Published:2009/7/13 5:02:00 Author:May | From:SeekIC
The DRL circuit is a variable-duty-cycle power oscillator that allows pulses of current to be applied to the headlights whenever the ignition of the vehicle is turned on. The 12-Vdc power that drives the circuit is taken from two sources in the vehicle. Power for the headlights is provided by the vehicular battery and generator system, and is input through fuse F1 to the circuit. The power source for the digital or logic part of the circuit is the + 12-V line that feeds the ignition system of the vehicle. When the ignition is turned on, the circuit is on, and when the ignition is turned off, so is the circuit. When the ignition is on, power is applied through D1 to operate IC1, a 555 timer that is connected as a free-running multivibrator. Pin 3 of IC1 provides the output of the oscillator; the duty cycle is determined by the following ratio: (R1 + R2) /R3 Potentiometer R1 allows the duty cycle of the negative part of the waveform to be adjusted from about 10 percent to almost 50 percent. Adjustment capability is provided so that the intensity of illumination can be set as desired. A set of logic gates (IC2), powered by the battery and generator, is used to control the operation of the circuit. When the ignition is off, the logic output of pin 3 of IC2-a is high. Under that high-logic condition, the transistors are not forward-biased and the headlights remain off. When the ignition is on, the oscillator is powered through D1. The pulse train output of IC1 appears simultaneously at the gates of Q1 and Q2. Those two transistors then feed + 12-V pulses through their respective diodes, D3 and D4, to the low-beam headlights of the vehicle. The diodes provide isolation between the DRL circuit and the electrical system of the vehicle. The design of the circuit makes it possible to add second pairs of hex-FET transistors and corresponding diodes to the circuit and have them driven by IC2-c. Thus, if desired, the taillights of the vehicle may also be used for DRL operation.
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