Home > Circuit Blox > Ultrasonic Tape Measure
UPDATED:.13:32 29 June 2014

This tape measure transmits a brief untrasonic pulse and uses the time it takes to bounce back as method of determining the distance from the sensors.

Ultrasonic sensors must be parallel to each other about 100mm apart.

Distances up to 5 meters can be measured if pointed at flat reflective surfaces.

Calibration of the unit is done using VR1 on the High Frequency astable. Set to 17015Hz

+Spike Generator:
Set jumper to positive polarity and VR1 to minimum

LF Astable:
Set Low Range, (0.7-18Hz) and VR1 to minimum

HF Astable:
Set High Range, (700-18KHz) and VR1 to maximum

Set Schmitt jumper to ON

Refer to SETTING UP procedure below..



The heart of the system is the LF Astable which generates a timing frame for all the functions to happen within.

The output from the LF Astable connects to a Spike Generator that produces a positive pulse on every low-high input. This pulse is about 190us long.

This pulse Resets the Latch and the Counter enabling the Counter to count the pulses from the HF Astable.

The pulse from the Spike Generator is also connected to the Reset of the Ultrasonic Astable. This will function when "-Rst" input is high, and the pulse is produced. It enables the Astable to produce about 7 or 8 pulses of 40KHz signal to the transducer.


Latch reset
Ctr reset
Rx signal
Latch set
Counter Enable

When this reflected burst of pulses is received by the Ultrasonic transducer it is amplified twice by the AC Amplifiers - the gain is about -30 for each amplifier at 40KHz giving an overall gain of about 900

The output signal from the second AC Amplifier is connected to the Comparator Switch which produces a high output pulse when the burst is detected.

This pulse is connected to the Set input of the Latch sending its output high and thus ending the timing of the returned signal. The Counter can only count when the enable is low and this is controlled by the Latch.

Adjusting VR1 of the HF Astable will calibrate the reading obtained. For correct measurement set 17015Hz. This is because:

  • To measure 1cm sound needs to travel 2cm (to the object and back).
  • Sound travels at 340.29m/s which means it takes 2.939us to travel 1mm.
  • So it will take 58.77us to travel 2cm where we require a reading of "1".
  • One pulse every 58.77us = 1/0.00005877 = 17015Hz

There is a problem with this design. The reset pulse to the counters impedes into the Count time which means that the counters will be prevented from counting for 190us, in which time sound will travel 64mm. Therefore when properly calibrated, the distance will always be in error by -3.2cm...but it means that a latch is not required and simpler circuitry overall.


Timing and measurement through sampling and gating.

How some things are measured relative to time

MPH = miles per hour
RPM = revolutions per minute
Hz = cycles per second

Investigation into other uses of Ultrasonics for fracture detection and cleaning applications

It is best if the Ultrasonic Transmitter and Reciever transducers have already been paired up and calibrated to work at each others optimum frequency. To do this, build the Doorway Monitor circuit and ensure the Ultrasonic Astable is "tuned" the the transducers.

Once tuned, connect up the circuit above and place the tranmitter and receievers in parallel with each other about 100mm apart. Place them facing a wall or large vertical flat object. Set the Blox as stated in NOTES section above. Now adjust the SENSITIVITY on the Comparator (VR1) until a pulse is shown on the LED. Continue to adjust this for further objects to improve distance.

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