Arrangement for a photodetector circuit for low power applications, and a corresponding method and a computer program product

Abstract

The present invention introduces an arrangement for enhancing the performance of an electronic circuit comprising a phototransistor (Q). Either a common-collector or a common-emitter connected phototransistor (Q) has a main resistor (R.sub.L), and at least one external bias resistors (R.sub.L2, R.sub.L3, R.sub.L4), each in parallel to one another. The microcontroller may directly control the voltage outputs or act via respective switches (S1, S2) regarding each respective resistor. When the electronic circuit with the phototransistor (Q) is switched on, at least one of the external bias resistors (R.sub.L2, R.sub.L3, R.sub.L4) are switched on. The voltage output rise time is short, and when the bias has been set, the external bias resistor(s) are disconnected functionally. This means that during the actual measurement with the electric circuit, only the main resistor (R.sub.L) is used in the connection.

Claims

1. An arrangement for controlling performance of a photodetector in an electronic circuit, wherein the photodetector is configured to be sensitive to light, the arrangement comprising: a positive supply voltage (Vcc) port and a ground (GND) port, wherein the photodetector is connected in the electronic circuit between the positive supply voltage (Vcc) port and the ground (GND), and wherein the photodetector is connected to the positive supply voltage (Vcc) port or to the ground (GND) port via at least a main resistor (RL); an external bias resistor (RL2) to be connectable in parallel to the main resistor (RL); a microcontroller configured to switch on at least one of the main resistor (RL) and the external bias resistor (RL2) by an output signal of the microcontroller; the microcontroller configured to connect the external bias resistor (RL2) between the photodetector and the positive supply voltage (Vcc) port, or between the photodetector and the ground (GND) port, for a time period enabling a bias to set for the photodetector based at least in part on the electronic circuit being switched on; and the microcontroller configured to disconnect functionally the external bias resistor (RL2) from the electronic circuit based at least in part on the bias being set.

2. The arrangement according to claim 1, wherein the photodetector is a phototransistor (Q), wherein a base (B) of the phototransistor (Q) is sensitive to light.

3. The arrangement according to claim 2, wherein a collector (C) of the phototransistor (Q) is connected to the main resistor (RL) and the external bias resistor (RL2), and an emitter (E) of the phototransistor (Q) is connected to the ground (GND) port or to a negative supply voltage port.

4. The arrangement according to claim 2, wherein a collector (C) of the phototransistor (Q) is connected to the positive supply voltage (Vcc) port, and an emitter (E) of the phototransistor (Q) is connected to the main resistor (RL) and the external bias resistor (RL2).

5. The arrangement according to claim 1, wherein the microcontroller is configured to activate at least one of the main resistor (RL) and the external bias resistor (RL2) through direct output voltages, or via a respective switch (S1, S2) connected in series with the respective resistor (RL, RL2).

6. The arrangement according to claim 5, wherein the microcontroller is configured to activate at least one of the main resistor (RL) and the external bias resistor (RL2) through direct output voltages, and the connection from at least one of the main resistor (RL) and the external bias resistor (RL2) to the ground (GND) port has been cut off.

7. The arrangement according to claim 1, wherein the electronic circuit applies one or more further external bias resistors (RL3, RL4) connected in parallel with the external bias resistor (RL2) and the main resistor (RL).

8. The arrangement according to claim 7, wherein the microcontroller is configured to activate at least one of the external bias resistors (RL2, RL3, RL4) during the biasing, wherein the microcontroller is configured to connect only the main resistor (RL) among all the resistors in the electronic circuit based at least in part on the bias being set.

9. The arrangement according to claim 8, wherein during the biasing, the main resistor (RL) is configured to be activated with at least one of the external bias resistors (RL2, RL3, RL4).

10. The arrangement according to claim 1, wherein the time period between the connecting and disconnecting instances is selected to be between 2-20 microseconds.

