NL1033451C2 - Driving circuit for switch state indicator, e.g. LED, comprises Wheatstone bridge and sensor for detecting when this bridge is out of balance - Google Patents

Abstract

A circuit branch (3) comprises a Wheatstone bridge (4) connected in series with the switch (S1), the bridge comprising two parallel branches (5a, 5b) each having a midpoint (6a, 6b), one of these midpoints being connected to a control pin (9). A sensor (7) is used to detect when the bridge is out of balance and actuates a control device (8) for turning the indicator (1) on and off. The circuit has a power voltage applied between two reference lines (2a, 2b), between which is a circuit branch including the switch. The control pin is used to provide a connection with a control circuit including a controllable switch component. An independent claim is also included for a system for driving six indicators, comprising six of these circuits, in which the first reference lines are common to all six circuits and can be connected with a first wire, the second reference lines are also common to all the circuits and can be connected with a second wire, and the control pins in each of these circuits can be connected with a wire.

Description

SWITCH WITH CONTROLLABLE INDICATOR AND REGISTERABLE

SWITCH

The present invention relates to a circuit for controlling an indicator, typically an LED, and for registering the position of a switch. The invention further relates to a system for controlling six indicators and registering the position of six switches. Finally, the invention relates to the use of such a circuit or system for certain applications.

Figure 1 illustrates a hospital application according to the state of the art in which two buttons (red and green with a built-in LED each) are provided for one bed. Such units are typically cabled per three beds. So there are six switches and six LEDs per three beds that must be controllable at the same time.

Such a button (switch) with built-in LED is controlled according to the state of the art with one of the circuits shown in figures 2A-2C. On the one hand, the LED can be switched on / off by applying a certain voltage Vc to the control pin. On the other hand, pressing the switch causes a certain voltage to be applied to the control pin. The behavior of the LED is thus influenced by pressing the button (switch). In the variant of figures 2B and 2C, the LED when it lights up will go out when the switch S is pressed. In the variant of Figure 2A, when the LED is not lit, it will come on when the switch S is pressed. In the variants shown in figures 2A-2C, the operation of the LED is thus disrupted by the closing of the switch.

For certain applications, however, it is desirable that the LED can be controlled independently of the position of the switch 1033451 2, the voltage Vc on the control pin allowing to derive the position of the switch. The circuits of Figs. 2A-2C are particularly not acceptable in situations where, for example, a button must be pressed for a certain period of time to cause a certain effect.

The present invention has for its object to propose a circuit with which an indicator can be controlled independently of the position of a switch, and wherein a limited number of connecting pins are used, including a control pin, via which control pin the position of the switch can be registered .

To this end the invention relates to a circuit for controlling an indicator, typically an LED, and registering the position of a switch, comprising: - a first and second reference line for applying a supply voltage therebetween; - a first branch between the reference lines; - a switch in this first branch; 20 - an indicator; - a control pin; characterized in that - the first branch comprises a Wheatstone bridge in series with the switch, which bridge consists of two parallel bridge branches with respective centers, a first of these centers being connected to the control pin; - a detection means for detecting the Wheatstone bridge's imbalance, i.e. for detecting a voltage greater than a determined threshold value between the centers of the bridge branches; - a control means controlled by the detection means for switching the indicator on / off.

3

Note that turning the indicator on / off is only determined by the Wheatstone bridge being in or out of balance, and therefore independent of the position of the switch S1. The indicator can be switched on / off easily by means of a control circuit, the output of which is connected to the control pin, such that the Wheatstone bridge can be unbalanced.

Advantageous embodiments of the circuit according to the invention are described in claims 2-14.

The detection means can for instance comprise a detection transistor which is connected such that the Wheatstone bridge is out of balance causes the conduction of this transistor. This state is then used by the control means to turn on the indicator. Such a detection means has the advantage of being very inexpensive. In other examples, an optocoupler or an OPAMP can be used as a detection means.

