Flat Feet Coach

Introduction

Flat feet—also known as fallen arches—affect over 2 billion people worldwide. This condition occurs when the arches of the feet collapse, causing the entire sole to touch the ground when standing. While common in infants and children, it often persists into adulthood, leading to a range of issues including knee, hip, and lower back pain, and in severe cases, permanent foot deformities or reduced mobility.

Current treatments rely heavily on custom-designed insoles that merely support the foot arch without addressing the root cause. Physiotherapy offers a more active approach by strengthening the muscles and joints that support the arch—but it requires strict daily commitment, patience, and long-term consistency. While these methods are proven, they are often passive, slow, and difficult to maintain.

Flat feet can be improved by activating specific muscles that naturally elevate the arch. This is done through a movement called pronation correction, where the bodyweight shifts from the inner foot (where flat feet apply pressure) outward toward the edge, restoring proper posture.

To make posture correction effortless and part of everyday life, I developed a smart, wearable insert that fits comfortably inside Crocs. Designed for simplicity and ease. In daily use, you simply wear your Crocs as usual. If you unknowingly shift your weight inward—common in people with flat feet—the device gently vibrates to remind you to adjust. This subtle feedback helps you correct your posture in real time, teaching your body to engage the right muscles naturally. Over time, this leads to better balance, improved alignment, and stronger foot arches—all without interrupting your routine.

Whether you're walking around the house, standing at school, or out for a casual stroll, the device works silently in the background to support healthy movement. This device when pair with your crocs as a normal croc jibbit, combines functionality with aesthetic design to make a wearable that will keep your feet healthy.

Disclaimer:

This device is a functional prototype developed as part of a personal project. It is not intended for commercial distribution. The system has been tested only on my own flat feet, and while it can be adjusted according to different people feet arches and elevation, its performance and effectiveness may vary from person to person. I am not a trained electronics engineering, and the circuit design is made as per my knowledge. If you encounter any problem while making this project you will need to use your own knowledge and logic to solve it although I will provide the solutions to problems I encountered while making this project. It is designed specifically for use during standing and walking activities only, and should not be used for running, sports. Use at your own discretion and always consult a medical professional for long-term treatment of flat feet or related conditions.

Theory of Operation: How the Device Works

Before we dive into the building process, it’s important to understand how the device actually works in simple terms. This will help you follow along and troubleshoot effectively if needed.

Flat feet, at their core, occur when the arch of the foot collapses, causing the entire sole to come into contact with the ground. When you pronate correctly, the arch lifts off the sole of your footwear (such as Crocs). But when the arch collapses, it sits flat on the sole.

The idea behind this device is to detect the distance between your arch and the sole. If this distance is too small (meaning the arch is collapsed), the device activates a signal, alerting the user with a vibration.

I explored several sensing methods—like pressure sensors, LDRs, and distance sensors—but ultimately chose a Hall effect sensor for its simplicity and low power consumption. Here's how it works:

1. A Hall effect sensor is placed on the sole of the shoe.
2. A small magnet is attached to the user’s arch (e.g., taped to the foot or in a strap).
3. As the magnet moves closer or farther from the Hall sensor (depending on whether the arch is raised or collapsed), the sensor outputs a different voltage.

This changing voltage is read by a comparator circuit, which checks if the sensor’s voltage crosses a certain threshold. If it does, a vibration motor is triggered to alert the user.

To make the system flexible for different users (since everyone’s arch height is different), a potentiometer is added to the circuit. This allows each user to adjust the threshold based on their unique foot anatomy and desired sensitivity.

The entire system is designed to be low power, so you won’t need to recharge the battery every few hours. By using a minimalistic design with the Hall effect sensor and simple logic, we maintain efficiency without sacrificing functionality.

3D Printing Settings

To protect and house your circuit, you’ll need to 3D print a custom-designed case. I’ve modelled the case to resemble a Crocs Jibbitz charm so that it integrates seamlessly with the footwear.

