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Design Example

Round Mirror LED Light

This is a personal beauty product.   It is a hand-held mirror that is back-lit with LEDs.    The product is battery driven and rechargeable and it is controlled by a metal touch-sense button.

We did not make the LED emitter board or the hardware for this project.  We only designed the controller and the driver circuits.

NOTE:  It was our suggestion to remove the normal battery wires.   This battery is ordered without wires and the metal tabs are soldered directly onto the circuit board.

A summary list of the core technical details is below.

3.7V Lithium-Ion battery - single prismatic pouch.   This battery is rechargeable with a standard 5V USB power source.

The battery charger runs independently - but status outputs are intercepted by the microcontroller for complete customization of 2 color indicators.   One red LED is used to indicate both charging and the low battery warning signal.  The other green LED is used to indicate charging complete.

The original software was designed to also indicate a charge level on the green LED -(by flashing a code upon removal of USB power), but that software was removed by the owner of the project.   Another feature for a software reset, trigger by plugging and unplugging USB power 3 times, was also removed

The unit contains two Asynchronous Boost current mode drivers which operate over the full range of the battery voltage.   Each driver produces about 40mA and a stepped-up voltage of about 26 volts.

These are a generic LED sold out of China.  These are a single die package LED and are surface mounted.   We push about 40mA through the LEDs in series.  

An 8-bit microcontroller generates a PWM modulated output to the drivers.   Control is based on a single wire touch-sense input.   The user touches a metallic tab on the unit and that touch is captured by the microcontroller.   There are 3 output light levels.   When the battery voltage falls close to 3V, a red indicator LED signals the customer to recharge the unit.