I want to create an interactive toy for my kids to play with me. We hold hands together then the RGB LED shows different colors. Hold tighter or lighter, or touch different parts of other's body will show different colors. i.e. different brightness of red, green and blue will mix a completely new color. That will be fun.

Step 1: The Final Product

There are four touch pads: one as common, three others for Red, Green and Blue.

I was planning to touch the common pad by myself, then my kids use one hand to touch red or green or blue, and the other hand to touch me so the RGB LED will show different colors. Soon I found this is a stupid idea. My kids found pressing me harder their controlled color will be brighter so they press me really hard yet sometimes pressing my nose.

So the game changed to: all kids use one hand to touch the common pad and the other hand to touch red or green or blue pad. They alter colors by pressing harder or lighter on the pads. Not on me :)

Step 2: What You Need and How the Circuit Works

BOM:
- pnp transistors x 3 (I used 8550)
- resistors x 3 (from 100 ohm to 300 ohm depending what you have)
- RGB LED x 1
- some wires and aluminum stickers
- battery (I used 1.2v x 3)
- one breadboard

The circuit is simple. Here is how it works:
- When one uses hands to touch the common pad (ground) and the color pad, the human resistant changes from several hundred k ohm to several m ohm
- The effect is like connecting pins B and C with different resistor allowing current pass through B and C
- The current is amplified by pnp transistor between pin E and C

So pressing the pads harder and lighter, the human resistant will be different that alter the current pass through LED. Different brightness of red, green and blue mix different color in the RGB LED.

Step 3: Assembly and Play

Assemble all parts into the breadboard as attached based on the circuit.

RGB LED is in fact a LED with three LED inside: red, green and blue LED. All LED share same common anode (in my case). Red, green and blue should connect in series with different resistors to get best brightness. But for this case the value of resistor does not matter. As long as the value is big enough to protect the LED, that is fine. i.e. any resistors with value 100 to 300 ohm should work.

Use a sand paper to brush the surface of the LED so you can see color diffusion that makes the color mixing effect matter.

After assembly, plug in power then try
- use one hand to touch the common pad
- use the other hand to touch red wire. You should see LED show red. Press harder then the LED will show red brighter
- touch the other green and blue wires and try the same
- at last try to use different fingers to touch different pads to mix colors. e.g. touch red and green evenly will show yellow. press red harder and green lighter will show orange

Well done. You may now ask your kids control different colors so they mix color together. Turn off your living room light for better effect.

Step 4: Alternatives If You Don't Have Pnp Transistors

If you don't have pnp but with some npn transistors, you can use the following circuit.

The circuit and effect are similar but the common pad is now connecting to positive power source and pin E is now connected to ground.

Have fun!