Here are the recommended 3D printing settings:

1. PLA material used
2. Nozzle Diameter: 0.4 mm
3. Infill: 100% (for maximum strength)

You can reduce the infill slightly if needed, but I do not recommend going too low. The case walls are relatively thin, and a lower infill could cause them to break under pressure, especially when attaching the case to your Crocs

I have divided the supplies you will need into 3 categories:

3D printed Parts:

1. The left case
2. The right case
3. The left lid
4. The right lid
5. The motor mount 2x
6. The motor mount lid 2x

The Electronics:

1. A multimeter
2. A oscilloscope (recommended for troubleshooting)
3. Perforated PCB board (If you are making the circuit on your own)
4. A PCB (If you are using the PCB layout provided as a file in the instructable)
5. Small 10mm by 10mm neodynium magnets 2x
6. 3.7v, 500mAh rechargeable Lithium-ion battery 2x
7. 4 Male connectors
8. MPN:HC-32 dip switch 2x
9. 4 female connectors
10. 100k ohms multiturn potentiometer 2x
11. LMV358 comparator 2x
12. 10k ohm resistor
13. 1 megaohm resistor
14. 270k resistor
15. 0.1 microfarad capacitor 2x
16. 2.2k ohm resistor
17. 2N2222 transistor 2x ( Surface mount if you are using the PCB, or Through hole if you are using a perforated PCB board)
18. Extra wires
19. Heat shrink sleeve
20. Coin vibration motors 2X
21. SS49e analog hall effect sensor 2x

The Tools:

1. Siccisors
2. Pliers
3. Blade Cutter
4. Wire stripper
5. Wire cutter
6. Soldering iron
7. Soldering wire
8. M2.5 machines screws 8x
9. Screws drivers
10. Pluckers
11. Desoldering pump
12. Soldering stand
13. Cello tape or Abro tape
14. crocs
15. socks

Let’s break down how the circuit works and where each component fits in.

Key Components Used

Comparator (LMV358):

A low-power dual comparator chip. We only use one of the two available comparators. It draws just 100 µA, making it ideal for our use case.

Hall Effect Sensor (SS49E):

After looking at its datasheet, this is a linear analog sensor that outputs a voltage based on the magnetic field’s strength and polarity.

The south pole of the magnet increases the output voltage.

North Pole decreases it.

We use the analog output because we need a continuously varying signal based on magnet distance.

Capacitors:

I have used a capacitor between the two motor terminals for voltage spike suppression.

I have used another decoupling capacitor between the VCC and the ground of the comparator.

Trim pot (Multi-turn Potentiometer):

Used to set the reference voltage for the comparator. A multi-turn pot is chosen for its precision, making the system easier to calibrate and much more forgiving to work with.

By doing so, we will create a non-inverting comparator. Which is a fancy way of saying that when the hall sensor voltage rises more than the reference voltage set by the pot, the comparator output will go high, which is exactly what we need to drive the vibration motor. This right here is the basis of the circuit. If you are able to understand this, making the circuit will become much easier.

Here are all the steps the circuit takes to make the device work:

1. The Hall sensor detects the magnet’s proximity and outputs a corresponding voltage.
2. This voltage is fed into the non-inverting input of the comparator.
3. The reference voltage from the trim pot goes into the inverting input.
4. When the Hall sensor voltage rises above the reference voltage (i.e., when the arch collapses and the magnet is close), the comparator output goes HIGH.
5. This HIGH signal is used to turn on the vibration motor through a transistor.

Stabilising with a Schmitt Trigger

A resistor is placed between the comparator’s output and the non-inverting input. Forming a basic Schmitt trigger to prevent false triggering or output flickering when the sensor voltage is close to the threshold.

Motor On Off control

The comparator output drives a 2N2222 transistor (SMD version), which acts as a switch to cleanly turn the vibration motor on or off. This transistor can supply enough current (80 mA) for the motor.

Building the Circuit

You can build this on a perforated board or use the custom PCB layout provided. I have provided the Schematic PDF file and JPEG in the Instructable. I have also attached the zip file of the PCB layout, which contains the Gerber files of each layer.

Note: The PCB shown in the photos is from an older project of mine and may not match the exact layout you will be using, but the functionality remains the same.

If you've reached this step, great job — the hardest part (building the circuit) is done! Now, let’s move on to placing everything into the 3D-printed case and setting it up on your Crocs.

Attaching Components to the PCB General Notes:

A photo of the PCB layout is provided — use it as a reference to know where each component connects.

1. Motor connections and signal pin connections are through-hole — this means you should insert the wires through the holes and solder them from the opposite side for a strong and secure connection. Potentiometer (Trim Pot) Mounting:
2. The wiper (middle) pin of the potentiometer is designed to be surface-mounted.
3. The VCC and GND pins are through-hole — this layout allows you to easily measure the reference voltage using a multimeter or oscilloscope while testing the circuit.

Important:

1. You will need to bend the wiper pin forward, past the VCC and GND holes.
2. Then, insert it through the designated hole and solder it securely.

Start with SMD (Surface-Mount) Components

1. These are usually the smallest and most delicate parts.
2. Solder them carefully before anything larger is added to avoid obstruction.

