Research on short-distance human signal detection and protection circuit for TV

    Abstract: A reliable and practical TV close range protection device is introduced. The device is mainly composed of human body signal detection and processing circuit, voice prompt circuit and control output circuit. When a human body approaches, the TV can issue a voice prompt and cut off the TV signal; when the human body leaves the detection range, it automatically restores the TV signal after a delay.

    Keywords: Vision protection, infrared sensor, voice prompts

There are many reasons for children's vision loss, such as incorrect posture for reading and studying, improper distance, etc. In addition, children watching TV too close to each other is also a reason that cannot be ignored. This is because the TV picture tube is a cathode ray tube. After the electrons emitted by the electron gun are focused, they are accelerated by the anode at high voltage and bombard the fluorescent screen with great energy, causing the phosphor to emit bright spots and radiate to the surroundings. emit harmful X-rays. Although TV sets produced in recent years generally have added anti-X-ray leakage circuits, which will not cause harm to the human body or vision within the normal viewing distance range, for preschool children, due to poor self-control ability, it is difficult to watch interesting TV During the program, I often can’t help but get close to the TV. During the development of children, the adjustment ability of the eyeballs and eye muscles is poor. When they are too close to the TV, the strong screen light will have a great irritating effect on the eyes. In addition, children are most sensitive to X-rays, so they are affected by X-rays. It is the most harmful, so watching TV at close range for a long time will cause vision loss, sometimes dizziness, vomiting, and memory loss.

The author developed a TV close range protection device through long-term research and multiple experiments.

1 Basic composition

The TV close range protection device mainly consists of a signal detection circuit, a delay circuit, a counting circuit, a voice prompt circuit and an output control circuit. Its basic composition is shown in Figure 1. The detection circuit detects the human body signal within 1.5 meters from the TV through an infrared sensor. After the signal is amplified, filtered out interference, and bidirectional amplitude detection by the signal processing circuit, the detection circuit outputs a high level, which drives the voice prompt circuit to send out the "distance" signal. Too close, harmful to eyes" prompts, and the counting circuit is started at the same time. Every time the signal detection circuit detects a signal, it will give a voice prompt and add 1 to the counter. When the counter reaches the set value (the set value here is 5), the output control circuit cuts off the TV signal and starts the delay at the same time. circuit, and after a delay of 3 minutes, the output circuit is controlled to restore the TV signal. Forcing children to watch TV only within a normal distance range serves the purpose of protecting children's eyes. The detection distance can be adjusted by changing the elevation angle of the detection probe, and the setting values ​​of the counter and timer can also be flexibly adjusted as needed.

2 Signal detection circuit

The signal detection circuit is mainly composed of a human body signal detection probe and a signal processing circuit.

2.1 Detection probe

The detection probe of this device uses pyroelectric infrared sensor SD02. Its structure is mainly composed of sensitive elements, field effect tubes, high-value resistors, etc. Its structure and appearance are shown in Figure 2.

The sensitive element is made of infrared pyroelectric material lead zirconate titanate (PZT). This material still maintains the polarization state after the external electric field is removed, that is, there is spontaneous polarization, and the spontaneous polarization intensity PS increases with the temperature T And decline. The sensitive element is plated with electrodes on a thin sheet made of pyroelectric material, forming two small polar capacitors in series, so the pyroelectric signal output with changes in temperature is also polar.

Usually the resistance of sensitive component materials is as high as 10 13 Ω . Therefore, field effect transistors are used for impedance transformation. Usually field effect transistors (such as: 2SK303V3, 2SK94X3, etc.) are used to form source followers. The function of the high-value resistor Rg is to release the gate charge so that the field effect transistor can operate normally. Generally, under the source output connection method, the source voltage is about 0.4~1.0V.

In order to make the sensor most sensitive to the human body, the sensor uses a filter as a window. The optical filter is made by plating multiple layers on a Si substrate. Every object emits infrared radiation, and the wavelength with the strongest radiation satisfies Wien's displacement law:

λm·T=2898(μm·K)

If the human body temperature is calculated as 36°C, the absolute temperature is 309K.

