Circuit diagram
Description
This circuit is similar to the fading eyes circuit above and is used to slowly brighten and fade interior lights of older cars.
The circuit is based around the LM324 low power opamp which draws
around 3mA of current, so it won't bother the battery if left connected
for extended periods.
The top two opamps (pins 1,2,3 and 5,6,7) form a triangle wave
oscillator running at about 700Hz while the lower opamp (pins 8,9,10)
produces a linear, 5 second ramp, that moves up or down depending on the
position of the door switch. The two transistors and associated
resistors serve to limit the ramp voltage to slightly more and less than
the upper and lower limits of the triangle waveform. These two signals
(700 hZ. triangle wave and 5 second ramp) are applied to the inputs of
the 4th opamp (pins 12,13,14) that serves as a voltage comparator and
generates a varying duty cycle square wave that controls the IRFZ44
MOSFET and lamp brightness. The 5 second fade time can be adjusted with
the 75K resistor connected to the door switch. A larger value will
increase the time and a smaller value will speed it up.
When the door switch is closed (car door open) the voltage on pin 8
slowly rises above the negative peaks of the triangle wave producing a
short duty cycle output and a dim light. As the ramp moves farther
positive, a greater percentage of the triangle wave will be lower than
the ramp voltage producing a wider pulse and brighter light. This
process continues until the ramp is 100% above the positive peaks of the
triangle wave and the output is maximum. When the door switch is open,
the reverse action takes place and the lamps slowly fade out.
The IRFZ44 shouldn't require a heat sink if the total load is 50
watts or less but the temperature of the MOSFET should be monitored to
insure it doesn't overheat. The on-state resistance is only 0.028 ohms
so that 4 amps of current (48 watts) is only around 100mW. For larger
loads, a small heat sink can be added to keep the MOSFET cool.
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