The control pin 9 is preferably connected to a control circuit which comprises a controllable switching element between the control pin 9 and a reference line. The switching on / off of this controllable switching element then determines the switching on / off of the indicator. This control circuit may also typically include a voltage detection circuit for determining the voltage on the control pin. The position of the switch can then be derived from this.

The invention also relates to a system comprising six circuits according to the invention, as defined in claim 15. Such a system can be fully connected using only one UTP cable, the eight wires of which are connected to the common first reference line, the common second reference line and the six control pins.

4

Finally, the invention relates to the use of a circuit as described above for: automotive applications, hospital applications, alarm systems.

The invention will be further elucidated on the basis of a number of exemplary embodiments which are described in detail below with reference to the figures in the annex in which:

Figure 1 illustrates a hospital application according to the prior art; Figures 2A-2C show three circuits according to the prior art, with an LED and a switch;

Figure 3 is a schematic representation of an embodiment of the circuit according to the invention;

Figure 4 is a diagram of a first embodiment of the circuit of the invention;

Figures 5A-5D illustrate the operation of the circuit of Figure 4 for the different positions of the switches S1, S2.

Figure 6 is a system for controlling six indicators and registering the position of six switches according to a first embodiment of the invention;

Figure 7 is a diagram of a second embodiment of the circuit of the invention;

Figure 8 is a diagram of a third embodiment of the circuit of the invention.

In the figures, the same or analogous parts are indicated with the same reference numerals.

Figure 3 shows schematically an embodiment of the invention. The circuit has three connection pins. A first connection pin is connected to a first reference line 2a, typically the supply voltage, and a second connection pin is connected to a second reference line 2b, typically the ground. The third connection pin 9 is a control pin. In a first branch 3 a switch S1 and a bridge of Wheatstone 4 are included in series with each other. The bridge of Wheatstone 4 consists of two parallel bridge branches 5a, 5b with respective centers 6a, 6b. Center point 6a is connected to control pin 9.

Between the centers 6a, 6b, a detection means 7 is provided for detecting the Wheatstone bridge being out of balance. Upon detection of the bridge's unbalance, an indicator, typically an LED, is switched on via a control means 8.

10 Now will. briefly describe the operation of the circuit of Figure 3. The control pin 9 is connected via a resistor R5 to a switch S2 which is connected to reference line 2b. To turn the indicator on, switch S2 is closed, leaving the bridge out of balance. This is detected by detection means 7 and the indicator is thus switched on. Furthermore, the switch S1 being open or closed has consequences for the voltage on control pin 9, and more particularly, closing the switch S1 causes a drop in voltage on control pin 9. From the value 20 of the voltage on control pin 9, it can thus be deduced whether switch S1 is open or closed. Note that the closing / opening of switch S1 has no influence on the Wheatstone bridge being in or out of balance and thus leaves the operation of the LED untouched.

A further elaborated embodiment will now be described with reference to Fig. 4 and Figs. 5A-5D. In this embodiment, the indicator is an LED. The detection means 7 consists of a bipolar PNP transistor whose emitter is connected to the first center point 6b of the Wheatstone bridge, the collector is connected to the second reference line 2b via a resistor R8, and the base is connected to a resistor R6 via control pin 9. The control means 8 for switching the LED 1 on / off comprises a bipolar NPN transistor T1 and two resistors R9; R10 in series with the LED 1. The base of the bipolar NPN transistor T1 is connected to the collector of the bipolar PNP transistor T2. The emitter of transistor T1 is connected to earth, and the collector 5 of T1 is connected to LED 1 via resistors R9, R10.

The control circuit 10 comprises a switch S2, here in the form of a bipolar transistor T3, the collector of which is connected to control pin 9 via a resistor R5.

