The project described here uses two laser pointers to detect a vehicle entering a controlled area. It will respond on entry to the area and will not activate when the vehicle departs. In my application it alerts me to a visitor entering the driveway while ignoring anyone leaving. Also included are counters which establish a time interval which minimizes false alarms. The output is a single pulse which activates a relay for .2 seconds. This provides a degree of isolation between the logic circuitry here and any additional equipment you may wish to use to respond to an activation.
How It works
Two laser beams are positioned to cross the driveway. These are positioned one in front of the other about 8 inches apart, and about 20 inches above the ground. Thus one will always be broken before the other depending on the direction of vehicular travel. For sake of discussion here we call the one closest to the road "A", and the one nearest the house "B". The intent is to activate when "A" breaks before "B", and to ignore the vehicle if "B" is broken before "A".
One concern is how ambient light may affect the sensors. This issue has been addressed by modulating the power to the lasers. The lasers are switched off and on at a 2 kiloHertz rate. Since ambient light is steady state, and the amplifiers used in the detector are capacitively coupled, only the pulsing laser beam is passed through. Direct sunlight into the sensor still may saturate the photocells and positioning should take this into consideration. Avoid locations where the sun can shine directly into the sensors, but normal light coming from an angle is generally not a problem.
The Post Lamp and Laser System as installed. The receiver is located in the box
at the base of the lamp, the lasers on the pedestal to the right of the driveway.
Below is a closer view of the Lasers and Receiver units. A cable buried under the driveway connects the two devices.
Refer to the schematic below as the description of circuit operation is explained. This circuit is built on a plug-in card for easy removal in event service is required. U4 is the Clock Generator which produces the 2 kHz square wave which modulates the lasers. Its output is fed to an open collector Laser Driver transistor which sends it along to the Laser Control in the Laser Box. More on that shortly. The 2 kHz signal is also divided down by U5 and U6 to provide timing for the pulse delay and generation.
The "A" and "B" laser beams strike their sensors where The signal is amplified by U1 and U2 for each channel. The output is detected and converted to a DC signal of 4 Volts, measurable on the designated Test Points. Level controls are provided to allow for proper setting of this level.
U3 provides comparator circuits that go high any time a signal of at least 2 Volts is not present on the detector Output. This occurs when a vehicle breaks the beams. If "A" breaks before "B" the NAND gate will pass the pulse along, setting the latch. This will place a positive signal on the data input of the shift register. Clocking will begin to shift this through each output. After about 2 seconds the outputs will be configured to allow the interval between two pulses to cause the output transistor to turn on. This will energize the external relay causing a valid detection to occur.
Once the vehicle passes through and both lasers are again active the comparator outputs will again go low. U9 will detect this condition forcing the Clear input on the shift register low. It also clears the latch removing the positive on the data input pin. This will reset the system for the next event.
Gating on the output of the comparator determines whether or not the latch is set. If "B" activates before "A", the action of the "A" comparator is prevented from setting the latch. Under this condition the shift register data input pin is low, thus no pulse is clocked through. This causes the system to ignore vehicles leaving the driveway.
The Lasers are located in the Laser Transmitter Box. The Laser Box contains two regulators, one for each laser. These are adjustable and are used to set the proper current for each laser diode. The 2 kHz signal is sent via cable from the Laser Driver mentioned previously. Here it is used to pulse the lase diodes on and off at the 2 kHz rate, modulating the beams. Physical adjustment is critical here; the lasers MUST be aligned to strike the receivers across the driveway. Failure to maintain this will result in false alarms or a failure to detect a vehicle's presence.
A board layout is also provided as well as external wiring needed to make the system operational. These drawings follow:
Board Layout and Component Placement
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