN transistor. Some of the common NPN transistors you can use are BC547, BC337 etc.
Arrange and solder the components as in circuit. I got old LED bulb that's commonly used in home and pried it open without damaging it and removed the existing led and its driver circuit.
The circuit have to be designed in such a way that everything fits inside the case. I used a small li-po battery I had to power, since it was small enough.
Solder the touch point leads to base terminals of the bulb case. Where you will be touching it to turn it on.
Or you can use your creativity to make something else with this circuit.
Step 3: HOW IT WORKS
I believe the BC548 acts as an amplifier here, that is a smaller current through base to emitter gives a higher collector to emitter current. This depends on the gain of the amplifier which ranges from 200 to 800 .
collector to emitter current = gain x base current
The 3.7v li-po battery I used measured, seemed to run 30 uA through my body. Considering the body resistance to be around 200k ohm at my fingertips. 30 uA or 0.03mA at base gives a maximum collector current of 24mA at gain = 800. 24mA is sufficient to light up an LED.
Slightly moisturizing the fingers with water will allow greater base current in turn greater collector currrent and thus LED will glow brighter. And finally if you short the touch contacts the base current may exceed the maximum limit prescribed in the datasheet (probably around 70 mA - 100mA and a maximum power of 500mW), so in order to protect the transistor a resistor may be used in series.
So this circuit can also be used as a water level indicator, led flasher etc.
So that's my take on how it works and feel free to add corrections.