Here is a simple LED blinker or flasher for decoration purpose. The 20 High bright LEDs flashes alternately giving a brilliant colour display. The circuit is low cost and derives DC directly from AC without using a step down transformer. The design is too simple so that the circuit can be assembled on a small piece of common PCB.
Power to the circuit is derived from 230 Volt AC through AC capacitor CX which is used to step down 230 volt AC to low volt AC. 474 K 250 Volt AC capacitor gives around 20 volts AC and 40 mA current. This low volt AC is rectified through the Bridge rectifier D1 through D4 and filtered by C1.
The LED driver is an Astable multivibrator using two NPN transistors T1 and T2.The circuit works on the principle of charging and discharging of capacitors C2 and C3. Current from the positive rail flows through first set of LEDs 1- 10 to the collector of T1 through resistor R3. Resistor R3 limits current through the LEDs to protect them.
The current through R3 and LEDs charges capacitor C2. It then discharges through the base of T2 and resistor R5. This gives base current to T2 and it conducts. As a result second set of LEDs 11-20 lights. As the Capacitor C2 discharges completely, T2 turns off and LEDs 11-20 also turns off. The same thing happens in the other side also. This gives alternate flashing of LEDs. Thus the flashing effect is produced through the switching of T1 and T2 by the charge from capacitors.
Connection: Connect high bright Red LEDs 1-10 serially from R3 to the collector of T1. Similarly connect High bright Green LEDs 11-20 from R4 to the collector of T2. Cathode of LED 10 and LED 20 should be connected to the collector of T1 and T2 respectively. The LEDs can be fitted alternately on the display boards or Photo frames by drilling holes. Connect the LEDs with the circuit using thin plastic wires.
Caution: This circuit is dangerous since it uses high volt AC which can be lethal if handled carelessly. Do not touch or trouble shoot when the circuit is connected to mains. Do not try the circuit if you are not an experienced person to handle AC circuits.