11. The arrangement according to claim 1, wherein a resistance of the main resistor (RL) is 1-4 kΩ.

12. The arrangement according to claim 1, wherein a resistance of the external bias resistor (RL2) is 100-400 Ω.

13. A method for controlling performance of a photodetector in an electronic circuit, wherein the photodetector is configured to be sensitive to light, the method comprising the steps of: connecting the photodetector in the electronic circuit between a positive supply voltage (Vcc) port and a ground (GND) port directly or indirectly; connecting the photodetector via a main resistor (RL) to the positive supply voltage (Vcc) port or to the ground (GND) port; connecting an external bias resistor (RL2) in parallel to the main resistor (RL); switching on at least one of the main resistor (RL) and the external bias resistor (RL2) by an output signal of a microcontroller; connecting the external bias resistor (RL2) between the photodetector and the positive supply voltage (Vcc) port, or between the photodetector and the ground (GND) port, for a time period enabling a bias to set for the photodetector based at least in part on the electronic circuit being switched on; and disconnecting functionally the external bias resistor (RL2) from the electronic circuit by the microcontroller based at least in part on the bias being set.

14. A computer program product for controlling performance of a photodetector in an electronic circuit, wherein the photodetector is configured to be sensitive to light, and wherein the computer program product comprises program code storable on a computer readable storage medium, the program code being configured to execute the following steps when the program code is run in a microcontroller or in an external processor: connecting the photodetector in the electronic circuit between a positive supply voltage (Vcc) port and a ground (GND) port directly or indirectly; connecting the photodetector via a main resistor (RL) to the positive supply voltage (Vcc) port or to the ground (GND) port; connecting an external bias resistor (RL2) in parallel to the main resistor (RL); switching on at least one of the main resistor (RL) and the external bias resistor (RL2) by an output signal of a microcontroller; connecting the external bias resistor (RL2) between the photodetector and the positive supply voltage (Vcc) port, or between the photodetector and the ground (GND) port, for a time period enabling a bias to set for the photodetector based at least in part on the electronic circuit being switched on; and disconnecting functionally the external bias resistor (RL2) from the electronic circuit by the microcontroller based at least in part on the bias being set.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows a schematic illustration of a typical common-emitter circuit system for a phototransistor in prior art,

(2) FIG. 1B shows a schematic illustration of a typical common-collector circuit system for a phototransistor in prior art,

(3) FIGS. 2A, 2B and 2C show schematic illustrations of common-emitter circuits controlled by a microcontroller according to the present invention,

(4) FIGS. 3A, 3B and 3C show schematic illustrations of common-collector circuits controlled by a microcontroller according to the present invention,

(5) FIG. 4A shows a schematic illustration of a common-emitter circuit controlled by a microcontroller according to the present invention,

(6) FIG. 4B shows a schematic illustration of a common-collector circuit controlled by a microcontroller according to the present invention, and

(7) FIG. 5A shows a common-emitter circuit example and FIG. 5B shows a graphic illustration of the set-up time and gain with different R.sub.L values in the common-emitter circuit example of FIG. 5A, as in the present invention compared to the prior art, where the performance of the present invention against two prior art solutions is shown in three different graphs.

DETAILED DESCRIPTION OF THE INVENTION

(8) The following detailed description illustrates embodiments of the present disclosure and ways in which they can be implemented.

(9) The present invention discloses a structure and principle for a circuit comprising a photodetector in a wearable device where gain and speed of the circuit can be enhanced with less consumed power. In an embodiment, the photodetector is a phototransistor.

(10) Concerning the other receiver electronics components like the amplifier, AD-converter and microcontroller, their consumed power can be saved by making the photodetector to wake up faster. Thus, the purpose of the present invention is to make and allow a photodetector to wake up faster and make its output less sensitive to noise and errors.

(11) The circuit according to the invention is implemented with discrete components in a tiny physical structure and with a very low power concerning the adjustment of the gain and shortening of the rise time of the phototransistor circuit. The use of a discrete phototransistor enables to select a thin and small-sized component to be fitted to a small device structure. By connecting a load resistance, R.sub.L, directly to the phototransistor enables to keep a number of components small and to save space in a small device structure.

(12) The idea is here discussed to be used in “common-emitter” type of circuit structures, but the concept can be utilized in common-collector circuits as well.

