As background to hacking an existing LASER rangefinder, here is an extremely good account of one man's Attempt tounderstand and serially interface a Aparkfun Prexiso LASER rangefinder - he failed but gives extremely interesting and probably useful information of what he found.
Sparkfun utrasonic rangwefonder module.
Expensive considering what a LASER tap measure costs.
Variants and basic specifications . 25 foot range claimed. Useful information.
A Doppler RADAR using RF operating at a few hundred MHz - possibly using a Gunn door opener module from days of yore, can have a range of 'very large'. I once owned an APN1 Radio Altimeter from a Bristol Freighter (aircraft) and it managed 1000's of feet with ground as the 'reflector' - lots of ground admittedly - and used acorn tube thermionic valves. You should be able to get as much range as your heart may desire using RF.
The APN 1 used a simple but useful method to NOT have to measure time of flight explicitly. Time of Flight (TOF) measurement is possible but at short ranges involves VERY short times. A nanosecond is a light-foot time wise!.
The APN1 transmitter was frequency swept and the received reflected signal was mixed with the current transmitted signal. The returning signal wis at the tx freq when the signal left and the tx freq has been swept to some other frequency by the tome the signal returns. The difference frequmcy, obtained buy mixing tx and reflected signal, gives a direct measure of range.
Discussion - while this relates to WW2 RF equipment it is directly applicable to a modern short range version
Basic principle:
Yee Ha!!!
You could use an angle measuring two points on baseline classical rangefinder. This could use a LASER to make two spots which you arrange to coincide under Arduino control. An olde worlde approach, but one which is eminently doable.
If you used a 1 metre baseline and one beam went straight out and the other was moved to conicide and you then measured tha angle of he movable spot.
At 5 metres the angle change for 1 metre increase is 78.7 degrees to 80.6 degrees = + 1.85 degrees
10 to 11 metres, angle change = 0.516 degrees
15 to 16 = 0.238 degrees
20 to 21 = + 0.14 degrees
25 to 26 ~= 0.1 degrees
30 to 31 degrees = +0.06 degrees
You can decide at what range the degrees change is too hard to read accurately.
A longer baseline decreases the position in the table.
eg a 2 metre baseline gives 30/2 =15 degree effective result.
Distance
........Degrees
................. Delta degrees
1... 45.0
2... 63.4... 18.4
5... 78.7 ...2.7
10..84.3 ...0.63
15.. 86.2... 0.27
25.. 87.7... 0.10
30.. 88.1... 0.07
ADDED:
I included the APN1 main circuit diagram as much for fun to show what could be done in 1940 with Acorn tubes BUT in gact it is potentially useful for ideas. The diagram above is readable if you look at it full size as supplied. (Right click and then copy or save or open).This is "as supplied" quality -somebody has scanned the original to a gif in 2 colour "black & white".
I have copied part of the circuit below and added notes. Something like this could be done with modern components with "relative ease" [tm].
The key special magic is provided by the modulator - here a voice coild driven variable capacitor wihich scans the transmitter across a frequency range. The modern equivalent is a varacror diode - variable capacitance with reverse voltage. The oscillator at right middle drives this modulator.
The transmitter is a pair of push-pull acorn tubes driving the transmit aerial at bottom left. A "sniff" [technical term :-) ] of RF is sent from the TX to the RX at top left. A balanced detector - here a pair of Acorn tubes V101 and V102 bu nowadays a Schottky diode ring mixer or similar take the TX sniff and the received echo and mix them to provide a difference signal at the output at top right. This is then amplified as the range signal. Implementing this with "solid state" parts would result in a simple and possibly even effective result. Transmitter beam width would be a major consideration. A GHZ or so oscillator and a lonnnnnng Pringles can antenna (or two) may be enough. Modern cellphone and WiFi components are so readily available (surplus if needs be) that the "hard" RF aspects could be covered by premade parts.
The BISS0001 is a not well documented (In English) Chinese IC. How it operates with certain changes is not well known. But the changes you made shouldn't affect it's life.
In the posted schematic, R13, along with RT1 (The Time potentiometer) and CY1 control the On Time of the Module's output. R33 and CY2 control the Minimum Off Time. There is also Jumper JP1 which handles the Trigger mode.
The Formula for the On Time is Tx ~= 24576 * Rx in kΩ * Cx in µF. This provides time in milliseconds. Multiply by 0.001 for seconds. R13 + RT1 = Rx. RT1 can be anywhere between 0Ω and 1MΩ (1000kΩ). CY1 = Cx. In the Schematic, CY1 is a 103 capacitor, which is 0.01 µF. Given this, we can calculate the two time ranges, when you shorted R13 out (Value of 0):
Tx = 24576 * (0kΩ + 0kΩ) * 0.01 µF * 0.001 ~= 0 seconds
Tx = 24576 * (0kΩ + 1000kΩ) * 0.01 µF * 0.001 ~= 245 seconds
The Off Time is similar. Ti = 24 * Ri * Ci, again times 0.001 for seconds. Ri = R33, Ci = CY2 (104 means 0.1µF). Since you shorted R33 out...
Ti = 24 * 0kΩ * 0.1µF * 0.001 ~= 0 seconds
Multiplying by 0 is easy. That said, we don't know how the BISS0001 handles the time, if it has a minimum period or not. Other PIR sensor ICs use a RC circuit to create an oscillator, so shorting the resistor would break the oscillator function.
Also consider JP1, which connects pin 1, A (Mode Select). If tied to Low/ground, it will not retrigger if motion is detected while the output is already high/on. If tied to High/Vcc, it will retrigger, meaning the On Time clock restarts, even if the sensor is already outputting high. So if someone is moving around in the field of view, it won't turn off in the middle of movement.
Also, don't forget to turn the Sensitivity potentiometer to it's highest setting.
But assuming you have On Time set to 1 second or less, and Off Time set to 0 seconds, and sensitivity potentiometer set to MAX, you still have to deal with the PIR sensor. The BISS0001 handles detection logic, but the sensor itself is an analog device that has it's own conditions for signaling a change. Short of it is, depending on the fresnel lens you have on it, which helps widen and divide the field of view, which direction you have the sensor pointed at (people walking directly towards the front of it will be less noticeable compared to people walking across it's field of vision.), and how quickly the sensor detects the changes based on it's composition, it does take time for the module to trigger. The PIR sensor AND the BISS0001 logic chip both [likely] have hysteresis to prevent over sensitivity and chronic triggering. The sensor requires time to reset when first turned on, and map the field of visions' default Infrared State. It only maps changes from one "picture" to the next.
Best Answer
How large is an obstruction? I think your options are probably either "build a RADAR set" (difficult, pricey) or "modify a laser rangefinder to do what you want". There exists an open source laser rangefinder base unit: http://www.lightware.co.za/shop/en/ - that $150 sounds expensive but realistically you're not likely to do it for less unless you have a really great supply of scavenged electronics and optics.