Here is the circuit of a “Selfie Light” for capturing beautiful selfies on your smartphone/tablet. The idea is an aftereffect of my frustration when I found that the front facing camera of my tablet would produce slightly blurred and underexposed photographs; it wasn’t up to the task of taking well-lit selfies in dark environments. This USB-powered portable LED-based light can be used with any smart phone/tablet offers a USB On- The- Go (USB OTG) feature. The amount of light emitted can be controlled, so there is no worrying that the light output will ever wash out the object’s face. Further, its overall dimensions can be reduced by employing chip (leadless) components in lieu of the traditional (leaded) components!
USB OTG INTRO
Portable computing products such as smartphones and tablets that today connect to the PC as a USB peripheral will benefit from having additional capability to connect to other USB devices. USB OTG is a new supplement to the USB 2.0 specification that augments the capability of existing mobile devices and USB peripherals by adding host functionality for connection to USB peripherals. Since USB has traditionally consisted of a host-peripheral topology where the PC was the host and the peripheral was a relatively dumb device, these new features were needed to upgrade standard USB technology for mobile devices. The specification and more information on USB OTG can be found on the world-wide web at http://www.usb.org/developers/onthego/
SELFIE LIGHT CIRCUIT
The LMC 555 (IC1) is configured here as a squarewave oscillator. Differing from the standard application is the addition of the BAT85 schottky diode (D2). Because of D2, two different timings can now be adjusted independently. The duration that the output is logic-High is determined by R1 and C2 and the time the output is logic-Low is determined by R2, P1 and C2. The duty cycle can be adjusted from near 30% to 96% with the indiacted part values (in principle, dimming to less than 30% has zero merit because the light output will be too low to be useful). Since the LMC555 is the CMOS version of the familiar timer IC type LM555 handles less current, an additional driver BS170 mosfet (T1) is used to drive the output load (LED x3), which can deal with a load current upto 500 mA without any problems.
According to USB-OTG specifications, the host must be able to supply at least 8mA of current between 4.4V and 5.25V. The OTG specification calls for at least 8mA and allows negotiation for higher currents if the peripheral needs more power. OTG devices can provide up to 500mA. In practice, handheld portable electronics such as smartphones and tablets don’t have 500mA to spare for additional/external loads. So, 100mA is a commonly accepted realistic maximum.
As shown here the newer mini and micro USB connections have an ID signal to distinguish the host device from slave devices. In a standard USB cable pin four is open, and in a USB OTG cable pin four is grounded. So, what you’ll need to do is jumper pin four (that’s the ID connection) to pin five (ground).
A Practical Failure?
After the spark of interest, doubts about power catering capability of a typical USB OTG port entered into my mind. So, at first I tried one NE 555 IC-based (readymade) pwm module with my Android tablet using a spare USB OTG cable, and after sureness jumped into this rattling project. Sneak eyeshot of the former experiment is shown below just as an initial proof of concept!