# Short-distance point-to-point rangefinding

I'm curious as to how well this would work. I'm sure it's technically possible, but I'm not sure it's practical or accurate enough.

I picked up these two gadgets a while back:

I was wondering if I could make a rangefinder with these - I'm looking for a fairly short range, between about 6 and 90 inches, and hoping for an accuracy of somewhere between 0.1 and 0.5 inches.

I'm familiar with how radio rangefinding works in principle, but I have no idea how it works in practice. Can someone with more experience with it than I tell me how silly my idea is?

EDIT: Had some sense smacked into me. Now looking to see if there's any other possible point-to-point method.
Radio's out since it's just silly at those ranges.
Ultrasonic/IR rangefinders that are prolific are out because I want a point, not a cone.
Laser range finders are out because of cost.
What else, if anything, is there available?

This comes up SOOOO often on here.

Radio waves travel at the speed of light. Any signal within the range of those modules will reach their destination in less than the time it takes for the arduino to process a single instruction.

It's like using a calendar to measure the speed of a vehicle on the freeway. The timescale is just totally impractical.

KenF:
This comes up SOOOO often on here.

Radio waves travel at the speed of light. Any signal within the range of those modules will reach their destination in less than the time it takes for the arduino to process a single instruction.

It's like using a calendar to measure the speed of a vehicle on the freeway. The timescale is just totally impractical.

That makes sense. Doing some math now that I've had my head properly smacked, I'd need something like 12 billion instructions per second to get even an inch's resolution from it. Rough glance says that my desktop computer wouldn't even be able to get a 0.1 inch resolution from it.

EDIT: What sort of rangefinding would work on that scale? Both ultrasonic and IR don't work for what I'd like (Point to point, they use cones and don't have the accuracy I'd need), and lasers are far too expensive, if right on the money for what I want.

Ultra sonic works well with some tweeking. You put a pinger on the fixed station. Have the reciever listen for the ping and then send a ping back. You can normally get to within a cm. If you want within .1 inches you've got your work cut out.

Within a cm is fine. And that pinger sounds like just what I need.

Would I be able to adapt a pair of HC-SR04 (Or similar) sensors, or would I need to build my own?

You can use a pair of modified HC-SR04 sensors for point-to-point distance measurement. Take the sending transducer off the receiver. Pulse the Trigger pin on both units at the same time and measure the pulse from the Echo pin of the receiver. That will give you the time-of-flight from sender to receiver. Use the speed of sound to convert from microseconds to the distance unit of your choice.

The problem is triggering both the sender and receiver at the same time. You might be able to use radio or IR for that with some adjustments. You will have to adjust for any delays between the two Trigger signals. If you can keep the delay down to a few microseconds it should not be hard. A two-channel oscilloscope should allow you to measure the time delay.

AndonRT:
Would I be able to adapt a pair of HC-SR04 (Or similar) sensors, or would I need to build my own?

I don't think building your own is in any way worthwhile for such things.

And more: Noted from that Amazon page:

Ships from and sold by Amazon.com. Gift-wrap available.