(13) In the present invention, the tuning of the circuit is performed in the front stage of the circuit. This is now discussed in more detail in connection to FIGS. 2-5.

(14) FIG. 2A shows a first schematic illustration of a common-emitter circuit controlled by a microcontroller according to the present invention. In other words, FIG. 2A discloses a first embodiment of the applied circuit structure comprising a photodetector which in this example is a phototransistor Q. The phototransistor Q has a collector C, a base B and an emitter E, where the emitter E is connected to the ground (GND), the collector C is connected to output voltage V.sub.out and the base B receives and detects light. Based on the detected light intensity, the phototransistor Q transforms the light information to an electric signal which can be seen as an emerging base current whose magnitude is directly proportional to the intensity of the light. The light is absorbed and detected by the photosensitive semiconductor base region. A collector current is produced based on the base current, and the phototransistor otherwise functions as a regular bipolar junction transistor (BJT).

(15) The supply voltage V.sub.cc is connected so that the main resistor R.sub.L is connected between the collector C and the V.sub.cc. Now going into the inventive circuit components of this embodiment, an external bias resistor R.sub.L2 is connected in parallel with the main resistor R.sub.L where the external bias resistor R.sub.L2 acts as an extra resistor. A microcontroller is further set to control a switch S2 where the switch S2 is connected in series with the external bias resistor R.sub.L2, i.e. between the V.sub.cc and the R.sub.L2.

(16) Now the functional working principle of the presented circuit structure is discussed. A faster bias is set to the photodetector Q by using the external bias resistor R.sub.L2. In other words, the photodetector Q has an extra connection to the power source V.sub.cc from the collector C with a smaller resistor. The resistance of the external bias resistor R.sub.L2 can be selected to be 5-30 times smaller than the resistance of the main resistor R.sub.L. The capacitor (not shown) including a capacitor from the base B to collector C, and from the collector C to emitter E, can be loaded quickly by switching the voltage through the external bias resistor R.sub.L2. When the capacitor is loaded, the voltage to the external bias resistor R.sub.L2 can be switched off, and thereafter, the measurement can be done with the main resistor R.sub.L (i.e. the first bias resistor) and its gain. In other words, the external bias resistor R.sub.L2 is switched on first or together with the main resistor R.sub.L. When the photoresistor Q is biased, the external bias resistor R.sub.L2 is disconnected functionally, and the measurement will be done with the main resistor R.sub.L only.

(17) The differences between the prior art (only R.sub.L used) and the present invention with the external bias resistor R.sub.L2 are illustrated and discussed in more detail later in connection with FIGS. 5A-B.

(18) FIG. 2B shows a second schematic illustration of a common-emitter circuit controlled by a microcontroller according to the present invention. In other words, FIG. 2B discloses a second embodiment of the applied circuit structure comprising a photodetector which in this example is a phototransistor Q as in the previous embodiment. The second embodiment comprises the phototransistor Q similarly connected as in the first embodiment of FIG. 2A. Furthermore, it comprises the main resistor R.sub.L, the external bias resistor R.sub.L2, the switch S2 and the microcontroller in a similar way as in the first embodiment. Furthermore, the second embodiment comprises additionally a switch S1 in series with the main resistor R.sub.L so that switch S1 and R.sub.L are chained in parallel connection with switch S2 and R.sub.L2 between the supply voltage V.sub.cc and the collector C. Microcontroller is set to control both switches S1 and S2. In the second embodiment, the microcontroller output signal is set to control biasing of the photoresistor Q by connecting either R.sub.L2, or both the R.sub.L2 and R.sub.L in parallel. In the latter case with both switches S1 and S2 closed, the combined bias resistance value is:

(19) $\begin{array}{cc}{R}_{\mathrm{total}}=\frac{1}{\frac{1}{R_{L2}}+\frac{1}{R_L}}& \left(1\right)\end{array}$

(20) When the photoresistor Q is biased, the external bias resistor R.sub.L2 is disconnected functionally by opening the switch S2, and the measurement will be done with the main resistor R.sub.L only through closed switch S1.

