JPS6128334B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photoplethysmogram detection circuit, more specifically, a circuit that shines light on biological tissues including arteries and detects arterial pulsation by changing the amount of reflected (transmitted) light based on the arterial pulsation. Regarding.

FIG. 1 shows a conventional photoplethysmogram detection circuit. In this conventional example, light from a light emitting circuit 1' is applied to a biological tissue (for example, a finger) 3' including an artery,
The light receiving circuit 2' detects changes in the amount of reflected (transmitted) light based on the pulsation of the artery.

However, in this conventional example, the amount of light emitted from the light source of the light emitting circuit 1', reflected (transmitted) by the living tissue 3', and input to the light receiving circuit 2' depends on the individual differences in the living tissue 3' and the amount of light emitted by the detector. The disadvantage is that the operating point of the light-receiving element (for example, a photoconductive cell) of the light-receiving circuit 2' changes because it changes depending on the pressing pressure against the living tissue 3', and therefore a stable pulse wave output cannot be obtained. It was hot.

The present invention has been proposed to improve the above-mentioned drawbacks, and will be described in detail below with reference to the drawings.

FIG. 2 is a block diagram showing the basic circuit configuration of the present invention. In the figure, 1 is a light emission drive circuit, 2 is a light reception detection circuit, and a light emitting element (such as a filament lamp or a light emitting diode) is connected to the output of the power amplifier in the light emission drive circuit 1 and emits light, and the amount of emitted light 4 is transmitted to the arteries. When the light hits the living tissue 5 containing the light, it appears as a change in the amount of reflected (transmitted) light based on the pulsation of the artery, and this changing amount of light 6 is input to the light reception detection circuit 2. Then, in the light receiving detection circuit 2, the changing light amount 6 is detected by a light receiving element (such as a photoconductive cell or a photovoltaic cell), amplified by a measurement amplifier, and a pulse wave is outputted from a terminal 7. Further, 3 is a feedback control circuit, and its function and operation are as follows.

Now, when detecting a change in the amount of reflected (transmitted) light based on the pulsation of an artery, it is possible to When the operating point of the light reception detection circuit 2 fluctuates more than the set value, the feedback control circuit 3 inserted between the light emission drive circuit 1 and the light reception detection circuit 2 detects the fluctuation level and corrects it. be exposed. As a method of detecting the fluctuation level, when the output of the light reception detection circuit 2 is passed through a low-pass filter with a frequency lower than the fundamental frequency of the pulse wave signal in the feedback control circuit 3, an operating point corresponding to the average intensity of the pulse wave signal is detected. It is possible to detect the fluctuation level of The detected fluctuation level is amplified by the control amplifier of the feedback control circuit 3, inputted to the power amplifier of the light emission drive circuit 1, and the operating point of the light reception detection circuit 2 is set to the set value by controlling the light emission output (light amount). Work to bring it closer. In other words, when the average level of the amount of reflected (transmitted) light decreases due to individual differences in the living tissue 5 or the pressing pressure of the detector, control is performed to increase the light emission output, and conversely, when it increases, control is performed to decrease it. be exposed.

FIG. 3 shows a concrete implementation circuit of the present invention, in which the light emitting drive circuit 1 includes a driving transistor Tr ₁
The cathode of this light emitting diode D is grounded, the anode is connected to the emitter of the driving transistor Tr _{1, the collector of this transistor Tr 1 is} connected to the power supply terminal, and the base is connected to the feedback control circuit. The transistor Tr _{2 is connected to the collector of the control transistor Tr 2} constituting the transistor Tr 3 , the collector is grounded via the resistor r ₁ , and the emitter of the transistor Tr ₂ is supplied with power. Also, transistor
The base of Tr ₂ is connected via a resistor r ₂ to a low-pass filter consisting of a resistor r ₃ and a capacitor C, and the low-pass filter is connected to the output terminal of an operational amplifier OP constituting the light reception detection circuit 2. amplifier
The non-inverting input terminal of the OP is grounded, the cathode side of the photodetector S such as a solar cell is connected to the inverting input terminal of the operational amplifier OP, and the anode side of the photodetector S is grounded. Further, a parallel circuit including a capacitor and a feedback resistor is connected between the inverting input terminal and the output terminal of the operational amplifier OP to prevent disturbance noise.

