Ultrasonic motion detector

4.5 5 2 Product

The motion detector utilizes the Doppler effect to detect motion. The maximum surveillance range is around 7 meters. Once any moving object is detected inside the surveillance area, a relay is activated which in turn can be used to activate a siren or any other device.

The detector produces ultrasounds at 40KHz and listens to the reflected sounds. The circuit is analog and does not use any microcontroller, so no programming is required.

The circuit

The electronic circuit of the ultrasonic motion detector is shown in Figure 1. The circuit consists of four stages:

  1. The transmitter
  2. The receiver
  3. A delay circuit
  4. A relay activation circuit

The transmitter is made from U1, U2 and the MX1 ultrasonic speaker. The receiver consists of the MR1 ultrasonic microphone, the Q1 FET,  the U3A and U3D operational amplifiers. The operational amplifier U3B is used to produce a delay at startup and the operational amplifier U3C, together with the Q2 transistor are used to activate the relay when any motion is detected in the surveillance area.

Ultrasonic motion detector electronic circuit
Figure 1.
The electronic circuit of the ultrasonic motion detector

U1 contains four NAND gates. An oscillator is made with the U1D gate and R1, L1 and C2, C3 elements. It produces a square waveform at 320KHz. After a division by eight from U2A, a frequency of 40KHz is produced.

The 40KHz signal is applied to the current amplifiers U1A, U1B and U1C. U1C and U1A produce an in phase signal at terminal 1 of the MX1 ultrasonic transducer while gate U1B produces a signal with a phase difference of 180° (inverted) at terminal 2 of MX1. Thus, MX1 emits a continuous ultrasound at 40KHz.

At the receiver side, the ultrasonic microphone receives the 40KHz sounds directly from the transmitter but also from reflections from any objects in the surrounding space. As long as the objects in the surrounding space remain stationary in relation to the transmitter, the sound from the reflections has exactly the same frequency as that of the transmitter. However, if an object moves, then due to the Doppler effect, the reflected sound has a slightly different frequency compared to the 40KHz,. The frequency difference depends on the speed of the moving object and its moving direction. For common low speeds, that difference is in the order of some few Hz.

In the receiver, the direct and the reflected sounds are received from the MR1 ultrasonic microphone and are subjected to electronic mixing from Q1. Due to mixing, sum and difference frequencies components are produced. When there are moving objects in space, the difference components are of the order of a few Hz and these frequencies are amplified by the U3A and U3D operating amplifiers. Actually, only the Q1 FET operates at 40KHz. All other stages of the receiver are actually amplifiers operating at low frequencies, from 1 to about 25Hz. The response in higher frequencies has been deliberately limited in order to avoid false alarms due to vibrations from windows or other common vibrations in the surrounding space.

The signal from the drain of the FET is rectified by the D6 and D7 diodes and under stationary conditions, a continuous voltage (DC) is produced at C13. However, when there are any moving objects in the monitored space, some AC at low frequencies occur and bypasses the C15. This AC signal is then amplified from U3A. The R20 trimmer resistor is used to adjust the gain of U3A, thus regulating the sensitivity of the detector.

The signal is then amplified by a second amplifier, the U3D. It is rectified from  D8 and D9 diodes and charges the C22 capacitor. U3C works as a comparator. When the voltage of C22 becomes higher than the threshold voltage at the inverting input of U3C, the output of the comparator goes to a high level and activates the relay and the D11 LED through the Q2 transistor. The threshold, and therefore the sensitivity of the detector, can be adjusted by the R6 trimmer potensiometer. Of course, C22 is only charged when there are any moving objects in the surrounding space. Otherwise, C22 remains uncharged or it is discharged via the R26 (if it had been charged at an earlier moment). The value of the C22 has been chosen in such a way that the charge time constant is relatively large so that the relay is not activated by imperceptible movements.

The U3B operating amplifier produces a time delay at startup so that the relay does not falsely activate when the circuit is initially powered. This works as follow: During startup, C5 is uncharged and starts charging via R4. Due to the charge current, there is a voltage drop in R4 and this is applied to the non-inverting input of U3B. This voltage is greater than the voltage of 4.5V that is permanently applied to the inverting input of U3B through the voltage divider of resistors R5, R6 and R7. As a result, the U3B output goes to a high logic level and resets U2A. This way the meter is blocked and the 40KHz signal cannot be generated. The output of U3B is also aplied through the diode D2 to the non-inverting input of U3C and changes the threshold voltage of the U3C comparator, so that the relay is not activated. After 10 to 15 seconds from startup, the voltage across C5 reaches at a significant level due to the  charge,  and the output of U3B goes into low logic mode and normal circuit operation is restored.

Finally, it is worth analyzing the role of R8 resistor. R8 extends the active time of the relay. This is because as soon as the relay is activated, the voltage at the Q2 collector drops to about 0V and through R8 this voltage reduces the threshold voltage of the U3C comparator. As a result, the voltage at the ends of the C22 must drop to a lower value to deactivate the relay, thus prolonging the excitation time.

Making the circuit

The circuit of the ultrasonic motion detector can be easily assembled on the circuit board provided below. The circuit board is double sided and all the components should be placed on the upper side of the board while the soldering should be done on the back side. The assembly can be done based on the assembly guide shown in Figure 2.

Assembly guide for the ultrasonic motion detector
Figure 2. How to assemble the ultrasonic motion detector

The MX1 ultrasound transducer and the MR1 ultrasound microphone are not soldered on the board but they must mounted on PCB terminal blocks attached to the board, as shown in Figure 2. In the prototype, we use ultrasound modules from Murata. Of course, you may use different ultrasound modules but they should be for the 40KHz frequency.

The circuit requires a 12V supply, which the Zener D1 reduces to approximately 9V. You should use a good, noise-free power supply.

All resistors in the circuit are of 1/4W type and of 5% tolerance or better. If you notice any reduced surveillance range, it will most likely be due to the fact that due to tolerances, the oscillator frequency will be different from 320KHz and a signal frequency other than 40KHz will be obtained in the ultrasonic converter. To correct the problem, you should use an oscilloscope or a frequency meter to adjust the frequency of the oscillator to its correct value.

Adjusting the device

The device is adjusted via trimmers R20 and R6. Trimmer R20 adjusts the gain on the receiver while trimmer R6 adjusts the threshold voltage of the comparator. The only way to make an accurate adjustment is through trial and error, in place where the detector will operate.

For large surveillance areas, R20 should be adjusted for large receiver gain, while in smaller areas or for range limiting, a smaller gain would be sufficient. Care must be taken when adjusting the threshold voltage via the R6 trimmer, as incorrect adjustment of the R6 can completely invalidate the receiver's sensitivity.

Finally, due to the time delay in the circuit, you will have to wait about 20 seconds after the circuit is initially powered on, before making any of these adjustments.



Printed circuit board Artwork for the ultrasonic motion detector


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