A two LED flasher is about as simple as it gets.A simple LED flasher is a good project to get started with AVRs. This ATtiny13 based circuit alternately flashes 2 LEDs (or however you want them flashed). Note: Circuit and firmware now available.source

The circuit is rather simple. Main parts are the microcontroller, the light sensor and the RGB-LED. The sensor and R4 are forming a voltage divider. An ADC (Analog Digital Conversion) channel at pin 3 is used to read the values of the sensor. The circuit is designed for 5V power supply. It has no integrated power regulator.source

This design uses a discontinuous mode flyback power supply fed from an input stage with minimal capacitance. With a fixed duty cycle over an AC line cycle, this allows the peak drain current envelope, and therefore the input current, to follow the input AC voltage waveform to give high power factor and low harmonic content. Although this simple configuration gives both output regulation and power factor correction in a single stage converter, it does require higher peak drain currents compared to a standard power supply with substantiation input capacitance.source

Using the Buck-Boost topology, the circuit shown in Figure above generates a non-isolated constant current (CC) output for driving LEDs. Typical uses include night-lights, neon sign replacements,emergency exit signs or any application utilizing LEDs for lighting.The AC input is rectified and filtered by D1, D2, C1, C2, RF1, and RF2. Two diodes improve both line surge withstand (2 kV) and conducted EMI. Resistor RF1 should be a fusible flameproof type, whereas RF2 can be fl ameproof only.source

Figure above shows a simple buck-boost converter, operating in open loop with no output feedback, being used as a constant current LED driver. The circuit relies on the internal current limiting function of the LNK302PN, which ensures constant current is supplied to the load. Typical uses include night-lights, neon sign replacements,emergency exit signs or any application utilizing LEDs for lighting.The AC input is rectified and filtered by D1, C1, C2, and RF2. For safety, resistor RF1 should be a fusible fl ameproof type, whereas RF2 can be fl ameproof only.source