LED Flasher III

Description: 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 junk box. Obviously, the 470 ohm resistor (R3) determines the LED's brightness and limits the current flow to about 20mA. A 390 ohm can also be used as a safe 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 higher duty cycle operation and limits the feedback at the base of Q1 to -0.7 volts. D1 may be omitted for low supply power like 6 - 9Vdc and low duty cycle operation.

The described circuit is a simple LED flasher that can be utilized in various applications, such as a visual alarm system or decorative lighting. The core components include a super-bright LED, resistors, a capacitor, and transistors configured in a feedback loop to create an oscillating signal.

The LED's brightness is controlled by resistor R3, which limits the current to approximately 20mA. This value can be modified depending on the specific LED color used, as different colors have varying forward voltage drops and current requirements. The use of a 390-ohm resistor as an alternative to 470 ohms provides flexibility in brightness adjustment, allowing for experimentation with different resistor values from available components.

The oscillation frequency, which dictates the flashing rate of the LED, is set by the combination of resistor R2 and capacitor C1. The formula for calculating the flash rate is approximately three time constants (3*R2*C1), meaning that the time it takes for the LED to turn on and off is directly influenced by these components. Careful selection of R2 and C1 will allow for tuning the flash rate to suit specific needs.

Transistor Q1 acts as a switch, controlled by the biasing provided through resistor R1. This resistor must be chosen carefully to avoid saturating transistor Q2 when the capacitor is disconnected, ensuring that the circuit can oscillate properly. If oscillation does not occur, adjustments to R1 or R2 may be necessary.

Diode D1 serves two primary functions: it enables higher duty cycle operation and limits the negative feedback at the base of Q1 to -0.7 volts, which helps stabilize the operation of the circuit. In applications where a lower supply voltage (6-9V DC) and low duty cycle are sufficient, D1 can be omitted, simplifying the circuit design.

Overall, this LED flasher circuit is versatile and adaptable, allowing for adjustments in component values to achieve desired performance characteristics while utilizing readily available electronic components.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.