Here in the US, the police are using laser-based speed detectors (LIDAR) to measure the speed of motorists. Light emission is not presently regulated by the federal government, so there is no federal law against emitting light at the right frequency and pulse duration to confuse LIDAR. However, individual states may enact legislation making it a crime to use such a device to jam LIDAR used by law enforcement, so it's up to you to insure that its legal to use in your jurisdiction or be willing to face the consequences.
Existing laser-based speed measurement devices work by emitting pulses at a rate of between 100 to 500 pulses per second and measuring the amount of time they take to return from a target. The amount of time they take to return corresponds to the distance between the LIDAR unit and the target. By comparing a number of measurements made at different times, they can use the delta of the distance divided by the delta in time to computer the vehicles speed with some accuracy. LIDAR units emit coherent light at a wavelength of approximately 900nm which is in the near infrared region (visible by CCD cameras).
A number of approaches have been tried to jam these devices with varying levels of sophistication. Here are some I am aware of:
- Leave headlights on -- some infrared energy is emitted by most headlights -- this raises the noise floor for the pulse received by the jammer and decreases the speed detection range somewhat
- An extension of the above -- put a number of infrared-emitting LEDs on a PC board and mount it on the vehicle pointed in the expected direction of the LIDAR device
- Modulate infrared-emitting LEDs on a PC board to emit pulses designed to confuse the LIDAR speed detection device
- Modulate an infrared-emitting laser to emit pulses designed to confuse the LIDAR speed detection device
The first two approaches are of limited usefulness because while they may reduce the effective range of LIDAR, it may not be sufficient to prevent the device from working at the ranges it is normally used (the manufacturers recommend not more than 1000 feet, but they apparently work considerably further out).
The fourth approach is advocated by a number of manufacturers of somewhat expensive jammers already on the market for this purpose ($200+).
The third approach is the one I have taken myself, for the following reasons:
- Lasers are much more expensive than LEDs
- It is not clear why a laser would be more effective than an LED. Consider that while the laser in the LIDAR unit may be focused into a narrow beam in order to strike only a single vehicle 1000 feet away, a laser being used in a vehicle attempting to jam a LIDAR unit will need to be aimed in a much wider pattern in order to be sure to strike the LIDAR device.
- If the receiver in the LIDAR unit were tuned to the precise frequency of the laser, then a laser would have an advantage over LEDs in that all of its energy would be tuned to the correct frequency. However, building a filter that would operate over a sufficiently narrow range of wavelength is likely to be non-trivial and very expensive. Besides, anecdotal evidence suggests that the existing LIDAR units use CCD arrays to detect the returned pulses. This is apparent by examining the displays of the LIDAR units where you can see a rectangle placed on the target vehicle showing where the strongest returned pulses are coming from.
- MinPulseRate(MHz) = 1000 / (2 * d)
I chose a minimum effective jam distance of 400 feet. This requires a pulse rate of 1.25MHz. Cheap LEDs are available with power levels up to 100mW that can be modulated at frequencies up to 12MHz and with peak emissions at or near 900nm. An equivalent laser would be very expensive. Furthermore, if the duty cycle of the pulses is kept low, the peak power levels in the LEDs can be increased considerably further multiplying their effectiveness.
Based on the above information, I've designed an inexpensive LIDAR jammer based on the following:
- I use a simple 74HC04 based crystal oscillator to generate a 12MHz clock.
- I feed the clock into another 74HC4017 decade counter to generate 1.2MHz pulses with a 10% duty cycle
- The output of the 74HC4017 drives a MOSFET which in turn drives the infrared LED
The above circuit can be modified in several ways to enhance the performance:
- Multiple LEDs can be driven in parallel by the same 74HC4017 output in order to increase the jam strength
- Multiple LEDs can be driven by different 74HC4017 outputs in order to further decrease the minimum jam distance provided the pulses emitted by the LEDs are stronger than the returned pulses from the LIDAR
At this point, I have built a prototype device on breadboard and tested it in several ways:
- Viewed the output with a camcorder to verify it is working and to compare relative output intensity with different amounts of current
- Using an oscilloscope, measured the peak current through the LEDs with different voltages and current limiting resistors in place to generate the peak output current allowed by the LED manufacturer for the 10% duty cycle and extremely short pulse duration being used.
- Used the breadboarded circuit to trigger the LIDAR detection circuitry of my commercial radar/laser detector mounted in my car to confirm that it is working.
Since I do not myself have access to a LIDAR unit, I cannot comment on the effectiveness of this home-built unit. However, there are people who specialize in doing this kind of testing and it may be possible to persuade them to test out one of these units against a commercial LIDAR if I were to provide one to them for such purposes.
This is as far as I have gone with this project at this point. The next step is to build it on perfboard and find a suitable enclosure for mounting it (which will be highly dependent on exactly where it will fit on my car). Since the front license plate is the chief reflector of light for an approaching car, it makes sense to mount the jammer as close to this as is possible, while at the same time keeping it discreet so that it is not visible to casual observers.
Some commercial jammers have detectors in them and only switch on when they detect an incoming laser beam. However, the amount of energy consumed by this unit is so small (a couple of watts), and the internal heating in the LEDs is so small that combined with their very long lifetime, I'm not convinced the extra complexity is worthwhile and am sticking with a simple on/off switch for the time being.
Let me know if there is any further interest in this. I can post the schematic and other details if Ian doesn't object. If anyone who is interested has access to more sophisticated equipment (spectrophotometer for example) that would be great, too.