λm=2989/309=9.4μm

That is, human body radiation is strongest at a wavelength of 9.4 μm . The infrared filter selects the 7.5-14 μm band, so it can effectively select the infrared radiation of the human body and filter out interference such as sunlight and lights. When the pyroelectric sensor is actually used, a Fresnel lens must be installed in front. The Fresnel lens is actually a lens group, and each unit lens on it generally has only a small field of view. The fields of view of two adjacent unit lenses are neither continuous. There is no overlap, but they are all separated by a blind spot. The infrared radiation passing through the lens is focused on only one sensor element, so the resulting signal is not canceled.

Within the detection range, the signal frequency output by the sensor is approximately 0.1 ~ 10Hz. This frequency range is determined by the characteristics of the Fresnel lens, human body movement rate and the pyroelectric sensor itself. For human body signals, a bandpass amplifier should be used to amplify them, and the general center frequency is taken around 1Hz. The bandwidth of the amplifier has an important impact on sensitivity and reliability. The bandwidth is narrow, the noise is small, and the false alarm rate is low; the bandwidth is wide, the noise is large, the false alarm rate is high, but the response to fast and slow movements is good. Therefore, appropriate bandwidth should be selected.

2.2 Signal processing chip

The signal processing is composed of integrated special chip SS0001. The structure of SS0001 is shown in Figure 3. It is mainly composed of operational amplifier, state controller, bidirectional amplitude detector, delay time timer, blockade time timer and reference voltage. The chip adopts a CMOS data hybrid structure, an independent high input impedance operational amplifier, and is matched with the sensor for signal preprocessing. The bidirectional amplitude detector composed of a voltage comparator can effectively suppress interference.

2.3 Signal detection circuit

The signal detection circuit is shown in Figure 4. A pyroelectric infrared sensor composed of a filter lens and an impedance matching field effect tube detects infrared radiation from the human body in a non-contact manner and converts it into an electrical signal. It is preamplified by the operational amplifier N1 in SS0001 and The second stage amplification of the operational amplifier N2 raises the DC potential to the built-in voltage Um and then sends it to the bidirectional amplitude detector composed of comparators N4 and N5 to detect the effective trigger signal Us. Since the built-in voltage UH≈0.7UDD and UL≈0.3UDD, when UDD=5V, the noise interference of ±1V can be effectively suppressed. N3 serves as a conditional comparator. When the input voltage Uc is less than the built-in voltage UR (≈0.2UDD), the output of N3 is low level and seals the AND gate N7, prohibiting the delivery of the trigger signal Us to the lower level. When Uc>UR, the output of N3 is high level, and the AND gate N7 is opened. At this time, if the rising edge of the trigger signal Us arrives, the delay timer can be started, and the output of U0 is high level. When the trigger control terminal A is connected to the "0" level, any changes in Us in Tx are ignored until the end of the Tx cycle, which is the so-called non-retriggerable working mode. When the Tx period ends, U0 jumps back to low level and starts the blockade timer at the same time and enters the blockade period Ti. Any change in Us during the Ti period cannot make U0 valid. If the trigger control terminal A = "1", Us can retrigger U0 to the valid state and remain valid during the Tx cycle. As long as there is an upward transition of Us in the Tx cycle, U0 will continue to extend one Tx cycle from the moment of the upper transition of Us, which is the so-called retriggerable mode. The selection of the comparator's threshold value is very important. If the threshold value is too low, it is easy to cause false alarms. If the threshold value is too high, the sensitivity will be low, so it needs to be carefully selected.

During the timing period Tx, the output terminal 2 of SS0001 is at high potential, then the transistor VT1 is saturated and turned on, its collector is at low potential, and the voice prompt circuit is in the playback state and sends out voice prompts; at the end of Tx, SS0001 enters the blockade During period Ti, its output terminal becomes low level, the transistor is turned off, its collector is high level, and the sound playback stops. During the transition between high and low levels at the output terminal, the counter automatically counts, and the calculation circuit is not shown here. Pin 1 (A terminal) of SS0001 is connected to the power supply, so that the signal detection circuit is repeatedly triggered. The elevation angle of the signal detection probe can be adjusted within a range of 120°, and the actual detection distance can be adjusted by changing the elevation angle.

