hugoender
New Member
Why not build one similar to an ultrasonic range finder?
The Tx unit sends a pulse which hits the opposite wall and the Rx unit receives the echo. You would sound the alarm if you didn't receive a return pulse, or the pulse came back too soon or the pulse came back too late.
If there is no wall to reflect off, then you would simply sound the alarm whenever you received a return echo from any distance. This would be easier to implement than the first example.
Well there is a wall nearby (about 2-3ft from the circuit) but if a person crossed, the signal would not be absorbed but instead reflected. Therefore you cannot make a circuit that detects when the circuit does not receive a return pulse.
To detect if a pulse came back too soon or too late is essentially what we have been discussing (the doppler effect).
The best would be if there was no wall to reflect the signal. Then I could make the circuit just detect when a signal is reflected back to the circuit by a person passing by. However, this is not the case and there is a wall. Maybe I could fine tune the power so that the signal is not powerful enough to travel to the wall and back... however, the difference in distance from the wall to the circuit and a person to the circuit is so small that it is probably not possible to fine tune it to such a degree (especially when running off a battery that is not a stable voltage source since it will run low).