An improved version of the simple 3 Red LED flashing bike rearlight.
The main improvement is to use PNP transistors to make the astable multivibrator stage and then to use an NMOSFET to drive the LEDs. That way the circuit does not waste over 20mA base current in just turning a on PNP bjt every time the the LEDs are turned on.
A second improvement is to use the Vishay TLCR5800 Ultrabright red LEDs:
The circuit can be modified to use white LEDs by adding an extra cell to bring the battery voltage up to 6V.
The circuit run at 4.5V will not drive white LEDs very well because they have a higher forward drop and so the values of R6 - R8 must be reduced with a consequent reduction in the accuracy of the current sharing between the LEDs.
This may result in significant brightness variation between the LEDs in a unit and from unit to unit.