Next, the operation of the present invention will be explained in more detail.

The light emitting drive circuit 1 is constructed by connecting a light emitting diode D to the emitter of a driving transistor Tr ₁ , and a negative voltage feedback is applied to the driving transistor Tr ₁ due to the voltage drop of the light emitting diode D entering the emitter, and the power supply voltage is reduced. Even if the value changes, the amount of light from the light emitting diode D hardly changes. In addition, the voltage applied to the light emitting diode D is
The voltage applied to the base of transistor Tr ₁ is kept at a voltage lower than the base potential of transistor Tr ₁ by the base-emitter voltage, so the light emission output (light amount)
control is performed. Note that at the start of operation, the voltage relationship of each part cannot be determined, so the amount of light from the light emitting diode D cannot be determined either, but if it is larger than the amount of light in a stable state, it will be smaller, and conversely, if it is smaller than the amount of light in a stable state, it will be smaller. It will move to become larger. In other words, when the light intensity of the light emitting diode D is small, the photocurrent generated in the light receiving element S is small, so the average value of the voltage appearing at the output terminal of the operational amplifier OP becomes low, and the resistance
r ₃ , a low-level voltage is applied to the base of transistor Tr ₂ through the low-pass filter by capacitor C, so the collector current of transistor Tr ₂ increases, increasing the base potential of transistor Tr ₁ and increasing the amount of light emitted. work. Conversely, when the light intensity of the light emitting diode D is large, the photocurrent generated in the light receiving element S is large, so the average value of the voltage appearing at the output terminal of the operational amplifier OP becomes high, and the low-pass filter by the resistor r ₃ and the capacitor C Since a high level voltage is applied to the base of the transistor Tr ₂ through the transistor Tr ₂ , the collector current of the transistor Tr 2 decreases, which lowers the base potential of the transistor Tr ₁ and reduces the amount of light emitted.

Now, when the amount of light emitted from the light emission driving circuit 1 hits the living tissue 5 including the artery, it is input to the light reception detection circuit 2 as a change in the amount of reflected (transmitted) light based on the pulsation of the artery. As described above, the light receiving detection circuit 2 is based on the operational amplifier OP, and is composed of a voltage amplifier having a light receiving element S and a feedback resistor. FIG. 5 shows the signal waveforms of each part in the light receiving detection circuit 2, where A shows the photocurrent generated in the light receiving element S (flows from the inverting input terminal side of the operational amplifier OP to the ground side), and B shows the calculation is the output voltage of amplifier OP. The input light causes a reverse current in the pn junction of the light receiving element S, and this is converted into a voltage by the operational amplifier OP. That is,
Both input terminals of the operational amplifier OP are at the same potential due to an imaginary short caused by negative feedback, and the input impedance is considered to be infinite in practical terms, so all the current flowing to the photodetector S is transferred to the operational amplifier.
The voltage flows from the output terminal of the OP via the feedback resistor, and the voltage drop appears as the output voltage. In addition,
A feedback resistor and a capacitor connected in parallel between the inverting input terminal and output terminal of the operational amplifier OP remove high frequency noise components and do not appear in the output signal.

