Sensor detects leaving, not coming

Thread Starter

shezza

Joined Mar 19, 2014
66
Hi all,

As the title reads... Im looking for a sensor that could sense a person leaving my property, but not coming.
So when someone enters, nothing happens, but when someone exits, it switches. And yes, the same path is used for entering and exiting. I know it sounds out there, but figured it couldnt hurt to ask...
 

MikeML

Joined Oct 2, 2009
5,444
A device based on Doppler Shift (using either radio waves or acoustic waves) can easily tell which direction (toward, away) an object is moving, and how fast.
 
Last edited:

crutschow

Joined Mar 14, 2008
30,075
Use two sensors (IR, ultrasonic, etc.) some distant apart looking across the path. Then detect the sequence of detection for the two sensors to determine the direction of movement.
 

Thread Starter

shezza

Joined Mar 19, 2014
66
Thanks for all the replies bar one (you know who Im referring to :p).

My understanding of electronics is fairly basic. So most of the suggestions have gone right over my head. Im guessing I need to understand IC's and that is well beyond me at the moment!
Im thinking of SPDT relays and a time delay switch to cancel out a second sensor, but without thinking it all the way through, it seems messy. Would be great if I could set something up fairly simple.

I will explain the gate and entry. Its the very front gate, 7ft high, tin with a metal frame. It has a electronic door strike and RFID pad connected to it. The door swings inwards towards the path and shuts via a spring. The entry has a garden on either side and its roughly 2 metres wide.

To help explain what I want, as Im approaching my front gate, I want a sensor to detect me, operate a linear actuator that rather than pulling the gate open, will just pop it open. At the same time, the electronic door strike will release to allow the gate to pop open. Though when someone enters and goes past the sensor, of course I dont want them opening the gate if they are already inside.
 

pwdixon

Joined Oct 11, 2012
488
Sounds like you really want a PIR detector facing out of the garden that triggers when someone approaches the gate from outside, someone inside would not be in the PIR beam and hence would not activate the gate.
 

Thread Starter

shezza

Joined Mar 19, 2014
66
Sounds like you really want a PIR detector facing out of the garden that triggers when someone approaches the gate from outside, someone inside would not be in the PIR beam and hence would not activate the gate.
Im not sure what you mean. The same path is used for exit and entry. There is no one place a sensor can be placed that a person wont pass through on exit and entry....
 

pwdixon

Joined Oct 11, 2012
488
If the person has exited through the gate then it's already open so triggering the sensor makes no difference but someone coming up to the gate from the outside would trigger the sensor and open the closed gate.
 

MikeML

Joined Oct 2, 2009
5,444
Its the other way around though. The gate is supposed to open for those leaving, not coming :)
Can you put an IR beam-break detector across the path to the gate on the inside of the fenced area? If detecting a car, you could use a buried-loop magnetic detector...
 
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