Components List:

This circuit will flash a bright or high-brightness red LED (5000+ mcd). Great for fake car alarm or other awareness getting equipment. Component values aren’t significant, attempt anything else first from the junkbox.Obviously, the 470 ohm resistor (R3) determines the LED’s brightness and limits the current flow to around 20mA. R3 value of 390 ohm can also be implemented as a save value.

If you determine to go with a green or yellow led, which take extra current, you might wish to change the 470 ohm with an proper value. Flash rate is determined by R2 and C1 and it 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. When the circuit does not oscillate, R1 may be too low or R2 too high. D1 allows for higher duty cycle operation and limits the feedback at the base of Q1 to -0.7 volts. D1 might be ommited for low supply power like 6 – 9V and low duy cycle operation.

LED Flasher 2-Transistors, circuit design by Tony Van Roon

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The supply voltage is 1.5V, so it can be operated using a 1.5V battery which can be easily found on market.

Component list:

LED1 _________ Red LED
C1 ___________ 100uf/16V
IC1 __________ LM3909
Battery ______ 1.5V AAA

The LM3909 is a monolithic oscillator specifically designed to flash Light Emitting Diodes. By using the timing capacitor for voltage boost, it delivers pulses of 2 or more volts to the LED while operating on a supply of 1.5V or less. The circuit is inherently self-starting, and requires addition of only a battery and capacitor to function as an LED flasher.

This circuit can be used for various applications:

• Finding flashlights in the dark, or locating boat mooring floats
• Sales and advertising gimmicks
• Emergency locators, for instance on fire extinguishers
• Toys and novelties
• Electronic applications such as trigger and sawtooth generators
• Siren for toy fire engine, (combined oscillator, speaker driver)
• Warning indicators powered by 1.4V to 200V

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