Electronics Puzzler Solution: Headlight Trap

By Forrest M. Mims III

What are some ways for government agents to design an automatic "headlight trap" to detect banned incandescent headlights?

Incandescent lamps are broadband sources, and LEDs are not. Conventional LEDs are modified to emit white light by coating a blue LED with a phosphor that glows green and red when stimulated by blue. The resultant colors merge together to form a whitish light, often with a visually obvious blue tint. An Ocean Optics USB2000 fiber optic spectrometer nicely shows the spectrum of light emitted by a typical white LED and an incandescent flashlight bulb.

Light Emission

Several methods can be used to automatically detect the differences in the spectral distribution of light emitted by incandescent lamps and LEDs. The key difference is that incandescent sources emit abundant near-infrared and white LEDs do not. One solution is to employ a miniature spectrometer that measures the intensity of light from the blue to the near-IR. This provides a sophisticated method to quickly and automatically (with suitable software) identify incandescent and LED sources. Government programs tend to prefer sophisticated solutions like this.

There is a much simpler and cheaper solution to the problem that does not require an expensive spectrometer and relies only on the main spectral difference between white LEDs and incandescent lamps. This method makes the headlight detector sensitive only to near-IR, which means the circuit will ignore LED headlights while instantly responding to incandescent headlights. The complete device can be made small enough to rest on the dashboard of a patrol car. A common silicon photodiode with an IR filter will provide a suitable detector. Many infrared remote control receivers detect the invisible signal from the transmitter by means of a silicon photodiode installed in near-infrared transmitting epoxy. This kind of photodiode should work well in this application. A silicon solar cell will work well so long as a near-infrared filter is placed over its sensitive surface. The photocurrent from the photodiode or solar cell can be amplified by an op-amp and coupled to a comparator that will switch states only when an incandescent head light is detected. The output of the comparator in the circuit below activates a bright red LED. A buzzer can be added to the circuit to awaken the headlight trap patrol officer.


Going Further

The Society of Authentic Antique Motor Cars (SAAMC) is unhappy about the arrival of headlight traps. Can you think of ways they can fool the headlight traps?

The National Bureaucracy of Nocturnal Vehicle Statistics wants the headlight trap to do double duty by counting all the vehicles that pass along a stretch of highway at night. How can the basic circuit above be modified to detect and count both LED and incandescent headlights?

A second circuit based on the one above can provide audio clues about the kind of incandescent headlight being detected if the car is driving along a relatively rough road. Before reading the next paragraph, do you know how?

This method ignores the spectrum of light emitted by the headlight and concentrates on its mechanical construction. The filament of an incandescent lamp is installed across two electrodes at the focus of a parabolic reflector. Very slight movements of the filament amplitude modulate the intensity of the beam by defocusing it. The beam from some incandescent headlights will be modulated when the vehicle passes over bumps in the road.

You can modify the circuit above to detect the modulated signal from bouncy headlights by inserting a 0.01 microfarad capacitor between pin 2 of the op-amp and the photodiode. This will allow the modulated photocurrent caused by the vibrating filament to reach the op-amp while blocking any nearby incandescent lights. Connect output pin 1 of the op-amp to the input of an audio amplifier made from an LM386 or similar chip or use an off-the-shelf karaoke or MP3 player audio amplifier. The end result will be very distinctive tone bursts from certain incandescent headlights if the road is rather rough.


This Puzzler is based on years of designing and testing various light wave communication systems, the first of which employed a receiver built into a 6-volt lantern light. Two back-to-back solar cells were mounted where a flashlight bulb once stood. The cells were connected to an audio amplifier installed inside the body of the lantern light along with a small speaker, power switch, volume control and 9-volt battery. During a night time test of this receiver and a light wave transmitter modulated by a radio signal, I was surprised to hear various pings and chirps when a car drove by. Other passing cars caused the same effect, which is when it became obvious that the headlight filaments were vibrating in response to bumps in the road. These observations were made along a country road in March 1966, when I was a senior at Texas A&M University (and should have been studying for the next day's classes).

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