Electronics

Initial Circuit Design and Part Orders

Our goal is to have our yo-yo light up when it spins. After looking at previous LED backlit Yo-Yo designs we settled on the Roller-Ball Tilt Switch as our mechanism for turning our circuit on only (in theory) when the Yo-Yo spins. There is a conductive metal ball inside of the switch’s cylinder that will short the leads, completing the circuit, when it rolls to the bottom of the cylinder. If we orient the switch with its contacts pointed radially outward the ball will short the leads when forced outward while the yo-yo is spinning. With only one switch this is not of very much benefit to us; whenever the yo-yo is held with the switch below the central horizontal axis the yo-yo will light. This is unfavorable because the yo-yo would have to be stored carefully to avoid draining the battery. If, however, we put two of these switches in series and opposite each other, both pointed radially outward, only one ball will be able to contact at a time unless the yo-yo is spinning.

 

Above: Circuit diagram. The circles are our 5mm LEDs, the central red circle is a coin cell battery, the two rectangle are our switches (in series between the battery’s ground and the LED ground ring), and the grey outlines are the inner diameter of our yo-yo and, inside of that, our circuit boundary).

  

We have developed a circuit for testing and ordered the parts to do so. Below is a basic schematic (for the purpose of calculating necessary voltage sources and current-limiting resistors).

I have ordered an assortment of 5mm Super Bright Water Clear LEDs- the bulk price was fairly good (300 LEDs for $13), and the assortment of colors will satisfy all members of our team (the one thing we could not agree on was a single color; we could, however agree that multiple colors [a single color per yo-yo] would be awesome). The circuit will be essentially the same for all colors, although the voltage drops are split into two different ranges depending on color, so we will have two different resistor values for optimal brightness.

Color	Forward Voltage (V)
Red	2.0~2.2
Green	3.0~3.2
Yellow	2.0~2.2
Blue	3.0~3.2
White	3.0~3.2
Purple (UV)	3.2~3.4

Above: Forward voltages for various colors of LEDs. For the purposes of resistor calculations I split them into two forward voltage categories: 2.2V and 3.2V.

The recommended current is 20mA. The voltage source is made up of 3V coin cell batteries. We will likely need two per yo-yo for the 3.2V LEDs (and may just go ahead and use the same for both for continuity). Using Ohm’s Law, R=V/I, I found that the resistors should be 30Ω for a 3V source and 2.2V drop, 190 Ω for a 6V source and 2.2V drop, and 140 Ω for a 6V source and 3.2V drop. I ordered 200 Ω resistors for testing purposes, as 190 Ω is an unusual resistor value. Additionally, we will need 200 Ball Tilt Switches (two for each of our 100 circuits). I ordered 230 to account for errors on our end and switches that arrive broken. Everything is scheduled to arrive over Spring Break which will give us time to test and optimize our circuit.

Expenses

Item	Amount		Cost ($)
22 AWG Solid Wire (Hook Up Wire)	25 feet		4.70
CR2032 Lithium 3V Batteries	50 batteries		5.25
LED Assortment Kit	300 LEDs 20 each R,Y,G,B,W,UV		12.99
Ball Angle Tilt Vibration Switch (SW-520D)	230 switches		49.87
Resistors (200Ω)	100		1.49
Total			74.30

What still needs to be ordered/tracked down: solder, copper braid, and violet LEDs (the LEDs can wait until we get our initial circuit working).