(21) FIG. 2C shows a third schematic illustration of a common-emitter circuit controlled by a microcontroller according to the present invention. In other words, FIG. 2C discloses a third embodiment of the applied circuit structure comprising a photodetector which in this example is a phototransistor Q as in the previous embodiments. The third embodiment comprises the phototransistor Q similarly connected as in the first two embodiments of FIGS. 2A-B. This time both the main resistor R.sub.L and the external bias resistor R.sub.L2 are connected directly to the microcontroller output ports where the microcontroller directly controls either or both output branches by supplying a desired voltage to each desired output port. The output voltages can be freely selected to each of the output ports so that the biasing procedure as explained in the previous embodiments, as well as the measurement itself can be obtained. Thus, the output voltage to its 1st output port may differ from the output voltage to its 2.sup.nd output port. Also, either or both outputs of the microcontroller can naturally be selected to be in a non-connected state or in very high impedance state, when desired. The voltage pattern as a function of time in each of the output ports can be defined in the microcontroller in programmable means, i.e. by executing a piece of software which is set to direct the voltage outputs as desired or to be non-connected i.e. a resistor is not connected to any voltage or ground.

(22) Now going into different kinds of circuit structures, FIGS. 3A-3C illustrate fourth, fifth and sixth embodiments of the invention, which have certain analogy with the FIGS. 2A-2C but they represent now common-collector circuits. In other words, FIG. 3A shows a first schematic illustration of a common-collector circuit controlled by a microcontroller according to the present invention. Thus, FIG. 3A discloses a fourth embodiment of the applied circuit structure comprising a photodetector which in this example is a phototransistor Q. The phototransistor Q has a collector C, a base B and an emitter E, where the collector C is connected to the supply voltage (V.sub.cc), the emitter E is connected to output voltage V.sub.out and the base B receives and detects light. Based on the detected light intensity, the phototransistor Q transforms the light information to an electric signal which can be seen as an emerging base current whose magnitude is directly proportional to the intensity of the light. The light is absorbed and detected by the photosensitive semiconductor base region. A collector current is produced based on the base current, and the phototransistor otherwise functions as a regular bipolar junction transistor (BJT).

(23) The emitter E is further connected so that the main resistor R.sub.L is connected between the emitter E and the ground (GND). Now going into the inventive circuit components of the fourth embodiment, an external bias resistor R.sub.L2 is connected in parallel with the main resistor R.sub.L where the external bias resistor R.sub.L2 acts as an extra resistor. A microcontroller is further set to control a switch S2 where the switch S2 is connected in series with the external bias resistor R.sub.L2, i.e. between the emitter E and the ground (GND).

(24) Now the functional working principle of the presented circuit structure is discussed. A faster bias is set to the photodetector Q by using the external bias resistor R.sub.L2. In other words, the photodetector Q has an extra connection to the ground GND from the emitter E with a smaller resistor. The resistance of the external bias resistor R.sub.L2 can be selected to be 5-30 times smaller than the resistance of the main resistor R.sub.L. The capacitor (not shown) can be loaded quickly by switching the voltage through the external bias resistor R.sub.L2. When the capacitor is loaded, the voltage to the external bias resistor R.sub.L2 can be switched off, and thereafter, the measurement can be done with the main resistor R.sub.L (i.e. the first bias resistor) and its gain. In other words, the external bias resistor R.sub.L2 is switched on first or together with the main resistor R.sub.L. When the photoresistor Q is biased, the external bias resistor R.sub.L2 is disconnected functionally, and the measurement will be done with the main resistor R.sub.L only.

(25) The differences between the prior art (only R.sub.L used) and the present invention with the external bias resistor R.sub.L2 are illustrated and discussed in more detail later in connection with FIGS. 5A-B, and this applies also for the fourth, fifth and sixth embodiments of the invention.

