How to build Fading Red Eyes

Two Light Emitting Diodes slowly light up and fade away due to the circuitry, giving it a feel of blinking red eyes in darkness.
It is the perfect way to set up your own horror show in Halloween. It can also be installed in series for the Christmas tree or during Diwali or a decorative power indicator for the daily appliance.

A 3 volt linear wave is generated at Pin 1 via LM 1458 IC and followed by a transistor which buffers the emission. A 47 K resistance coupled with a 22uF capacitor is joined at Pin2 to limit the frequency to 0.5Hz. The rate can be varied using a 100K pot(potentiometer) replacing the 47K resistor.

Two operational amplifiers govern the circuit. The transitional voltage is produced by the first op-amp which slowly modulates voltage between 3 to 6 volts. The second op-amp supplies alternating voltage of 2 and 7 volts to charge the capacitor and discharge it via constant current.

Two 47K resistors establish a fixed voltage when joined across Pin 3 & 6 to keep the reference voltage constant. One op-amp works as an inverted amplifier connecting a capacitor across Output Pin 1 and Pin 2 which inverts the output. One of the op-amp compares the voltage to regulate the output in Pin 7. The reference voltage needs to be lower than the input at one point and needs to be higher with the reference at the higher end. A 100K resistance provides a positive feedback and controls the switching point via a capacitor that changes the direction of current. The op-amp also changes its directional output. The triangular waveform at Pin 1 is achieved which moves up or down keeping input constant at 4.5 Volts.

The Point when the LEDs go off can be adjusted by altering the resistance value across Pin 3 and grounding Pin 6. Using a 9 Volt Battery will ensure long operational hours.

Please note that LEDs have a narrow viewing angle of 30 degrees and appears brightest if viewed directly from the front or it might loose the desired effect of the eerie feeling it can generate.

Circuit diagram

The circuit here shows a double pair of LEDs that operate intermittently such that while a pair illuminates, the other pair fades out.

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