Simple LED Flasher Circuit: Perfect for Novice Experimentation
This LED flasher circuit comprises merely five components, making it an excellent choice for beginners to explore and experiment with. Understanding its operation is straightforward. When a battery is connected, capacitor C1 begins charging through the 1 MΩ resistor R1. The capacitor is linked to the emitter of the PNP transistor (BC557). The base junction of this transistor is connected to the positive 9 V supply via an LED. Therefore, the voltage at the base junction equals the supply voltage minus the forward voltage drop of the LED. Typically, a red LED has a forward voltage drop of about 1.6 V, resulting in a voltage level on the transistor’s base junction of 7.4 V (9 V – 1.6 V).
Creating LED Flashing Effect: Circuit Operation Explained
When the voltage on the capacitor rises to a level that forward biases the base-emitter junction of the PNP transistor, current flows through its emitter-collector junction. This current flow forward biases the base-emitter junction of the NPN transistor, turning it on. Now, the NPN transistor conducts, bringing its collector close to ground potential. This action grounds both the LED cathode and the base of the PNP transistor, reinforcing the ON state of the PNP transistor and allowing a relatively high current to pass through the LED, causing it to flash.
Efficient and Long-Lasting Flashing Circuit: Practical Application
Upon discharging the capacitor, the transistors turn off, and the cycle repeats. With the specified values in the circuit diagram (C1 = 1 µF, R1 = 1 MΩ), the LED flashes briefly once every two seconds. Remarkably, the circuit operates even with a battery voltage as low as 2 V and consumes so little current that a fresh 9 V battery can sustain continuous flashing for many months.
Versatile Modifications for Different Functions: Metronome and Tone Generator
Even depleted 9 V batteries unsuitable for other applications can power this circuit. The second diagram illustrates a modification where a low-power 8 Ω loudspeaker is connected in series with the LED, transforming the circuit into a metronome or tone generator. Depending on the values of capacitor C1 and resistor R1, the loudspeaker produces either a repetitive click or a tone.
Adjusting the values of R1 and C1 alters the oscillation speed; smaller values result in a faster oscillation. In this modified circuit, R1 and C1 have values of 22 kΩ and 100 nF, respectively.