Automobile Lamp Dimmer Circuit

Many folks have voiced a desire to have an aftermarket dimmer circuit for their autos. Such add-ons as puddle lights are made more dramatic if they dim automatically rather than just extinguish when the door is closed.

This circuit provides a variable delay after the door switch is opened (door is closed) and a variable rate of dimming of the lights. The adjustable rates are independent of load. The Field Effect Transistor output provides a load capacity of up to 75 watts, thereby allowing multiple incandescent lamps to be dimmed simultaneously. Quite literally, hundreds of LED lamps can be simultaneously controlled.

It's operation is as follows:

Initially,the output of the comparator, U1, is held low by virtue of the pull-up resistors R1 and R2. This reverse biases the disconnect diode R2. That allows the pull-down resistors R5 and R6 to hold the enhancement mode FET Q1 in cutoff.

When the door switch, SW1 is activated (shorted - door open), capacitor C1 is immediately charged and comparator U1 switches to a high output, diode D2 conducts, and Diode D1 limits the voltage at the gate of the FET to about 4V preventing damage to the FET while simultaneously turning it on hard and discharging capacitor C2. The load then has the full system voltage (nominally 12V) applied to it.

When the door switch SW1 is deactivated (open - door closed), capacitor C1 discharges through R1 and R2 until pin 2 of U1 reaches approximately half of the system voltage whereupon U1 output switches to low, diode D2 disconnects and capacitor C2 discharges through R5 and R6 until Q1 turns off. Diode D3 is optional and, if used, disconnects the circuit from all other loads that may be connected directly to the door switch preventing them from becoming a parallel discharge path for capacitor C1.

The circuit comprised of D2, C2, Q1, R5 and R6 is a Miller Integrator. That is, the drain (pin 4) of the FET attempts to rise in voltage as the gate (pin 1) falls in voltage. Capacitor C2 holds the gate at a nearly constant voltage as it (C2) discharges linearly through R5 and R6. This causes the voltage at the drain of Q1 to rise linearly until it reaches system voltage. At that point, C2 has allowed the gate voltage (pin 1) to fall to cutoff for the FET and current stops flowing to the load altogether as the FET has effectively become an open switch.

R1 adjusts the delay of the start of turn off from about 1.5 seconds to about 15 seconds. It's value can be scaled to affect the maximum delay. R2 controls the minimum delay and and also can be scaled, but should not be reduced below 10K or the variable resistor R1 could sustain damage.

R6 adjusts the turn off dimming period from about a half of a second to about 5 seconds. It's value can be scaled to affect the maximum dimming period. R5 controls the minimum dimming period and also can be scaled, but should not be reduced below 10K or the variable resistor R6 may be subject to damage.

All resistors can be 1/8 watt carbon film. the capacitors should be rated at 25 Volts to assure they are not damaged by transient voltages present in the automobile environment. (35V wouldn't hurt). The comparator U1 and FET Q1 are specified as surface mount devices but substitution of LM321 in an 8 pin DIP package for Q1 and NDP6020 in a TO220 package for Q2 is acceptable.

A circuit board has been designed for the surface mount devices and can be made available as a design file. Kits of parts and circuit boards can be made available if demand exists. Ditto completed populated boards. r.riis@rriis.com

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