Two Transistor LED Flasher

Parts List
R1 = 10M, 5%
R2 = 1K - 100K, 5%
R3 = 470, 5%
C1 = 0.47uF - 10uF/25V
D1 = 1N914
Q1 = 2N3904
Q2 = 2N3906
Led = High Brightness Red LED

This circuit will flash a super-bright LED (5000+ mcd). Good for fake car alarm or other attention getting device.
Component values are not critical, try anything else first from your junkbox.
Obviously, the 470 ohm resistor (R3) determines the LED's brightness and limits the current flow to about 20mA. 390 ohm can also be used as a save value. If you decide to go with a green or yellow led, which draw more current, you may want to replace the 470 ohm with an appropriate value. Flash rate is determined by R2 and C1 and is approximately three time constants (3*R2*C1). R1 provides bias to Q1 which should be low enough not to saturate Q2 with the capacitor disconnected. If the circuit does not oscillate, R1 may be too low or R2 too high.
D1 allows for highter duty cycle operation and limits the feedback at the base of Q1 to -0.7 volts. D1 may be ommited for low supply power like 6 - 9Vdc and low duty cycle operation.

Back to Circuits page
Page Copyright © 2001 - Tony van Roon