Experiment on making a demonstration device for mobile phone electromagnetic radiation

It is well known that mobile phones generate electromagnetic radiation when in use. How can we observe this phenomenon? This production can demonstrate the electromagnetic wave signals emitted by mobile phones. Put it near the mobile phone, and every time the mobile phone makes a call or receives a call, it will emit light and sound signals to indicate the status of the mobile phone emitting electromagnetic waves. Through this small device, we can observe the state of the mobile phone radiating electromagnetic waves, and at the same time, we can do some interesting experiments to experience and understand the electromagnetic radiation phenomenon of mobile phones.

Circuit Principle

See the schematic diagram below. The circuit consists of four parts: high-frequency amplifier, detection, shaping, and display. The GSM mobile phone transmits electromagnetic waves at a frequency of 900MHz. After receiving the signal through the antenna, the signal is first amplified by the high-frequency amplifier circuit composed of the V1 transistor 9018, and then input to the detection circuit composed of the V2 transistor 9018 for detection. After detection, a pulse signal encoding the communication information is obtained. Through the shaping circuit composed of the 4069 NOT gate circuit, it becomes a display pulse signal, which drives the display circuit to emit sound and light to demonstrate information.

　　
1. High-power circuit

The 9018 triode with the highest operating frequency in the 9000 series is used to form a non-tuned high-frequency amplifier circuit. In order to effectively suppress the interference of high-frequency signals in the lower frequency band, the circuit uses an emitter DC feedback circuit and does not add an AC bypass capacitor. In this way, this stage of the amplifier circuit has a strong negative feedback function for the AC signal while obtaining a relatively stable DC operating point, and its AC amplification factor is very low.

But for the 900MHz mobile phone high-frequency signal, the situation is different. We know that the emitter and the feedback resistor R2 connected to it have a distributed capacitance to the ground, and the magnitude of this capacitance cannot be ignored for the mobile phone high-frequency signal; so for the electromagnetic signal emitted by the mobile phone, the negative feedback of this stage of high-frequency amplification is relatively small, and a relatively large amplification factor can be obtained. The high-frequency amplified signal is coupled to the next stage detection circuit through a 12pF capacitor C1.

2. Detection circuit

Adopt triode detection. Use the nonlinear characteristics of the base of 9018 under small current to extract the pulse signal carried by the high-frequency signal. The base resistor R4 of this circuit is 4.7MΩ, and the voltage across it is about 2.5V; the static working current of the base is 0.5uA.

At this time, the collector current of the transistor is 40uA, and a voltage drop of about 2.8V can be generated through the load resistor R5. The collector-to-ground voltage is about 3.2V. When the base detects the signal and generates the signal current, the collector current rises, the voltage drop on the load resistor increases, and the collector-to-ground voltage drops. The detection signal is output to the 4069 shaping circuit through the resistor. In order to make the detection circuit more stable and to improve the detection efficiency, a voltage doubler detection diode D1 is connected in parallel to the base.

3. Shaping circuit

The signal waveform output from the detector is a digital pulse signal; its characteristic is that the duty cycle is very small, that is, the pulse is relatively narrow; through experimental detection, it is known that the communication signal pulse width of a GSM mobile phone is about 0.5 milliseconds, and the shortest period is about 5 milliseconds; several such pulse signals constitute a pulse signal group.

Such a signal contains less energy and cannot directly drive the display circuit. We use a shaping circuit to convert it into a pulse signal with a larger duty cycle.

Under normal conditions, the signal voltage output by the collector of the detection transistor is 3.2V. Once the circuit receives the electromagnetic wave signal emitted by the mobile phone, the voltage output by the detection stage will drop below 3V. In this way, the output voltage of the NOT gate will flip. After passing through the 3-level NOT gate circuit, the detection signal passes through the current limiting resistor R7 and the rectifier diode D2, and is added to the RC circuit composed of resistor R8 and capacitor C2. By adjusting the time constant of the RC circuit, each narrow pulse signal can be converted into a signal with a pulse width of about 5 milliseconds. In this way, the narrow pulse signal group obtained by the detection becomes a wide pulse signal of unequal length. The shaped signal is output to the display circuit through the 2-level NOT gate circuit.

4. Display circuit

The display circuit is composed of a 9014 transistor DC amplifier circuit, which drives the light-emitting diode and a miniature active single-tone buzzer. If a 3V buzzer is used, a blue light-emitting diode can be connected in series in the circuit; if a 6V buzzer is used, a red light-emitting diode can be connected in series in the circuit.

