Single-Cell LED Flasher

This circuit flashes a group of paralleled light-emitting diodes (LEDs) from a single 1.5V alkaline cell. It utilizes a charge-pump technique in which a two-phase sequence alternately charges a large capacitor and then places the charge in series with the cell and the LEDs. Without such a charge-multiplying scheme, most LEDs will not illuminate with the lower voltage obtained from a cell alone.

An astable multivibrator consisting of transistors Q1 and Q2, capacitors C1 and C2, and resistors R1, R2, R3, and R4 produces a positive pulse waveform at Q1's collector (øA) with an approximate duration of 25mS and period of 375mS, correlating to a flash rate of 160 per minute. An inversion of this output appears at Q2's collector (øB).

During the time that øA is in the high state, transistor pair Q3 and Q4 are driven into conduction, applying the positive cell voltage (+1.5V) to the top of capacitor C3. Simultaneously, the low state at øB drives transistor pair Q5 and Q6 into conduction, connecting the bottom of C3 to ground. C3 charges to the supply voltage, excepting the small saturation voltages for transistors Q4 and Q6.

When the two phases reverse state, transistor pairs Q3, Q4 and Q5, Q6 are cut off. Simultaneously, the low state at øA drives transistor pair Q7 and Q8 into conduction. This grounds the top plate of C3 through Q7, connecting C3's negatively-charged lower plate to the paralleled LED (DS1 through DS6) cathodes. Since the LED anodes are connected to the positive cell voltage, the potential across the LEDS rise toward the sum of the cell voltage plus the capacitor charge (totaling approximately 3V for a fresh cell), and limited by the forward voltage of the LEDs at the peak current.

In the 1993 prototype, the brightest LED available for this design was Gilway Technical Lamp, Inc. type E-171, exhibiting a typical forward voltage 1.85V for a current of 20mA. The Gilway LED provided a minimum luminous output of 700mcd with a half-power angle of 50°. The LED specified here, Cree, Inc. type 503B-RAN-CY0B0AA1, exhibits a slightly higher forward voltage of 2.1V for a current of 20mA, but provides a far-greater minimum luminous output of 5860mcd with a half-power angle of 15°. It is expected that the luminous efficiency of this circuit with the Cree LEDs will far exceed the version built with the older LEDs.

To ensure that all six LEDs share the current equally, it is important that they are obtained from the same lot and have reasonably-matched forward voltage and current characteristics.

Current consumption for the 1993 prototype was measured at 150mA when the LEDs were on, and 500uA when the LEDs were off. An alkaline AA cell provided at least several days of bright operation. The circuit ceased operation at a cell voltage of about 1V. At this voltage, the sum LED-on current was about 25mA.

October 8, 2013

Text and image ©2013 by Arthur Harrison

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