The
555 circuit below is a flashing bicycle light powered with four C,D or
AA cells (6 volts). Two sets of 20 LEDs will alternately flash at
approximately 4.7 cycles per second using RC values shown (4.7K for R1,
150K for R2 and a 1uF capacitor). Time intervals for the two lamps are
about 107 milliseconds (T1, upper LEDs) and 104 milliseconds (T2 lower
LEDs). Two transistors are used to provide additional current beyond
the 200 mA limit of the 555 timer. A single LED is placed in series
with the base of the PNP transistor so that the lower 20 LEDs turn off
when the 555 output goes high during the T1 time interval. The high
output level of the 555 timer is 1.7 volts less than the supply
voltage.
Adding the LED increases the forward
voltage required for the PNP transistor to about 2.7 volts so that the
1.7 volt difference from supply to the output is insufficient to turn
on the transistor. Each LED is supplied with about 20mA of current for a
total of 220mA. The circuit should work with additional LEDs up to
about 40 for each group, or 81 total. The circuit will also work with
fewer LEDs so it could be assembled and tested with just 5 LEDs (two
groups of two plus one) before adding the others. Link
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