LIDAR Signal Detection to warn against Laser Speed Traps

[OvaR8ed]

I know that LIDAR Speed Trap Detectors exist since it is used for signal jamming. In my country that's illegal, but we are supposedly allowed to detect if signals are present.

Information on these LIDAR devices are very limited, but I have found the following:

When a car enters the trapping range (typically 200-300m / 600-900ft) the gun is fired at a good reflective surface on the car (e.g. the number plate). The gun sends a number of pulses with a laser and times the return of the pulses to receiver. Each pulse records time of flight of the respective laser pulse and thus it basically measured multiple distance measurements sequentially over time. The change in these distances are used to calculate the speed of the car.

Laser power: 50uW
Laser cone: 4 milliradians (translates to a 1 square meter / 9 square feet footprint on the car at 300m)
Laser Wavelength: 904nm
Laser Carrier: Unknown and may differ between manufacturers.
Pulses per Second: Differences between manufacturers, but 45Hz to 250Hz are common.

Now the first problem is that we don't know the carrier (and it may vary between different manufacturers). Now, unless we want to record and reproduce the signal for jamming (which I don't) we simply have to detect whether there is a man-made signal or not.

Here is the concept:

Use an IR Phototransistor to sense IR light. This will pick up IR rays in daylight as well, but this reading without a man-made signal will present itself as a slightly noisy DC Constant that changes slowly over time. If IR pulses are present, it will be seen as an oscillation on top of this DC bias (provided the daylight doesn't max out the output of the transistor and thus clipping the signal)

A DC Blocking Capacitor (AC Coupling) will remove the DC Bias caused by daylight. If we further filter out frequencies outside of the range 30-300Hz using a Band-Pass Filter we will have a very simple device scanning for pulses in the range 30-300Hz.

We can feed the output signal (after the filters) to a a timer like the 555 to trigger a Monostable pulse beeping a buzzer and warning the driver that man-made IR signal in the range 30-300Hz may be present.


Anyone have any thoughts to further refine the concept?