Automatic 12 Volt Lamp Fader

This circuit resembles the "Fading Red Eyes" circuit found in the LED section, designed to fade a pair of red LEDs. In this iteration, the lamps are dimmed by adjusting the duty cycle, allowing for the use of higher power incandescent lamps with minimal power loss. The switching waveform is created by comparing two linear ramps of differing frequencies. The higher frequency ramp waveform, approximately 75 Hz, is generated from one section of the LM324 quad op-amp configured as a Schmitt trigger oscillator. The lower frequency ramp dictates the fading rate and is produced by the upper two op-amps, similar to the "fading eyes" circuit. The two ramp waveforms at pins 9 and 1 are compared by the fourth op-amp, which generates a rectangular waveform with a varying duty cycle to drive the output transistor. A second transistor inverts the waveform, causing one group of lamps to fade while the other group brightens. The 2N3053 transistor can handle up to 500 milliamps, allowing for the connection of 12 strings of 4 LEDs each (totaling 48 LEDs) with a 220-ohm resistor in series with each group of 4 LEDs, resulting in approximately 250 milliamps. Alternatively, three 4-volt, 200 mA Christmas tree bulbs can be connected in series. For higher power 12-volt automotive lamps, it is necessary to replace the transistor with a MOSFET capable of handling several amps of current. Refer to the drawing below the schematic for potential connections.

The circuit operates by utilizing an LM324 quad op-amp, which is versatile and can function in various configurations. The first op-amp is configured as a Schmitt trigger oscillator to generate a high-frequency square wave at about 75 Hz. This square wave is essential for creating the rapid on-off cycling needed for the fading effect. The second and third op-amps generate a lower frequency ramp signal that controls the fading rate of the lamps. These ramp signals are linear and can be adjusted based on the desired fading speed by altering component values in the circuit.

The fourth op-amp compares the two ramp signals, producing a rectangular waveform whose duty cycle varies based on the input from the ramp signals. This varying duty cycle is crucial as it determines how long the output is high versus low, effectively controlling the brightness of the lamps. The output from this op-amp drives a transistor, specifically the 2N3053, which acts as a switch to control the power supplied to the lamps.

In the case of higher power applications, such as automotive lighting, the 2N3053 may not suffice due to its current limitations. Therefore, a suitable MOSFET should be selected that can handle the required current load. The circuit also includes provisions for inverting the output signal to create a complementary fading effect between two groups of lamps, enhancing the visual impact of the design.

For implementation, careful consideration should be given to the resistor values used in series with the LEDs or lamps to ensure they operate within safe current limits. The use of a 220-ohm resistor for each group of four LEDs is a good starting point, but adjustments may be necessary based on the specific characteristics of the LEDs or lamps being used. Additionally, the schematic should be followed meticulously to ensure all connections are correct, particularly when integrating different types of lighting components.This circuit is similar to the "Fading Red Eyes" circuit (in the LED section) used to fade a pair of red LEDs. In this version, the lamps are faded by varying the duty cycle so that higher power incandescent lamps can be used without much power loss.

The switching waveform is generated by comparing two linear ramps of different frequencies. The hi gher frequency ramp waveform (about 75 Hz. ) is produced from one section of the LM324 quad op-amp wired as a Schmitt trigger oscillator. The lower frequency ramp controls the fading rate and is generated from the upper two op-amps similar to the "fading eyes" circuit. The two ramp waveforms at pins 9 and 1 are compared by the 4th op-amp which generates a varying duty cycle rectangular waveform to drive the output transistor.

A second transistor is used to invert the waveform so that one group of lamps will fade as the other group brightens. The 2N3053 will handle up to 500 milliamps so you could connect 12 strings of 4 LEDs each (48 LEDs) with a 220 ohm resistor in series with each group of 4 LEDs.

This would total about 250 milliamps. Or you can use three 4 volt, 200 mA Xmas tree bulbs in series. For higher power 12 volt automobile lamps, the transistor will need to be replaced with a MOSFET that can handle several amps of current. See the drawing below the schematic for possible hookups. 🔗 External reference