Rising Popularity of Portable Devices
The trend toward portability in electronic devices is undeniable, with many running on battery. Consequently, a significant number of flat batteries end up in the trash prematurely. When a group of batteries can no longer power a specific device, such as a flashgun, it doesn’t mean the cells are entirely depleted. Surprisingly, seemingly unusable AA-size cells, when placed into a radio-controlled clock with an LCD display, can power the clock for months or even years.
Optimizing Battery Use with a Night Light Circuit
While not every partially discharged battery can be repurposed for use in a clock, the circuit introduced here offers a clever solution to extract the remaining energy from your batteries and create a free ‘night light.’ This circuit leverages a TBA820M, an affordable audio power amplifier that can operate on remarkably low supply voltage. In this setup, it functions as an astable multivibrator running at a frequency of around 13 kHz. Combined with two diodes and an electrolytic capacitor, it forms a DC-DC converter capable of nearly doubling the voltage derived from a series of four to eight AA, C, or D-sized cells, or a PP3-style battery.
Enhancing Battery Voltage for LED Brightness
Following the DC-DC converter, there’s a constant current source responsible for driving the LED. This component serves to safeguard the valuable white LED, particularly in scenarios where the voltages from old batteries can significantly fluctuate. By incorporating the DC-DC converter and a 20 mA constant current source, a wider range of usable input voltages is attainable. Especially beneficial when dealing with old batteries operating at the lower end of their voltage range. Using the constant current source alone, the white LED would not achieve adequate brightness when powered by low voltages.
Key Features of the TBA820M Amplifier
The TBA820M is a monolithic integrated audio amplifier packaged in an 8-lead dual-in-line plastic format. Its primary application is as a low-frequency class B power amplifier, catering to a broad supply voltage range from 3 to 16V. It’s commonly employed in portable radios, cassette recorders, players, and similar devices. Noteworthy features of this amplifier include a minimum working supply voltage of 3V, low quiescent current, a minimal number of external components, effective ripple rejection, the absence of cross-over distortion, and low power dissipation.
Output power: Po = 2W at 12V/8Ω, 1.6W at 9V/4Ω, and 1.2W at 9V/8Ω.