Next, Install the Trim Pot (Potentiometer)

1. Be careful: The middle pin is the wiper, which should go to one of the comparator's input pins.
2. The side pins are for VCC and GND — solder these after positioning the wiper correctly.

Parts Marked in Blue:

1. These are cut-off sections or not used.
2. Do not connect or solder anything to these areas.

What You Need to Do

3D Print the Case:

Use the provided 3D model file to print the case. It’s designed to curve along the edge of your Crocs.

Elongate the wires of the hall sensor from the module such that it can be tapped to its place( the position is mentioned in the next step ).

Place the Circuit in the Case:

Fit the battery, comparator, and sensor wiring into the 3D-printed case.

Arrange them as shown in the reference photos, ensuring everything fits neatly and that the wires are not pulled unnecessarily. You can use zip ties to tie the wires near the hole so that the wire is pulled and doesn’t break.

To ensure that the device works, the placement of the magnet is important.

Here you can use two approaches:

By using tape or Velcro:

You secure the magnet to the roof of your arch, taping it as shown in the photos, or you can glue velcro to the north pole of the magnet and attach that to your sock fabric.

How to make the Magnet Bend:

You can make a simple magnet bend with the help of a hair band. With the help of a strong glue, you can stick the magnet to the hairbend, which would then slide over your foot.

Placement points to note:

It’s not too important where exactly you place the magnet, but what matters is how you align it:

1. The magnet should vertically align with the Hall sensor
2. The south pole of the magnet must face the sensor
3. When your arch is lowered (foot presses down), the magnet should move into the detection range( typically 5mm to 7mm for the SS49e) of the sensor. When your foot arch is elevated (foot off the ground), the magnet should move out of the detection range.
4. Because this sensor is a linear sensor, when your foot presses down, the magnet should be directly above the sensor.
5. A good place is the highest point on the roof of your foot arch. Tape the magnet there so that it lines up with the sensor during movement. Photos and videos provided cover where to place the sensor in depth.

I have attached a video that shows these steps being executed for better understanding.

After you have placed your magnet where it should be, you can just mark the spot right below it, and that would be your position to tape the hall sensor.

Vibration Motor Attachment

You can place the motor anywhere it’s comfortable and easy to feel.

I have made a CAD file of the vibration motor mount that can be pushed through one of the holes of the Crocs. You can then close it off with a lid I have provided by glueing it to the mount.

Video of the complete calibration is provided in the file below, and the photos are above.

Sit down and wear the Croc with the sensor under your arch.

Rest your foot naturally — your arch should be at its lowest (close to the sensor).

Find the main potentiometer (trim pot) on your device.

Adjust the screw on the potentiometer slowly:

Counterclockwise (left) → Increases the threshold.

Clockwise (right) → Decreases the threshold.

Two Possible Cases When You Power the Device:

Case 1: The Motor stays OFF when you plug in the battery

This means the threshold is too low (below the Hall sensor's reading when your foot is resting).

Turn the potentiometer counterclockwise slowly to raise the threshold.

Watch for the moment the motor starts vibrating — this is when your threshold matches your foot's natural resting position.

Calibration is complete — now, lifting your arch slightly should stop the vibration.

Case 2: The Motor vibrates immediately when you plug in the battery

This means the threshold is too high (above the Hall sensor's reading, even when your foot is fully down).

Turn the potentiometer clockwise to lower the threshold until the motor just stops vibrating.

BUT — your foot is still pressing down at this point, so the threshold might be slightly too low now.

To fix this, turn the potentiometer slightly counterclockwise again until the motor just starts vibrating.

Calibration is complete — now, lifting your arch slightly should stop the vibration.

I have provided two videos which show how to calibrate the device in both cases mentioned above for better understanding.

Congratulations! You’ve successfully created your own flat feet coach. I have attached some videos showing the device in action. The goal was to design a device that seamlessly integrates into daily life while addressing the challenges of flat feet. I aimed to make this device as accurate, low-power, and reliable as possible.

By using a simple comparator circuit that operates on very low current, we kept the design efficient and easy to build. However, there’s still room for improvement. For example, adding a microcontroller chip can make the device more compact, more power-efficient, and further increase its functionality.

This was a proof of concept — a solution that hasn’t been done like this before. I hope you learned something new from this Instructable! I’ve tried to break down the complexities of building and calibrating the device simply and understandably so that you can follow along easily.

If anything remains unclear, please don't hesitate to leave a comment — I’ll be happy to assist you.

Thank you! 😊