This is a simple but useful LED flasher that can be used as power on indicator or along with alarms. Simply you can use it as a Mock Alarm in the gate or door to baffle the intruders. Do this very simple circuit and it can be an evening project for you that take only 10 minutes to complete.
The circuit is a simple Relaxation oscillator using two complementary transistors BC 547 and BC 557. Capacitor C1 is doing the trick and it gives the positive feedback to start the oscillation of T1 and T2. R1 and C1 form the timing components of the oscillator and with a 330 K resistor (R1) and 10 uF capacitor (C1) the LED blinks around 1 Hz rate. By varying the value of either R1 or C1, the blinking rate of LED can be changed. That means, lower value gives fast blinking and higher value gives slow blinking.
What happens in the circuit is like this. When power is applied, C1 starts charging via R1. When C1 fully charges, T1 conducts and pulls the base of T2 to ground potential and it also conducts. The collector of T2 gives power to LED and it lights. At the same time C1 discharges through R3. So T1 and T2 turn off. This turns off LED. Again C1 starts charging and the cycle repeats making the LED blinking continuously.
Flashing LEDs are available now which needs only a current limiting resistor. If it is not available, you can use this circuit. Anyway, it is a simple circuit for the beginner to enjoy its fun.
Bread board assembly of Flasher Circuit
Smallest Flasher ever seen
Circuit build through point to point soldering
You can use any type LED. The 3 volt White and Blue LEDs also work well. Transparent LEDs give bright flashes than opaque LEDs. High bright LEDs like the one used in Traffic signals is the best to give long range light.