(26) FIG. 3B shows a second schematic illustration of a common-collector circuit controlled by a microcontroller according to the present invention. In other words, FIG. 3B discloses a fifth embodiment of the applied circuit structure comprising a photodetector which in this example is a phototransistor Q as in the previous embodiments. The fifth embodiment comprises the phototransistor Q similarly connected as in the fourth embodiment of FIG. 3A. Furthermore, it comprises the main resistor R.sub.L, the external bias resistor R.sub.L2, the switch S2 and the microcontroller in a similar way as in the fourth embodiment. Furthermore, the fifth embodiment comprises additionally a switch S1 in series with the main resistor R.sub.L so that switch S1 and R.sub.L are chained in parallel connection with switch S2 and R.sub.L2 between the emitter E and the ground GND. Microcontroller is set to control both switches S1 and S2. In the fifth embodiment, the microcontroller output signal is set to control biasing of the photoresistor Q by connecting either R.sub.L2, or both the R.sub.L2 and R.sub.L in parallel. In the latter case with both switches S1 and S2 closed, the combined bias resistance value is:

(27) $\begin{array}{cc}{R}_{\mathrm{total}}=\frac{1}{\frac{1}{R_{L2}}+\frac{1}{R_L}}& \left(1\right)\end{array}$

(28) When the photoresistor Q is biased, the external bias resistor R.sub.L2 is disconnected functionally by opening the switch S2, and the measurement will be done with the main resistor R.sub.L only through closed switch S1.

(29) FIG. 3C shows a third schematic illustration of a common-collector circuit controlled by a microcontroller according to the present invention. In other words, FIG. 3C discloses a sixth embodiment of the applied circuit structure comprising a photodetector which in this example is a phototransistor Q as in the previous embodiments. The sixth embodiment comprises the phototransistor Q similarly connected as in the previous two embodiments of FIGS. 3A-B. This time both the main resistor R.sub.L and the external bias resistor R.sub.L2 are connected directly to the microcontroller output ports where the microcontroller directly controls either or both output branches by supplying a desired voltage to each desired output port or to be in a non-connected state. The output voltages can be freely selected to each of the output ports so that the biasing procedure as explained in the previous embodiments, as well as the measurement itself can be obtained. Thus, the output voltage to its 1.sup.st output port may differ from the output voltage to its 2.sup.nd output port. Also, either or both outputs of the microcontroller can naturally be selected to be in a non-connected state or in very high impedance state, when desired. The voltage pattern as a function of time in each of the output ports can be defined in the microcontroller in programmable means, i.e. by executing a piece of software which is set to direct the voltage outputs as desired. In the sixth embodiment, the connections to the ground GND are removed.

(30) FIG. 4A shows a schematic illustration of a yet another common-emitter circuit structure controlled by a microcontroller according to the present invention. This time the circuit structure applies a group of parallel resistors with different resistance values. This seventh embodiment is a generalization of the third embodiment (FIG. 2C). The circuit structure is the same as in the third embodiment but there are additional resistors R.sub.L3 and R.sub.L4 connected separately in parallel with the parallel connection of resistors R.sub.L and R.sub.L2. Thus, the microcontroller is connected to all four resistors, and it can control each of these resistors by an individual voltage output. Preferably, the main resistor R.sub.L and the external bias resistors R.sub.L2, R.sub.L3 and R.sub.L4 are selected to have different resistance values. In one example, the values could be selected as follows:
R.sub.L=4000 Ω
R.sub.L2=⅕*R.sub.L=800 Ω
R.sub.L3= 1/10*R.sub.L=400 Ω
R.sub.L4= 3/80*R.sub.L=150 Ω

(31) With such a resistor pattern, the output voltage from the microcontroller can be a fixed value of V.sub.cc in each of the four branches, or alternatively zero. When the biasing is performed, the desired resistor or a group of resistors are activated through output signals from the microcontroller. After the biasing has been achieved, the three lower control signals from the microcontroller are disconnected from V.sub.cc, for example setting to an indefinite state i.e a non-connected state or very high impedance state, and only the main resistor R.sub.L is activated through supply voltage V.sub.cc from the microcontroller. It is notable that during the biasing of the phototransistor Q, the microcontroller can select any one of the resistors R.sub.L-R.sub.L4, or any two of the four resistors, or any three of the four resistors, or all four resistors of the circuit for the connection. During the actual measurement after the biasing, only R.sub.L is activated by the microcontroller.

