Short distange battery operated(door bell type) alarm

[HVACTech35]

Hello all! I'm new to the forum and was wanting a little insite on a project for my family.
I'm looking to fab a distance alarm like a cheap batt operated front door alarm. Distance is the key. Having said that Would need to be sensed is 1,2,3,4 feet from each other when being separated. Max battery 4.2v dc. I'd like it to be compact if possible. What would i need to achieve this setup?
Any insite and help on this is greatly appreciated. Thank you all for your time.

 

[Audioguru]

The size of the sensed object affects the sensitivity of an IR motion detector or ultrasonic distance detector very much. The result will show the distance plus or minus the size. IR does not work in daylight and ultrasonic does not work where there are noise sounds.

 

[Alec_t]

Welcome to EP!
Perhaps a vehicle-reversing alarm/aid could be adapted?

 

[Bluejets]

We do "walk tests" with pir devices in daylight and they work fine right up to the rated 12 metre range.

 

[Audioguru]

He wants 4 of them to have accurate fairly close distances detected.

 

[HVACTech35]

Thank you all for your input. At this point it's all still theoretical. Ideally it would be nice to nave one with selectable distances, but if that is not an option then I'll be ok with more then one to obtain the desired range. Tho an infared setup would be easy its not going to fit the application. I would use a door alarm but i cant figure how to achieve the desired distances.

 

[HVACTech35]

Welcome to EP!
Perhaps a vehicle-reversing alarm/aid could be adapted?

Maybe so I'll try to figure it out.

 

[ChosunOne]

I'd like to help, but I have no idea what it is you're trying to detect. A "door alarm" usually means a sensor that detects a door opening/closing. Do you want a sensor that doesn't trip until a door or hatch or gate opens 1,2,3,4 feet? That's not very useful for a front door alarm, but might conceivably be for a driveway gate. Are we talking a swinging door on hinges, like a front door, or a sliding glass door, or garage overhead door (panels, or roll-up?), or what? Different applications take different solutions. More information on what you're doing would be helpful.