3 Voice prompt circuit

The voice prompt circuit is composed of CS53108, a voice recording and playback integrated circuit chip with complete functions, good performance, cheap price and easy to use. The chip integrates a microphone amplifier, automatic gain control circuit, D/A conversion circuit, low-pass filter circuit and buffer, etc. It has recording, playback and erasing functions, and the recording and playback time is 10 to 12 seconds. It can directly drive speakers, has few peripheral components and low operating voltage (3~5V). It has two packaging forms. The one chosen here is a 19-pin soft package. The functions of each main pin are as follows: pin 1 is the positive terminal of the VCCA analog circuit power supply; pin 2 is the positive terminal of the SPKP speaker; pin 3 is the negative terminal of the SPKN speaker; pin 4 is the negative terminal (or ground) of the VSSA analog circuit power supply; pin 5 The amplifier output terminal is for the MICOUT microphone, and a 0.1 μF capacitor is connected to pin 9; pin 6 is the input terminal for the MICIN microphone preamplifier; pin 7 is the MICREF microphone input bias voltage, and an external 0.1 μF bypass capacitor is connected; pin 8 is the AGC Automatic gain control, the maximum loop gain can be obtained when grounded; pin 9 is the ADI fixed gain amplifier input terminal; pin 10 is the RECb recording control terminal; pin 11 is the PLAYb playback control terminal; pin 12 is the ERASEb erasure control terminal; 13 The pin is the AUTO automatic playback control terminal. When this terminal is connected to VDD, it is in the automatic playback mode. When it is connected to the ground, it cannot automatically play the sound. Pin 14 is the TESTb test terminal. Pin 15 is the BUSY recording and playback indicator terminal. Pin 16 is the VCCD digital circuit source. The positive terminal can be connected to VDDA; pin 17 is left floating; pin 18 is the negative terminal of the VSSD digital circuit power supply and can be connected to VSSA; pin 19 is the audio output terminal for AUD playback. The voice prompt circuit of the TV close range protection device is shown in Figure 5. Two buttons are connected to the recording terminal 10 and the erasing terminal 12 of the CS53108 respectively. S1 is the sound search button. When button S1 is pressed, the recording control terminal RECb is low level, and the voice circuit can record. The voice is input through the microphone (MIC), and the voice signal is coupled to pin 6 of the microphone preamplifier input terminal through capacitor C22. After being preamplified, it is output from pin 5, and then amplified and output by a fixed gain amplifier. The S2 erase button is used to erase notes. When button S2 is pressed, the erase terminal is at low level, clearing the recorded voice; in order to make the voice prompt loud enough, a transistor is added between terminals 2 and 3 of CS53108 and the speaker. A signal amplification composed of transistor VT2 and a power amplification composed of LM386 are added between terminals 2 and 3 of CS53108 and the speaker, and the output volume can also be adjusted through the potentiometer RP. When the human body is close to the TV, the output terminal 2 of SS0001 is high level, then the transistor VT1 is saturated, its collector is low level, and the playback terminal (PLAYE) is also low level. The voice signal output by the voice circuit passes through the transistor. After signal amplification and power amplification, the speaker is pushed to issue a voice prompt. The prompt volume can be adjusted through the potentiometer RP to meet the sound volume requirements.

Experiments have shown that the protection device has high sensitivity. As long as the human body enters the detection range, there will be a signal output immediately. It is easy and flexible to use. The detection distance, voice prompt time, and delay time to cut off the TV signal can be flexibly adjusted according to needs. It is resistant to It has strong interference ability and can still work normally under the conditions of lights and sunlight.

In order to further improve the reliability of human body detection signals and enhance the ability to resist white light interference, other effective measures should be taken, such as pulse counting, patch components, shielding boxes, double-layer filters, etc.