(32) FIG. 4B shows a schematic illustration of a yet another common-collector circuit structure controlled by a microcontroller according to the present invention. This time the circuit structure applies a group of parallel resistors with different resistance values, as in the previous embodiment. This eighth embodiment is a generalization of the sixth embodiment (FIG. 3C). The circuit structure is the same as in the sixth embodiment but there are additional resistors R.sub.L3 and R.sub.L4 connected separately in parallel with the parallel connection of resistors R.sub.L and R.sub.L2. Thus, the microcontroller is connected to all four resistors, and it can control each of these resistors by an individual voltage output. Preferably, the main resistor R.sub.L and the external bias resistors R.sub.L2, R.sub.L3 and R.sub.L4 are selected to have different resistance values. In one example, the values could be selected as follows:
R.sub.L=4000 Ω
R.sub.L2=⅕*R.sub.L=800 Ω
R.sub.L3= 1/10*R.sub.L=400 Ω
R.sub.L4= 1/20*R.sub.L=200 Ω

(33) With such a resistor pattern, the output voltage from the microcontroller can be a fixed value of V.sub.cc in each of the four branches, or alternatively being in a non-connected state. When the biasing is performed, the desired resistor or a group of resistors are activated through output signals from the microcontroller. After the biasing has been achieved, the three upper control signals from the microcontroller are functionally disconnected and only the main resistor R.sub.L is activated through connecting it to GND or to other fixed voltage value below V.sub.cc by the microcontroller. It is notable that during the biasing of the phototransistor Q, the microcontroller can select any one of the resistors R.sub.L-R.sub.L4, or any two of the four resistors, or any three of the four resistors, or all four resistors of the circuit for the connection. During the actual measurement after the biasing, only R.sub.L is activated by the microcontroller.

(34) It is highlighted that the above resistor values from the 7th and 8th embodiments are merely examples, and any other appropriate resistance values can be applied in the present invention.

(35) FIG. 5A shows a used circuit example with selected resistance values and FIG. 5B shows its graphic result illustration of the set-up time and gain with different R.sub.L values in a common-emitter circuit, as in the present invention compared to the prior art. The situation can be picked from the seventh embodiment in FIG. 4A, where R.sub.L2 and R.sub.L3 are functionally disconnected or removed and thus not used. The exemplary values for the resistors R.sub.L=4000Ω and R.sub.L4=150Ω are used in this context regarding the resulting graph of FIG. 5B, the resistor values shown in the common-emitter circuit diagram of FIG. 5A. Now let's discuss three different scenarios with the selection of the resistor branches.

(36) The first method is to use only R.sub.L=4000Ω for both the biasing and for the measurement. This means the method according to prior art. The resulting output voltage V.sub.out as a function of time is shown as the solid line 51 in FIG. 5B. It illustrates a slow set-up time, a bit over 100 μs in this graph. As it can be seen after the voltage rise time or the set-up time, the behavior of V.sub.out show a pulse signal (related optical pulsing input to the base B, not shown) with a relatively high gain because the ratio of the maximum output voltage and the minimum output voltage is relatively high.

(37) The second method is to use only the resistor with the smaller resistance, i.e. R.sub.L=R.sub.L4=150Ω; also part of the prior art. This resistor results in the graph according to the dashed line 52 in FIG. 5B, where the set-up time of the output voltage is significantly shorter, in this case around 5 μs. As it is shown in FIG. 5B, the reduction in the set-up time of the output voltage compared to the first method can be in the range of 80-150 μs (the set-up time difference between the solid 51 and dashed 52 lines). This means that time is saved due to the faster set-up time, and it further means smaller noise and bias voltage fluctuation. However, as it can be seen from the behavior of the dashed line 52 later as a function of time, the amplitude variation of the pulse signal (related optical pulsing input to the base B, not shown) is very small, meaning a very low gain. This is a clear disadvantage.

(38) The third method is the method according to the present invention. This time we use the seventh embodiment with the above values of R.sub.L=4000 Ω and R.sub.L4=150Ω, with the signal branches R.sub.L2 and R.sub.L3 as functionally disconnected (i.e. unused). We use first only the smaller resistance 150Ω for the biasing. The result can be seen in the dot-dash line 53 of FIG. 5B. The set-up time for the V.sub.out is very short, only around 5 μs. As already discussed above, the reduction in the set-up time of the output voltage according to the invented method compared to the first method can be in the range of 80-150 μs (the set-up time difference between the solid 51 and dot-dash 53 lines), which is a great advantage. After the set-up time has passed from the initiation, the phototransistor is biased, and the microcontroller can right then be arranged to switch the resistors. Thus, after ˜5 μs, the larger resistance 4 kΩ is connected for the actual measurements. This means that the V.sub.out will behave as a pulsed signal with a high gain value, shown especially after 100 μs when the dot-dash line 53 has clearly notable variation between its maximum and minimum values. This indicates a good signal gain value, which is now combined with the earlier result of a very short set-up time of the signal. This is an advantageous result within the circuits discussed here in connection to physically small, and wearable health and sleep monitoring devices. The present invention indeed achieves both these crucial characteristics by the intelligent switching of the resistors according to any of the above embodiments. Therefore, they represent the advantages of the applied circuitry and its connection logic.

(39) Back to the results emerging from the connection according to the invention, the bias can be set 5-30 times faster, meaning for example in 5 μs instead of 100 μs. This means extreme power saving as the other components do not need to wait the bias setup so long, meaning that this theoretically saves 80-96% of the total power. In practice as the total saving time cannot be found, the power saving is still remarkable 20-80%. Due to the fact that the bias is set faster, the bias is also more stable and not so sensitive because of temperature or other external conditions and therefore, the noise will also be reduced. These are great advantages of the invented solution.

(40) Furthermore, because the I/O output port of the processor directly controls the resistor branches, the tuning of the measurement circuit is indeed performed in the front stage of the measurement circuit. This represents a clear difference to the usual circuit solutions where the gain is adjusted in a subsequent gain control block in the latter stages of the signal processing circuit.

(41) Also because the invented solution is implemented with discrete electrical components within a physically tiny structure in the context of wearable health and sleep monitoring devices, the circuit structures implemented with ASICs are not well suited to the context of the invention. In a preferable embodiment, the wearable health and sleep monitoring device is manufacturing in a form of a wearable ring by a human user. All necessary electrical components and sensors and even a battery can be implemented in such a small physical structure. A charging device can be implemented in a form of a table charger, where the ring can be placed for wireless charging of the battery. A further smartphone app may be connectable to the ring and the charging unit for transferring the measured data from the human user (i.e. from the ring-shaped monitoring device) to be visualized to the user him/herself through the personal smartphone screen. Appropriate radio transceivers are thus applied as well for the data transfer between the ring and the smartphone (or any other personal device, such as a tablet or a PC).

(42) A further advantage is that the circuitry with such an advantageous slow set-up time and high gain can be implemented in a tiny structure allowing various other application areas than just the ones discussed above. Because the present invention can be implemented with a small number of discrete components, and the circuit structure is also simple, a tiny microcontroller is also able to control the invented circuit. All this enables the fact that the desired components can indeed be implemented in a small ring-shaped device which is wearable and convenient for the human user even during his/her sleeping time.

(43) Summarizing the advantages once more for the present invention, the invented circuit structure consumes a very low power level, although a very high sensitivity is indeed needed. It is also notable that the phototransistor detects here pulsed light so there is a need to switch the phototransistor and the whole analog path and the microcontroller off very often and thus, as fast as possible. This further means that the photoreceiver must be switched on within short time periods, which means that the single rise time of the voltage output of the phototransistor must be very short. The presented circuit structure and microcontroller output control logic indeed allow these requirements to be fulfilled.

(44) Thus, the voltage output rise time is very short, good gain characteristics are obtained, and power savings are a notable result of the invented method and arrangement.

(45) The present invention comprises an arrangement, a corresponding method and a corresponding computer program product. All these aspects of the invention comprise the same sub-features, sub-parts and sub-functionalities which are comprised in the dependent arrangement claims.

(46) The present invention is not restricted merely to the embodiments disclosed above but it may vary within the scope of the claims.