ultrasonic proximity detector

Using ultrasonic piezoelectric ceramic transducers, a matching pair working at 40kHz forms a transmitting and receiving network, and an ultrasonic proximity detector composed of an external power supply, as shown in the attached figure. This kind of device that can detect human proximity has a wide range of uses. For example, at a booth in an exhibition hall, when an exhibitor who is interested in a product approaches the product booth, it can immediately and automatically trigger the playback of a pre-recorded audio/video introducing the displayed product. information. It can also be used in security places to ensure safety and prevent idlers from approaching.

Figure 1 is the transmitter circuit, which consists of the CMOS time base circuit IC7555 (IC1) to form an ultrasonic transmitter. A bistable multivibrator is composed of 7555 and external components. Use the potentiometer UR1 to adjust the frequency to the 40kHz resonant frequency emitted by ultrasonic piezoelectric ceramics. T1 and T2 complementary pairs of tubes form the buffer exciter of transducer TX1. Excite TX1 to directionally emit ultrasonic waves. Because the transducer absorbs the peak current of IC1, the circuit oscillation is very stable.

Figure 2 is the receiving circuit. In order to reliably absorb the weak ultrasonic signals that are close to reflection by the human body, the pre-stage of the receiving circuit is used. A two-stage high-gain amplifier is composed of N1 and N2, which amplifies the ultrasonic reflection signal received by the ultrasonic receiving transducer RX1 in advance (80 times the gain of each stage). After the amplified output signal is detected by D1, it enters the filter circuit composed of C5 and R10, which bypasses the instantaneous environmental noise (clutter) and the 40kHz ultrasonic fundamental frequency in the ultrasonic area, and only takes out the negative level of the reflected signal.

The negative level is sent to the inverting input terminal of the comparator composed of N3. After comparing with the reference level of the positive phase terminal, a positive pulse is output. That is, the irregular reflected signal is shaped into a very regular trigger pulse signal, and LED1 lights up at the same time.

Use this pulse. It can trigger the timer or monostable circuit and convert it into the required wide pulse control signal to control the corresponding application as mentioned above.

N1, N2, and N3 here use four operational amplifiers LM324, which not only has low price and good performance, but also has high open-loop gain, small bandwidth and small bias current, and can operate on a single power supply. Single op amps such as CA3130 or CA3140 can also be used.

The circuit is installed on a printed board (PCB), and a common universal test PCB board on the market can be used during trial production. It is best to choose the standard type with a pad hole pitch of 2.54mm to facilitate the insertion of integrated chips. However, the transmitting and receiving circuits should cooperate with each other for calibration. The method is to first relatively adjust the transmitting and receiving frequency (adjust VR1), and gradually adjust the distance to achieve the best match; then adjust the ultrasonic transmitting signal to select those approaching at a specific location. The best receiving position where the reflected signal is the largest and the interference is minimal.

Figure 3 is the T1 and T2 pin configuration diagram. Figure 4 is a schematic diagram of the installation of an ultrasonic piezoelectric ceramic transducer (40kHz).