Short distance measuring without clear view (family biking project)

Hi

I need some suggestions on how to measure distance between two points that don't have clear visibility to each other. The distance does change all the time, can be up to tens of meters, the measurement doesn't have to be very precise (around a meter resolution is good enough), and each end can have a transmitter/receiver or whatever it takes attached to it.

Here's why - we often go cycling with wife and kids. I usually lead the group, kids in the middle and wife in the end. I would like to know, without having to turn my head all the time, how far behind they are - sometimes a kid drops off, wife stops to help him and I only find out minutes later and have to come back with the other kiddies, etc. So my plan is to attach a small gadget on each bike, periodically measure the distance, display the readings and raise an alarm on my bike if they're too far.

However first I need to measure the distance. Any ideas? I was thinking of GPS modules but that sounds like an overkill. I'm sure there must be a simpler solution.

Thanks for any ideas!

How about a low power radio frequency transmitter?
The strength of the RF signal received goes as the inverse square law, i.e. the signal strength falls off as the square of the distance. You can build a receiver that indicates the signal strength received and this can trigger an alarm when the signal strength falls away.

MrChips said:

How about a low power radio frequency transmitter?
The strength of the RF signal received goes as the inverse square law, i.e. the signal strength falls off as the square of the distance. You can build a receiver that indicates the signal strength received and this can trigger an alarm when the signal strength falls away.

Click to expand...

What about the obstacles? It will be ok with free line-of-sight, but ever changing obstacles (after curves for examples, or trees or buildings) I think it would not be very precise.
I think I would use GPS, even if it seems to be overkill . If you have smartphones there is software you can use that shows the position of each other phone.

Check post #7 here...... http://forum.allaboutcircuits.com/showthread.php?t=80032

Is there some reason you don't want to use a pair of FRS walkie-talkies - one for you and one for your wife?

BillB3857 said:

Check post #7 here...... http://forum.allaboutcircuits.com/showthread.php?t=80032

Click to expand...

Thanks. I guess this sets off on some signal strength threshold. 25 m range is not enough but could be a starting point.

tracecom said:

Is there some reason you don't want to use a pair of FRS walkie-talkies - one for you and one for your wife?

Click to expand...

The primary reason - it's not cool enough. Same for the above link.

If I could measure the distance I would then make a small device with a display, showing things like "Kid 1: 17m, Kid 2: 29m, ...". Maybe more later.

What I was hoping for was something in the likes of ultrasound distance meters - send some kind of signal from bike1 to bike2 and back and compute how far apart they are. Ultrasound is probably no good for say 100m in open space though. Any other way to do that?

If you want something cool, try infrasound.

If you want really cool, how about bikes that communicate with each other using synchronized real-time clocks that send timestamps back and forth, distance can be determined by transit time....

... or you could just buy some cheap GPS modules... it'd save you a lot of time...

Tracecom's idea of using frs radios is the best idea also much more useful, your idea of a distance display is not doable on a hobbyists budget.

The Garmin Rhino GPS/FRS Radios allow both speech, and show the other users on a topo map relative to the transmitter.

Use them hunting all the time, they work great to make sure somebody hasn't wandered downrange.