When the Wheatstone bridge is pulled out of balance by closing the switch S2, the base-emitter voltage of transistor T2 increases above a threshold value, causing transistor T2 to conduct. As a result, a voltage is built across resistor R8, and transistor T1 starts to conduct. When pulling out the Wheatstone bridge 15, LED 1 will light up.

Furthermore, control pin 9 is connected to a voltage detection circuit 11 for determining the voltage on the control pin. From this it can then be deduced what the state is of switches S1 and S2, as will now be explained with reference to figures 5A-5D:

Figure 5A: SI open. S2 open

No current flows through the Wheatstone bridge and the voltage on control pin 9 is: 25 UI = Ud

Figure 5B: SI closed. S2 open

The Wheatstone bridge guides and is balanced: U1 - U2 = UE1 = 0 when R1 x R4 = R2 x R3 30 U1 = (R2 / (R1 + R2)) X Ud 7

Figure 5C: SI open. S2 closed

The Wheatstone bridge is out of balance and the voltage on the control pin 9 is given by: UI = Ud x R5 / ((((((R2 + R4) // RE1) + R3) // R1) + R5) 5 where RE1 = R6

Figure 5D: SI closed. S2 closed

The Wheatstone bridge is out of balance. The Thevemin equivalent is: 10 (R1 // R2 // R5) Rei (R3 // R4) or 15 Ud (R2 // R5) / ((R2 // R5) + R1) Ud (R4 / (R3 + R4) ))

From this follows a value for U1 that differs from the other cases, so that the value of U1 can unambiguously deduce the position of S1 and S2.

In the circuit of Figure 4, the resistors may, for example, have the following values:

R1 = R2 = R3 - R4 = 4.7 kQ R5 = 12 kü R6 = 10 kQ R7 = 68 kQ

R9 = R10 = 820 Ω Ril = 100 kQ R12 = 47 kQ R13 = R14 = 33 kQ 30 R15 = 1 ΜΩ

R16 = R17 = 47 kΩ

For these resistance values and for a supply voltage Ud = 24 V, the following voltages can occur at control pin 9: 8

S1 open, S2 open: U1 = 24 V SI closed, S2 open: U1 = 12 V SI open, S2 closed: U1 «18 V SI closed, S2 closed: U1» 10 V

Figure 6 illustrates the use of the circuit of Figure 4 in a system, for example for a hospital application. This system comprises six circuits as shown in Figure 4, wherein the first reference lines 2a are common to the six circuits and are connected to the supply voltage V, and the second reference lines 2b are also common to the six circuits and are connected to the ground. The six control pins of each of the six circuits are connected via wires 91-96 to a control circuit for controlling the different LEDs 101-106. The number of wires required for this system is therefore eight wires, such that one UTP cable can be used to connect the system.

A second and third embodiment of the circuit according to the invention will now be discussed with reference to Figs. 7 and 8.

In the embodiment of Figure 7, an optocoupler OC is used as a detection circuit to detect the unbalance of the Wheatstone bridge. When the voltage between the centers 6a and 6b becomes greater than a certain critical threshold value, the LED of the optocoupler starts conducting, whereby an electrical signal is generated at the output of the optocoupler. Those skilled in the art will understand that this output may suitably be connected to the indicator to be controlled. The advantage of using a detection circuit with optocoupler is that a galvanic isolation with the indicator is realized.

In the variant of Figure 8, the detection circuit comprises an OPAMP circuit. The negative input U ~ of the OPAMP is 9

via a resistor R20 connected to center 6b of the Wheatstone bridge 4. The positive input of the OPAMP is connected via resistor R22 to center 6a of the Wheatstone bridge. The positive input U * is further connected to the second reference line 2b via resistor R23. Finally, the output of the OPAMP is connected via a resistor R21 to the negative input of the OPAMP. According to a possible embodiment, the values of the resistors can be for example: R20 = R22 = 10 kQ 10 R21 = R23 = 100 kQ

For this example, the voltage at the OPAMP output will be:

Vout = 100 kQ / 10 kQ x (U + - U)

Those skilled in the art will understand that the invention is not limited to the exemplary embodiments illustrated above and that many variants and modifications are possible without departing from the scope of the invention. The scope of the invention is only determined by the following claims.

1033451

Claims (17)

Circuit for controlling an indicator (1), typically an LED, which allows registering the position of a switch (S1), comprising: - a first and second reference line (2a, 2b) for applying a supply voltage (Ud) between them; - a first branch (3) between the reference lines; - a switch (S1) in this first branch; 10. an indicator; - a control pin (9) for connection to a control circuit with a controllable switching element; characterized in that - the first branch (3) comprises a Wheatstone bridge (4) in series with the switch (S1), which bridge consists of two parallel bridge branches (5a, 5b) with respective centers (6a, 6b) wherein a first of these centers is connected to the control pin; - a detection means (7) for detecting the Wheatstone bridge being unbalanced; - a control means (8) controlled by the detection means for switching the indicator on / off. Circuit according to claim 1, characterized in that the indicator is an LED. Circuit according to claim 2, characterized in that the control means (8) controlled by the detection means is a switching element which is arranged in series with the LED between the first and second reference line (2a, 2b). Circuit according to claim 3, characterized in that the switching element (8) is a transistor (T1) with a control input, such as the gate for a MOS transistor or the base for a bipolar transistor, which control input is connected in such a way with the detection means (7), which conducts said transistor 1033451 11 tor (T1) in detecting the imbalance of the Wheatstone bridge. Circuit according to one of the preceding claims, characterized in that the detection means (7) comprise a detection transistor (T2), which detection transistor is connected in such a way that the Wheatstone bridge is out of balance causing its conduction. Circuit according to claim 5, characterized in that the detection transistor (T2) is a bipolar PNP transistor whose emitter is connected to the first center point (6b), the collector is connected to the second via a resistor (R8), lowest reference line (2b), and the base is connected to the control pin (9) via a resistor (R6). 7. Circuit according to claims 4 and 6, characterized in that the switching element (8) is a bipolar NPN transistor (T1) whose base is connected to the collector of the detection transistor, the emitter is connected to the second reference line (2b) and the collector is connected to the indicator (1) via a resistor (R9, R10). Circuit according to one of the preceding claims, characterized in that the detection means (7) comprises an opamp. Circuit according to one of the preceding claims, characterized in that the detection means (7) comprises an optocoupler. Circuit according to one of the preceding claims, characterized in that the control pin (9) is connected to a control circuit (10) for controlling the indicator (1) - 11. Circuit according to claim 10, characterized in that the control circuit comprises a controllable switching element (S2) between the control pin (9) and a reference line. A circuit according to claim 11, characterized in that the switching element is a transistor (T3) with a control input, which transistor (T3) is connected in series with a resistor (R5) between the control pin and a reference line. Circuit according to one of the preceding claims, characterized in that the control pin (9) is connected to a voltage detection circuit (11) for detecting the voltage on the control pin. 14. Circuit according to claim 13, characterized in that means are provided for determining with the aid of the detected voltage whether the switch is open / closed and whether the indicator is conducting. 15. System for controlling six indicators (1), typically formed by LEDs, and registering the position of six switches, comprising six circuits according to one of the preceding claims, wherein - the first reference lines (2a) are common to the six circuits and connectable to a first wire (20a); - the second reference lines (2b) are common to the six circuits and can be connected to a second wire (20b); - the control pin of each of the six circuits can be connected to one wire (91-96). 16. System according to claim 15, comprising one UTP cable whose eight wires (20b, 20a, 91-96) are connected to the common first reference line (2a), the common second reference line (2b) and the six control pins (respectively) 9). 17. Use of a circuit according to any one of the preceding claims 1-12 to simultaneously control an indicator, typically an LED, and to record the position of the switch (open / close) for one of the following applications : automotive applications, hospital applications, alarm systems. 1033451