Transistor Bling: the Analog LED Accessory

This ultra-compact wearable LED tag is designed to blink in vibrant patterns, making it perfect for festivals, cosplay, nighttime visibility, or creative fashion. With its rechargeable battery and sleek 3D-printed enclosure, it easily attaches to clothing, bags, or accessories using Velcro.

Whether you're signaling your presence in low-light conditions, adding a dynamic touch to your outfit, or integrating it into smart wearables (like earrings or badges), this tiny tag brings both style and safety. Bonus: it can even be upgraded to support microcontroller control for custom blinking patterns!

For Prototyping

1. Breadboard
2. Jumper wires
3. Capacitors (10µF, 100nF – depending on your circuit)
4. LEDs (any color) – for testing wave pulses
5. Transistors (e.g. BC547 or similar NPN)
6. Resistors (based on your multivibrator values)

For PCB design + enclosure

1. PCB Design Software – KiCad / Altium
2. Enclosure Design Tools – Autodesk Fusion 360 / TinkerCAD

Final component list

1. 4x Capacitors and ICs (BMS - BQ24092DGQR )
2. 12x SMD Orange LEDs (0603 package or similar)
3. 4x Potentiometers - frequency and brightness adjustments
4. 1x Lithium Polymer Battery (3.7V, 100–150mAh)
5. 1x JST Connector – matching your battery type
6. Velcro Strip or Dot – for wearing or attaching
7. 3D-Printed Enclosure – [STL file provided]
8. Optional: Microcontroller (if you want to make it smart/programmable)

This project is built around a classic dual-transistor astable multivibrator circuit. It’s a clever and timeless design that uses the charging and discharging of capacitors along with the gain characteristics of transistors to create a continuous blinking effect.

To understand the theory behind this circuit in detail, check out this excellent resource

Dual Transistor Multivibrator Circuit

In simple terms, this circuit alternates the ON and OFF states of two transistors. As one turns on, the other turns off, causing the connected LEDs to blink in a rhythmic pattern. It’s a beautiful example of how analog components can produce digital-like behavior!

Before diving into the PCB design, it’s essential to verify the core functionality of your circuit using basic components. That’s what this step is all about — making sure your idea works in real life.

What I Did:

1. I started by building the dual-transistor multivibrator circuit on a breadboard using common NPN transistors (like the 2N3904), resistors, and capacitors.
2. I tested the LED blinking pattern, ensuring it was rhythmic and visually smooth.
3. I also measured the voltage and current requirements, which helped me select a suitable Li-Po battery pack for the wearable version and also calculated suitable PCB trace routing and required BMS and Voltage Booster ICs

This is a crucial stage where you can tweak values and experiment with different timings. For instance:

Changing the capacitor value adjusts how long it takes to charge and discharge, which directly affects the LED blink rate.

To make the circuit even more versatile, I added four tiny potentiometers directly on the PCB

2x Potentiometers for LED Brightness. These control the base current to the transistors, allowing you to adjust how bright the LEDs appear, perfect for adapting to ambient lighting or battery-saving modes.

2x Potentiometers for Blinking Frequency. These tweak the RC time constants (resistor-capacitor pairs) in the multivibrator circuit, letting you control how fast or slow the LEDs blink.

You can see the placement and function of these potentiometers in the PCB image notes. Adjusting them gives you real-time control without reprogramming or swapping components!

Designing a PCB is straightforward and Fun until you try making it really small. Compact wearables pose a fun but challenging puzzle. How do you fit all the functionality in the tiniest possible space?

PCB Specs

1. Diameter: ~2 cm - compact!
2. Thickness: ~5 mm (including components) (can be optimized further depending on your enclosure and use case)

To make this design wearable, I kept the layout minimal and efficient, using SMD components and carefully routed copper traces.

The first two images show the 3D render model of the final PCB design.

The next two images showcase the layout view and routing, highlighting how the components and connections are packed efficiently in a tight footprint.

This custom round PCB was designed using KiCad, a free and open-source PCB design suite. It gave me full control over part placement, trace width, and via positioning, all critical in space-constrained designs like this.

A functional circuit is only half the story — for wearable tech, the enclosure is just as important. It must protect the electronics, look great, and be comfortable to wear.

What I Designed The enclosure was created to perfectly house the round PCB along with the battery and connector, while keeping everything slim and stylish.

Features:

1. Material: The main body is designed to be 3D printed using ABS plastic, which is lightweight, durable, and skin-safe.
2. Diffuser: A translucent front cover allows the orange LEDs to shine through evenly, creating a smooth glowing effect — ideal for visibility or decoration.
3. Battery Compartment: A dedicated cavity inside holds a slim LiPo battery snugly.
4. Attachment Method: The back includes room to mount a velcro pad, magnetic plate, or cloth clip — allowing you to easily attach it to jackets, bags, or costumes.

Specs:

1. Diameter: ~2.5 cm
2. Thickness: ~1.5 cm

The compact design ensures it sits comfortably without bulk, making it perfect for wearables, earrings, costumes, or even safety gear for night visibility.

This tiny wearable tag is just the beginning!

With a little imagination, the same core concept can be extended in creative and practical ways:

1. Microcontroller Control: Replace or augment the multivibrator with a microcontroller (like an ATtiny or ESP32) to enable programmable LED patterns, sensor inputs, or remote control.
2. LED Matrix Upgrade: Swap the LEDs with a miniature dot matrix display to show scrolling text, symbols, or notifications.
3. Rechargeable & Expandable: The current version is compact, lightweight, and rechargeable, making it perfect for wearables like earrings, festival badges, or cycling safety lights.
4. Automotive-Inspired Tech: This classic multivibrator circuit is widely used in automotive indicators and pulse generation systems. With more RC stages and transistors, you can build chaser lights, timers, or sequential turn signals.
5. Light Animation Effects: Add more LEDs and customize the layout to create smiley faces, heart shapes, or patterns that pulse and wave — ideal for cosplay, festivals, or fun science demos.

*In the above instructables, I've used AI-generated Images for Concept Illustration