A green light-emitting diode is connected to the last NOT gate of the display driver circuit; it serves as a power switch indicator. When there is no electromagnetic radiation signal from a mobile phone, only the green light is on; when there is an electromagnetic radiation signal from a mobile phone, the green light goes out and the red light is on.

Production and Experimentation

1. Assemble using breadboard

When using a breadboard, you should pay attention to the impact of the breadboard's quality on the work. It is recommended to use a breadboard with good quality, which is better than a cheap breadboard; the main advantage is that the metal reed has good elasticity and is very convenient to plug in; and the high-frequency performance of the plastic board is good. We know that there is distributed capacitance between the electrodes of the breadboard, and the Q value of this capacitance is related to the performance of the high-frequency circuit.

2. Antenna

The wavelength of 900MHz is 33cm, and a quarter wavelength is 8.2cm. The circuit uses a 0.6mm hard-core copper wire as the antenna, and the antenna length is between 6 and 8cm. The antenna can be coupled directly or through a capacitor. If through a capacitor, the capacitor is about 10pF.

3. Experimentation and debugging

When the power is turned on, the green light of the circuit is on; the red light is off and the buzzer is off, indicating that the circuit is working properly. Put a GSM mobile phone next to it and dial a phone number (you can dial your own phone number), then the circuit will emit a short beep and the light will flash.

After the power is turned on, if the red light is on and the buzzer is beeping, the circuit is not working properly. At this time, the collector voltage of the transistor V2 in the detection circuit should be checked first. When the power supply voltage is 6V, the voltage at this point should generally be around 3.2V: this voltage should be greater than half of the power supply voltage. If it is equal to 3V or less than 3V, it is abnormal and the circuit cannot work. At this time, the condition of the V2 circuit should be checked. The collector voltage can be adjusted to the normal range by adjusting the base resistor R4 or the resistance value of the collector load resistor R5. The detection output voltage is the input voltage of the shaping circuit; since the turning voltage of the 4069 NOT gate of the shaping circuit is half of the power supply voltage, this voltage must be higher than 3V. The circuit can work properly. The closer this voltage is to 3V, the more sensitive the entire circuit is; but at the same time, the stability becomes worse. Therefore, in order to maintain the stability of the circuit, the voltage should not be lower than 3.2V (referring to the case of 6V power supply voltage).

4. Relationship between sensitivity and anti-interference

The sensitivity of this machine is that it can detect the electromagnetic radiation signal emitted by GSM mobile phones within a range of 1 meter. The main interference sources in home use come from other household appliances and mains power lines. Through experiments, this machine can completely resist the interference of mains power lines. Among various electrical appliances, energy-saving lamps have the greatest interference. Generally, this circuit can be free from interference when it is 50cm away from energy-saving lamps. In this circuit, the coupling capacitor C1 uses 12pF. If this capacitor is too large, the circuit will work unstable.

5. Demonstration experiment of electromagnetic radiation from mobile phones

If the assembly is successful, we can conduct several experiments. The first is the experiment of making a phone call. After we make a call, we can gradually increase or decrease the distance between the phone and the antenna, and finally get a judgment on the sensitivity of this circuit. Generally, this circuit can detect the range of the phone within 1 to 2 meters.

Of course, this is related to the signal strength of the base station where the call is made; if the signal is strong, the mobile phone's transmission power will be reduced; otherwise, it will be enhanced. This will directly affect the judgment of the sensitivity of the machine.

On the other hand, we can judge the general situation of electromagnetic wave pulses emitted by the mobile phone at each stage of communication. From the light and sound display, we can judge that when the mobile phone makes a call, it emits several short radio waves. When talking, it emits more coherent radio waves.

Also, if we place a mobile phone next to an open circuit for a few hours, we will observe some phenomena that we didn’t know before, such as the circuit occasionally making a few beeps and flashing a few times.

In most cases, this is not other electromagnetic interference, but the mobile phone exchanging information with the base station.

Interested readers may wish to conduct experiments in this area by themselves and find some regular phenomena.
In addition, it should be noted that the chirping and light display status emitted by the circuit only roughly reflect the "outline" of the electromagnetic radiation of the mobile phone, not the actual situation. Why is this? Because the circuit uses a rectifier and shaping circuit to convert its original pulse signal into a pulse envelope signal. Interested readers can save the received signal to the computer through a recorder and observe its waveform.

6. Experiments with different mobile phone standards

We can also conduct the same experiment for Xiaolingtong and CDMA mobile phones. The experimental results tell us that compared with GSM mobile phones, the electromagnetic radiation of Xiaolingtong mobile phones is negligible. Only when the antenna of Xiaolingtong is close to the antenna in the circuit can the circuit send out a display signal.