FM as non contact switch device ... possible?

vixtr

Can an FM radio transmitter / receiver combination be configured to detect the presence of the radio signal as it passes along a path perpendicular to an “isolated” receiver?
1) A 200 foot FM broadcast range radio will pass within 10 feet of the receiver at a rate of not more than 10 meters / second
2) Can an FM receiver can be isolated in such a way that it can act as a laser in order to start a timing devise when the FM signal crosses a boundary i.e. can the receiver be shielded in a way that it could detect the transmission when the radio passes a certain point in relation to the receiver?
3) Multiple persons may pass in front of the system, We want to detect an individual outfitted with a FM broadcaster set to a particular (unused locally) frequency.

ElectricAye

vixtr wrote: »

...Can an FM receiver can be isolated in such a way that it can act as a laser....

You might have better luck using an IR emitter and a simple IR sensor. Must it be radio?

localroger

With FM, if there are multiple signals you only receive the strongest one. You don't get them mixed up, because the discriminator in the receiver can only follow one at a time; this was considered one of its great strengths when it was invented because if you had two AM signals, one 1/4 as strong as the other, on the same frequency then you'd hear both of them and each would ruin the other, but with FM you hear only the stronger signal with perfect clarity.

Otherwise, while I think you're using a few words wrong what you're probably trying to do is possible in a general sense. A simple way to do this would be to have the FM transmitter send an audio modulated signal (FM modulated tone). The receiver audio output could then be fed through an active filter or even just rectified and straight (through a current limiting resistor LOL) into a digital input of a microprocessor to provide detection.

Mike Green

You are not going to get the kind of effect you're looking for at broadcast FM frequencies. The wavelength is too long to get the directionality and resolution on the physical scale you want.

vixtr

Must be radio - no. My main project involves one-way radio "verbal instructions" I was thinking there might be a way to use the WHOLEHOUSE(.com) FM system as a timing device also. The moving target height and orientation are not controllable.
We're timing runners, cyclists Etc on a closed track and want to get start/stop times over various distances depending on where we set the "units". We have non participants in the loop so our individuals need to carry identifiers. Can IR be configured to detect a moving target as it passes a predetermined point?

ElectricAye

vixtr wrote: »

Must be radio - no. ...

One quick question: are your participants able to pass by something to identify themselves? Your scenario sounds almost like a marathon in which some runners are getting timed. If the runners want to be timed, can they pass within a foot or so of a detector? I was thinking perhaps a RFID system might be of some help. If you've got a bunch of random people wandering around, your IR detection systems might have some troubles. A RFID could possibly identify each participant individually, but I'm not sure what the range is on something like that.

vixtr

Most of the RFID foot race systems use a platform detector. The time starts/stops when the shoe mounted RFID contacts the "pad" We need to time independently of the foot strike zone ... not necessarily the leading edge.