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?

Concept / Outcome: I'm planning to drive a laser pointer slowly using a simple geared down 12V DC motor through an angle of about 70 degrees using output from PC. The gearbox is designed such that the laser travels very slow (i.e. 70 degrees = 5mins). I want to place LDR sensors at the limits ofthe sweep to detect the laser and give feedback to my software to stop the laser and initiate other stuff on the software side. Interface and I/O: I don't have a micro-controller so I'm planning to simply hack a USB keyboard for the chip. Windows already has drivers for it, it has 3 binary output channels (CAPS LOCK LED, NUM LOCK LED and SCROLL LOCK LED) and more input channels than there are buttons / keys. When the laser hits an LDR it opens a transistor circuit to switch a Solid State Relay connected to a specific key. This hotkey will stop the motor and change the state of one of the outputs to change direction of the motor. The motor can then be started again from the software with the other LDR waiting on the other side for the cut-off signal. Potential Problems: The LDR recieves a contious signal from the laser and the computer input only wants a short pulse so I decided to use a RC differentiator circuit to send a short pulse when the laser first hits the LDR. The motor stops over the LDR and even though the laser keeps the LDR circuit in ON mode, the signal (hotkey) to the PC will be off after the short pulse. The solid state relay that initiates the hotkey for the keyboard chip requires between 1-5V and 10mA to switch on so I designed the following circuit to send the pulse. (See pic/link below) Any comments and/or suggestions are very welcome, thanx!