Hand Held LED Mirror
The mirror is not shown in these photos, but it fits on top of the circuit board with the LEDs. This is a product for the beauty industry. It is an LED back-lit mirror. This circuit was made to accommodate 2 products, each with a slightly different plastic case. It was very smart of the client to make double use of the design. The client received a 2-for-1 value for the invested design dollars.
Designs like this take about 6 to 8 weeks to create (the circuit) provided there are no surprises. This design passed our electromagnetic emissions testing in the first round.
NOTE: You'll see that a few of these boards are black. We suggested to the client to use a black solder masking (rather than green) to reduce some light reflection that was making its way the back of the plastic case.
Design elements are described below...
3.7V Lithium-Ion battery - single prismatic pouch. This battery is rechargeable with a standard 5V USB power source.
The charger runs independently.
One interesting feature of this product is that the charge indicators were located behind the mirror and exposed through the glass in a thin line cut by a laser. When the indicators were active, their light peaked through this tiny slot in the glass.
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.
One variant of this circuit board also uses a Hall Effect Switch to detect a magnet which comes close to the circuit board when the product is closed (closed lid). That magnet helps reduce power consumption when in the off state. Software in each variant is exactly the same, just one part is added to one product to differentiate it.
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