Spring Switch Detects Motion in Flashing Shoes

My child’s favorite pair of sneakers is the kind that contains LEDs that blink as you walk, stomp, jump, and run. Eventually, though, the battery runs down, the shoes wear out, and the feet grow too big.

I fished the decorative athletic shoes out of the trash and discretely cut them open with a pair of tin snips to find out what kind of sensor determines when the shoes are shaking. Anyone who has been a parent understands that performing surgery or any form of reuse on a favorite thing is a delicate undertaking, and is best performed out of sight.

Light-up shoe with LEDs is cut open to reveal a circuit in the heel of the sole

Light-up shoe with LEDs is cut open to reveal a circuit in the heel of the sole.

Light-up shoes contain individual solid-state colored LEDs sewn into the upper fabric or embedded into a clear rubber window in the heel. The circuit, sensor, and battery are contained in the sole so as not to cause discomfort to the wearer’s sensitive foot.

LEDs pulled out of blinking shoes

LEDs pulled out of blinking shoes.

For this flashing shoe, there are three LEDs (two red LEDs and a yellow LED). They are soldered to thin, circular, fiberglass boards to hold the lights in position when installed in the upper.

One wire provides a common ground, while separate wires provide positive voltage to each LED. This arrangement allows the chip to blink each LED separately. This pleasantly surprised me, as I thought they would have simply used blinking LEDs without a chip and powered them using a single pair of wires.

The four wires come together in a ribbon cable that leads to the circuit in the heel.

Epoxy-encapsulated circuit with a coin cell

Epoxy-encapsulated circuit with a coin cell.

The power source (a disc-shaped cell) and the brains (a black-epoxy coated chip) are completely molded into the rubber shoe heel. That means the battery is in a non-serviceable location and is not replaceable.

As you might expect, the heel of a shoe is under considerable force from the weight and movements of the wearer. Furthermore, molten rubber is hot and may be slightly conductive. Therefore, to prevent the circuit and battery from being damaged, they are stored in a clear plastic case. Additionally, to make it crush-proof (and possibly to prevent moisture damage) the inside of the plastic case is filled with epoxy that fills in the voids.

In the circuit, there aren’t any capacitors and there are only two visible resistors. I expected the resistors to be tiny surface-mount packages, not through-hole.

I also expected to see one resistor per LED. Instead, I suspect the 68-ohm resistor is a shared current-limiter for all three LEDs. The circuit probably never enables more than one LED at a time. Because they are blinking rapidly, the eye never notices.

I have no idea what the almost 1-megohm resistor is doing.

The vibration sensor spring switch trips the blinking lights in a light-up shoe.

The vibration sensor spring switch trips the blinking lights in a light-up shoe.

The feature that I am most interested in is the method of detecting the shoe wearer’s movement. It turns out to be a simple spring that bounces on a contact.

This form of vibration sensor is not much different from any other electrical switch or pushbutton. The spring is connected to an input on the chip and the contact is connected to the battery. When the spring arm touches the contact, the circuit is complete and the input on the chip changes voltage.

The change in voltage probably wakes the chip up out of sleep mode. The chip then spends a programmed amount of time quickly turning on and off each LED Then, the chip returns to low-power sleep mode. 

Interestingly, the plastic case containing the circuit has a label indicating which side is up. This orients the spring-based sensor as shown in the above photograph. I suppose they found that gravity assists the spring to connect with the contact.

There isn’t a paper pull tab to keep the battery fresh before use. So, either a shipload of shoes blink in the cargo hold during their voyage across the Pacific ocean, or the common method of packing shoes on their sides in shoeboxes is a much less sensitive orientation for this sensor.