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Description

This design mirrors the "Fading Red Eyes" circuit (within the LED gallery) intended for dimming a set of crimson incandescent lamps. In this rendition, the illumination is modulated through adjusting the pulse width, permitting the utilization of higher-wattage incandescent bulbs with minimal energy waste. The modulating signal is created by comparing two escalating linear waveforms operating at differing frequencies. The higher-frequency ramp signal (approximately 75 Hz.) originates from one portion of the LM324 quad operational amplifier configured as a Schmitt trigger oscillator. The lower-frequency ramp governs the dimming pace and is generated by the uppermost two operational amplifiers, akin to the "fading eyes" circuit. The two escalating waveforms, applied at pins 9 and 1, are compared by the fourth operational amplifier, producing a fluctuating duty cycle rectangular signal to power the output transistor. A secondary transistor is employed to invert the waveform, ensuring that one cluster of lamps diminishes in brightness while the other group increases in luminance. The 2N3053 is capable of managing up to 500 milliamps, allowing for the connection of 12 strings of 4 LEDs each (48 LEDs) with a 220 ohm resistor in series with each group of 4 LEDs. This configuration would draw approximately 250 milliamps. Alternatively, three 4 volt, 200 mA Xmas tree bulbs in series could be utilized. For high-power 12 volt automobile lamps, the transistor will necessitate substitution with a MOSFET that can manage several amps of current. The details are shown in the illustration below the schematic.

Circuit diagram

Other possible hookups

More about components

The LM324 quad operational amplifier plays a critical role in this circuit's operation. These integrated circuits are versatile and commonly used for amplifying signals and generating waveforms. The LM324 is a four-channel operational amplifier, meaning it contains four independent amplifier stages within a single package. This reduces board space and simplifies circuit design. It is typically used in a variety of applications, including oscillators, amplifiers, and filters. The specific configuration – as a Schmitt trigger oscillator – utilizes the LM324's ability to switch rapidly between two states based on input voltage, which is fundamental for generating the ramp waveforms required for controlling the lamp fading rate. It is a popular choice for hobbyists and professionals due to its low cost and availability. Further specifications for the LM324 include a supply voltage range of 3V to 15V, a bandwidth of 1 MHz, and a slew rate of 0.8 V/µs, making it suitable for a wide range of signal processing tasks. The quad op-amp is chosen for its symmetrical output characteristics, vital for generating accurate and consistent ramp waveforms. The IC is provided with ten pins, eight for the amplifier channels and two for power supply. The output impedance of the op-amps is low, which is essential for efficient waveform generation. This allows for minimal signal loss when driving the output transistor. The input impedance of the op-amps is high, so the circuit is less sensitive to noise and interference.