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Return to BlogSingle Cell-Powered LED Driven by Astable Multivibrator
Posted Jan 27, 2012 at 1:55 pm
Figure 1
A typical multivibrator that operates in astable mode is capable of driving an LED with the use of a single cell.
The ability to use a 1.5V cell to light up a LED is a big challenge since the forward voltage of LEDs is higher than the cell’s. This can be simply achieved by using a step-up DC/DC converter which is a reliable alternative especially for low cost applications.
The circuit is utilizing a standard astable oscillator formed by Q1 & Q2. The PNP switching transistor Q3 is being turned ON/OFF by the collector of Q2 that is driven by square wave signal. Simultaneously, the inductor L1 is charged upon turning ON Q3 but when it turns OFF, L1 discharges the stored energy via the LED. This happens during flyback which allows any color and type of LED to light up. The LED is forward-biased by the inductor’s reverse polarity and discharges a constant voltage almost equal to the forward voltage of the LED.
The LED always appears ON as the cycle repeats at a high rate. Depending on the average current, the brightness of the LED is proportional to peak value.
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