Brief analysis of the self-made scheme of simple body fat measuring instrument

Just weight does not mean health, people are increasingly worried about the fat in their body, so various body fat meter products appear in the mall.

The correct method for detecting the content of body fat, that is, the fat rate, is to completely immerse the human body in water, measure the drainage rate (volume), and then calculate the specific gravity of the human body based on the body weight, thereby inferring the body fat percentage. But this method is not realistic in everyday situations.

The human body has a water content of about 70%. Most of this water is present in the blood, lean meat and internal organs, while the water content in the fat is extremely low. The water in the body exhibits low electrical resistance due to the dissolution of various components, but the fat exhibits high electrical resistance. Therefore, the ratio of fat and water in the human body affects the electrical resistance of the human body. If the synthetic resistance of fat and water is considered, the body resistance of the fat is large. The value is high. At present, almost all kinds of human body fat measuring instruments available on the market estimate the amount of fat by measuring the body resistance. FIG. 1 is a method for detecting a product in Japan.

Here is a self-made simple body fat measuring instrument.

The detection method is shown in Figure 2. Considering the simplicity and ease of implementation, the signal source uses a 5 kHz sine wave, and does not use a single-chip microcomputer, digital display, etc., and considers the same person to use, as long as you understand the changes in your body fat, and therefore do not consider the height of the person. The effect of weight on testing. Figure 3 is a full circuit made.

The whistle bridge oscillator is composed of T1 and T2, and a sine wave of 5KHz is generated, which is sent to one foot electrode via C5, and the other foot electrode is connected to the negative pole of the power source, that is, the resistance and signal source between the two legs of the human body. in parallel. Since the resistance between the two legs of the human body is generally several hundred kΩ or more, the subsequent detection circuit input adopts the emitter follower T3 with high input impedance; the signal outputted by T3 is rectified by the diode to take out the DC voltage related to the body fat percentage. It is output to the voltmeter via the first-stage emitter follower T4. Since the signal input to the detection circuit is small, and the conventional diode rectification circuit is not suitable for small signals, the resistor R12 is used to apply a bias voltage to the diode, so that the diode still has current when there is no signal, and the pair can be increased. The response capability of the signal; correspondingly, a zeroing circuit by R15 and VR2 is added to the circuit to offset the influence of the aforementioned bias voltage on the measurement result. Since the digital voltmeter is used to measure the detection result, the amplification circuit is not designed after the detection circuit. The two-legged electrode can be cut into four pieces of wafers with food packaging aluminum foil, two pieces per foot; one piece of insulating board is taken, the size is equivalent to the commercially available human body scale, and the double-sided adhesive of the same size as the electrode piece is attached to the two feet of the board. Through the perforation under the plate, the multi-strand wire is inserted through the double-sided tape, and the wire ends are spread out on the surface of the double-sided tape, and then the electrode sheet is pressed tightly on the double-sided tape to be in close contact with the wire to form a measuring pedal. The whole circuit is powered by a 3V battery.

Adjustment:

Make sure the oscillator is working properly. Use the audio signal transmission line to connect the C1 output terminal and the ground terminal to the stereo amplifier. If the oscillation is normal, the audio sound can be heard from the speaker; adjust the VR1 to hear the frequency change.

The detection circuit should be first zeroed, connected to the mV file of the digital multimeter at the output of the detector, and adjusted to VR2 when the electrodes of the two legs are suspended, so that the voltmeter indicates 0V. In the upper part of Figure 3, a 1M resistor is connected in series between the two electrode sheets, and then the two palms are respectively pressed against the upper and lower electrode plates, and the voltage meter should have a voltage indication of 10"20mV.

In actual use, the feet stand barefoot on the two-leg electrode position, and the voltmeter should have a voltage value, such as 9mV, which can be used as a reference value for measuring the fat rate in the future. Generally, people with more fat have higher voltage values ​​and lean people have lower voltage values.