This is not an original idea, two weeks ago i found the minty Beating Valentines Heart, where i begin, this is a little remodelated instructable i have mounted to gave a very special friend, My contribution is the acelerated beating rate depending on the parallel resistance of the skin.
I include photos and schemes to help me describe the assembly. This is a very simple circuit, like you will see.
PD: I also include an origami beating heart, from here
Step 1: Parts
You will need a few electronic parts and some waste materials.
1x 555 timer
1x 27kOhm resistor
1x 3,3MOhm resistor
1x 470nF condensador
2x AAA batt
thin cable (like USB data cable)
1x CD Lens (with coil)
Let's go with the circuit
Step 2: Integrated Circuit 555 Working in Astable Mode
555 timer is cheap and completly suitable for this instructable. From notional semiconductor datasheet you can find the external parts for te astable mode operation. I'll borrow some pics from national semiconductor datasheet.
First pic show how you have to connect resistors and the capacitor to enable astable mode operation. where:
C = 470nF
C1= 0.01uF ;this is recomended, but I didn't mount it
RL' sera la bobina de la lente del lector de CDs
RL no va a estar presente.
The second pic show timing graphic and basic equations.
Third pic is the pin description of LM555 timer
Step 3: Boarding
It's time for a little boarding, you have to put the scheme into a little pieze of island board. I have put five pins to connect the "main" pulse generator to the power supply, the coil and the resistive contact sensor.
I hate a little island board, but I always use them.
Step 4: Resistive Sensor
Conceptually is easy, in practice is easy too. the eq resistance of the finger's skin is about Mega Ohm, if you put this parallel with RA t1 will decrease because of the eq resistance is lower. Thats all! not complicated.
In the circuit I have connected Vcc to chasis and 555.7 pin to a aluminium sheet. When you hold the minty box and touch the aluminium sheet the cardiac ritme increase.
Step 5: Finishing
I have put all the circuit into the mint box. I drilled a hole to transmit movement of lens to the beating heart. I used a piece of tin. the lense is glued to the box in correct position.