On the other hand, when detecting changes in the amount of reflected (transmitted) light based on the pulsation of arteries, calculations are performed based on the individual differences in the living tissue 5 and the pressing pressure of the detector consisting of the light emitting diode D and the light receiving element S against the living tissue 5. Although the operating point of the amplifier OP tends to fluctuate beyond the set value, the level of this fluctuation is detected by the low-pass filter of the feedback control circuit 3. Low-pass filter is a resistor
r ₃ and a capacitor C, and has a cut-off frequency lower than the fundamental frequency of the pulse wave signal, so that a signal indicating the average intensity of the pulse wave can be detected. In other words, the output of the operational amplifier OP
Capacitor C is charged through r ₃ and resistors r ₂ , r ₃
Since it is discharged at a moderate speed through the , it acts as a low-pass filter that allows lower frequency signals to pass through more easily, and the output of this low-pass filter corresponds to the average level of the pulse wave, as shown by the broken line in Figure 5 (b). It becomes a signal. Then, the fluctuation level detected by the low-pass filter is amplified by the control transistor Tr ₂ , and by controlling the base potential of the driving transistor Tr ₂ , the light emitting output (light amount) of the light emitting diode D is adjusted. Bring the operating point of circuit 2 close to a predetermined set value. In other words, when the average level of the pulse wave decreases due to individual differences or pressure applied to the detector, the output level of the low-pass filter formed by resistor r ₃ and capacitor C decreases, and transistor Tr ₂
The base potential of the light emitting diode D is lowered, the base current increases, the collector current increases, the terminal voltage of the resistor _r1 , that is, the base potential of the transistor _Tr1 of the light emitting drive circuit 1 increases, and the voltage applied to the light emitting diode D increases. The amount of light is increased by increasing the amount of light. Conversely, when the average level of the pulse wave increases, the output level of the low-pass filter formed by resistor r ₃ and capacitor C increases, the base potential of transistor Tr ₂ is raised, the base current decreases, and the collector current decreases. However, the terminal voltage of the resistor _r1 , that is, the base potential of the transistor _Tr1 of the light emitting drive circuit 1 decreases, the voltage applied to the light emitting diode D decreases, and the amount of light is suppressed.

FIG. 4 shows another embodiment of the invention. This circuit differs from the circuit shown in FIG. 3 in the light emission drive circuit 1, in which the drive transistor _Tr
The collector of is connected to the power supply via the light emitting diode D, and the other light receiving detection circuit 2
The feedback control circuit 3 is the same as that shown in FIG. That is, in this embodiment, the light emitting drive circuit 1 includes a light emitting diode D connected between the collector of the driving transistor Tr ₁ and _the power supply terminal. is driven depending only on the base current of transistor _Tr1 , regardless of fluctuations in the power supply voltage. Therefore, even if the power supply voltage changes due to some influence, the amount of light from the light emitting diode D hardly changes. The base current of the current transistor Tr ₁ supplied to the light emitting diode D is multiplied by a current amplification factor, and the light emission output (light amount) is controlled by the voltage applied to the base of the transistor Tr ₁ .

As described above, the present invention extracts a part of the output of the light reception detection circuit and supplies it to the light emission drive circuit by means of a feedback control circuit consisting of a low-pass filter and an amplifier. Even if the variation occurs due to the difference and the pressure applied to the detector, it has the effect of eliminating the variation.

[Brief explanation of the drawing]

FIG. 1 shows a conventional photoplethysmogram detection circuit, FIG. 2 shows a photoplethysmogram detection circuit of the present invention, FIGS. 3 and 4 show specific implementation circuits, and FIG. 5 shows signal waveforms. 1...Light emission drive circuit, 2...Light reception detection circuit, 3
...Feedback control circuit, 4...Light amount, 5...Biological tissue, 6...Change light amount, 7...Output terminal, Tr ₁ ,
Tr ₂ ...transistor, D...light emitting diode,
S...Photodetector, OP...Operation amplifier, _r1 to _r3 ...
...Resistance, C...Capacitor.

Claims (1)

[Claims] 1. A light emitting drive circuit including an NPN transistor in which a light emitting diode is inserted between an emitter and ground or between a power supply terminal and a collector, and a light receiving circuit in which a non-inverting input terminal is grounded and an inverting input terminal receives light from the light emitting diode. A light receiving detection circuit includes an operational amplifier that is grounded through an element, and a parallel circuit of a capacitor and a feedback resistor is connected between the inverting input terminal and the output terminal; The base is connected through a range filter, the emitter is connected to the power supply terminal, and the collector is connected to the NPN of the light emission drive circuit.
and a feedback control circuit having a PNP transistor connected to the base of the transistor, and obtaining a pulse wave from the output terminal of the light reception detection circuit and controlling the light emission output of the light emission drive circuit via the feedback control circuit. A photoplethysmogram detection circuit featuring: