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Build a 13-Way Optocoupler Fog Lamp Sensor Circuit
Description
Over the past several years, a rear fog light has become a legal requirement for trailers and recreational vehicles, aiming to enhance visibility during periods of reduced visibility caused by fog. When this auxiliary light is activated, the fog light of the accompanying vehicle must be deactivated to prevent distracting glare. To accomplish this, a mechanical switch is now integrated into the 13-way female connector, facilitating the disabling of the fog light in the pulling vehicle and enabling the activation of the fog light in the caravan or trailer. For individuals utilizing a 7-way connector, this switching functionality can also be implemented electronically, utilizing the circuit depicted below.
Circuit diagram:
This circuit, employing a P521 optocoupler, monitors the status of the caravan or trailer’s fog light. Should the trailer’s fog light be illuminated, a current flows through diodes D1 and D2, activating the LED within the optocoupler. Consequently, the phototransistor conducts, thereby energizing relay T1. This relay then disables the fog light of the vehicle pulling the trailer. Individuals with limited technical expertise can easily construct this compact circuit on a small perforated circuit board and subsequently mount it near the rear lighting assembly of the towing vehicle.
Optocoupler P521
The P521 optocoupler is a solid-state device designed to isolate electrical circuits. It typically consists of an LED and a phototransistor housed within a single package. The LED emits light when current flows through it, and the phototransistor detects this light. When light strikes the phototransistor, it generates a current, enabling the two circuits to be separated while still maintaining communication. It is widely used in automotive applications where electrical isolation is crucial for safety and reliability. The device has a switching frequency up to 100 kHz and operates at a supply voltage of 5V. It has a quiescent current of 2mA. The P521 optocoupler can be used in circuits where a change in one circuit must affect another one while ensuring that the two circuits are electrically isolated.