Calling auto headlights “automatic” might be over-acknowledging their technological sophistication, for they rely on a fairly straightforward electronic system. Its primary element is something called a photoelectric cell (also known as photocell or photosensor), usually located on the windshield above the rear view mirror, or on top of the dashboard. The photoelectric cell must not be obstructed, because as its name suggests, it relies on light to produce electricity.
Incidentally, a wide range of photoelectric devices are commercially available off the shelf for a variety of industrial applications. Also, solar-powered calculators make use of photocells that depend on sunlight or artificial lighting to supply “battery” current for the electronic circuit board.
The photocell is made from the same silicon crystals that go into semiconductors and transistors, and all of them come under the category of solid-state components. These things are small, contain no moving parts, and experience no wear and tear. They rarely fail, except when subject to extreme heat or extreme cold, or they suffer a short circuit.
When light falls on a photocell, electron flow is initiated, resulting in electrical current being generated. This current flow means the photocell can be used to drive any other electronic device, on the basis of light availability. In the case of a car’s auto-activated headlights, the photosensors are specifically designed to sense natural light, so that road lamps are excluded. The current from the sensor runs a relay (an electrically actuated switch) that controls the lighting circuit.
At dusk (or in a dark basement carpark), when there is no longer sufficient light to initiate current flow in the photo cell, the relay is disconnected, so to speak. Unlike a simple switch, a relay has the unique capability to switch on one electrical circuit when energised and another when de-energised. Thus, when the headlights’ photocell cluster stops supplying current to its relay when there is no (or low) ambient light, the front and rear lamps of the vehicle are switched on automatically. These days, of course, the electrical signal from the photosensor is sent to an electronic control unit which does the rest.
In theory, a photocell can also be employed as a “night mode” trigger for any driving function. Perhaps then, there should be one dedicated sensor to remind the driver that it’s time to switch on his headlights!
Judging by the number of cars running around in the night “without” headlights, it would be reasonable to assume that there are drivers in Singapore with impeccable night vision. Not so, of course. The problem, really, is the combination of a permanently lit instrument panel and the sheer lack of awareness on the part of the driver.
In the past, an unlit dashboard was a sign that the headlights were off. But in the majority of cars today, the instruments light up when the ignition is switched on – in the day or night. Admittedly, the excellence of Singapore’s street lighting does play a part in “fooling” some motorists, but surely the vehicle’s headlights (especially on modern cars) spread enough illumination ahead to be blatantly obvious when it’s absent?
Even more surprising is that many of these cars have auto headlamps that are literally idiot-proof – they light up when the sun goes down! However, said feature also has an “off” switch, which is an override position that certain “unseeing” drivers seem to prefer. I hope they read this story.