R1_______33K 1/4W Resistor
R2,R3____47R 1/4W Resistors
C1_______10΅F 25V Electrolytic Capacitor
D1,(D2)__LED(s) (Any type and color)
Q1_____BC560 45V 100mA PNP Transistor
Q2_____BC337 45V 800mA NPN Transistor
Power supply can vary from 2 to 4.5V.
Add D2 to obtain two LED alternate blinking.
D1's on-time is shorter than off-time. The opposite regarding D2.
You can also use D2 only, shorting D1.
Don't change resistors' values.
Flashing frequency can be varied changing C1 value from 2.2΅F to 100΅F and higher.
This circuit is very efficient when driving a 3.2V lamp. In this case omit the LEDs and R3, connecting the lamp at Q2's collector and to positive supply, further reducing parts counting.
Source: Electronics Lab