Introduction: Distance Measurement With Radio Waves

First of all, we want to excuse us for our bad English. (German pupils :D)
We invented a new, inexpensive device to measure distances up to 1.5km (about 1 mile) with accuracy about ±5 Meter (15 feet). The use of radio waves makes it possible to measure without the target being in sight. This means, you can measure distances through whole buildings.There are many rangefinders available, which are working with sound waves or lasers. A disadvantage of distance measurement with laser rangefinder is that you must center up the beam to the receiver and ensure that there are no obstacles along the laser beam.
Schematics and layouts are 100% own work, no copy and paste, only the transmitter and receiver modules had been bought.We already took part with this project in a German youth science competition called „Jugend-Forscht“ and won the 1st prize.

Step 1: Step 1: Basic Idea

Step 1: Basic idea
To put it simply, the main part is an exact stopwatch, which measures time with a resolution in nanoseconds. It is used to stop the time the emitted radio wave is travelling. Because the spreading rate of radio waves is identical with velocity of light, you can calculate the distance between the two devices (measuring points) by a given travel time of the radio waves.The stopwatch contains a crystal with a clock rate of 30 Megahertz and a couple of decade counters (High- Speed CMOS). To display the stopped time, binary outputs of the decade counters must be converted to be easier readable on 7-segment-displays. The process of a single measurement:
1) The measurement is being initiated (started with a button) by the user at the basic station (1st point)
2) Counter starts, at exactly the same time a 434 MHz AM transmitter module emits out a 1st radio wave
3) The radio wave gets into the receiver at the 2nd point, and immediately starts the 2nd transmitter at a frequency of 868 MHz
4) The 868 MHz wave is being received at the basic station and stops the counter
5) The travelling time can be read on the display.

Step 2: Step 2: Calculation of Distance

Step 2: Calculation of Distance
The formula:
Δs = (Δt * c) / 2

Δs: Distance in m
Δt: Wave travelling time
c: Speed of light (299,792,458 m/s)

For Example, the Display shows ‘17’ (leading zeroes are shown, too.), the radio wave has been 560ns (nanoseconds) on the way.
(5,6*10-7s * 3,0*10^8m/s) / 2 = 85m
Inserting the values in upper formula, you’ll get a distance about 85 meters.

Step 3: Step 3: Problems/Accuracy

Step 3: Problems/Accuracy
The use of a 30MHz crystal does not allow creating a very precise time base. The aftermath will be an error on the distance about ±5m. Increasing the frequency of the time base will improve the accuracy: The higher frequency, the more precision . Although there are several crystals available on the market with frequencies up to 100MHz (5th harmonic), the actual limit is the maximum clock frequency input of the 74HC4510 counters with 53MHz. Farnell offers counters up to 1200MHz, but they are expensive, only available for companies and come in boxes with hundreds of it. Another problem is that free frequency bands (434 and 868 MHz in Europe) can be used by any other equipment like walkie-talkies. This means the device will not work, because the receivers will get some other signal that is in the air.
The picture shows on the left side the distance measured by out device. The real distance is plotted on the ground. To prevent errors, several measurements had been performed. The fist one is shown by the blue line, the second by the red one an the real distance is located by the orange one. 

Step 4: Step 4: Parts

Step 4: Parts
5x 74HC4510
5x 74HC4543
3x 74HC4040
1x 4093
2x 40106
2x 78L05
1x BD175
1x BC556
32x 1N4148
1x LED red small
5x 7-segment-display

All resistors are 1% metal film, unless otherwise specified.
40x 390R
1x 1k
1x 2,2k
1x 8,2k
2x 10k
1x 12k
1x 15k
4x 22k
1x 27k
1x 1M
1x 10M

2x 2,7pF
1x 470pF
5x 10nF
4x 100nF
1x 0,22µF
2x 470µF
2x 100µF

1x 30 MHz crystal
1x Transmitter module 434 MHz
1x Transmitter module 868 MHz
1x Receiver module 434 MHz
1x Receiver module 868 MHz
1x Push Button (to start the measurement)
1x Momentary Switch (to reset the clock watch)
1x small switch (to choose between permanent or temporary display)

Step 5: Step 5: Schematic

Step 5: Schematic
We thought about uploading layouts for making a PCB, but it it’s not worth it, because the pin connections and alignment of transmitter/receiver modules are always different. (We got very exotic ones from a company in Italy, produced in Taiwan :D) If you decide to buy one – with a high probability they wouldn’t fit in our PCB.

Step 6: Step 6: Trilateration

Step 6: Trilateration
Suppose you built such a unit, you may ask yourself if distance measurement only is a bit boring. Good news: If you built three of these, you can use them for trilateration.

Because a radiowave spreads spherically, at 3 given distances a point can be clearly defined:
A circle is simply the set of all points (infinite) with the same distance to a point. The intersection of two spheres is a circle. The intersection of three spheres is one point.

This can be used tracking an object for example a car (GPS uses exactly the same principle.)

Step 7: Step 6: Conclusion

Step 6: Conclusion
This new technique of measuring distances is a great alternative to conventional laser methods. But the largest advantage is that you can measure through whole buildings. And the way it is built up is also simple enough to push this project easily forward with minimal knowledge of electronic. And you can use it without any knowledge of measurement technics.
To sum it up:
It's easy to use, it's easy to built, it's cheap and it's a new revolutionary method of distance measurement

3rd Epilog Challenge

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3rd Epilog Challenge