GB1569264A

GB1569264A - Circuit for the visual monitoring of voltage

Info

Publication number: GB1569264A
Application number: GB4287876A
Authority: GB
Original assignee: Harvey P D
Current assignee: Harvey P D
Priority date: 1976-10-15
Filing date: 1976-10-15
Publication date: 1980-06-11

Description

(54) CIRCUIT FOR THE VISUAL MONITORING OF VOLTAGE (71) I, PHILIP DAVID HARVEY. a British subject of 1, Formby Avenue, Thatto Heath, St. Helens, Merseyside, formerly of 193A Rathbone Road, Liverpool.

Merseyside, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed to be particularly described in and by the following statement: The invention relates to visual monitoring circuits and is applicable to the monitoring or comparing of variable functions such as voltages.

According to the present invention there is provided a circuit for the visual monitoring of voltage comprising means for deriving at first and second circuit nodes respectively different proportions of a voltage to be monitored, first visual indication means connected between a reference voltage source and the first node for indicating visually when the monitored voltage exceeds a first predetermined level and second visual indication means connected between the second node and the reference voltage source for indicating when the monitored voltage is below a second predetermined level.

Each said visual indication means mav comprise means for providing unidirectional current flow and indicating such flow. e.g.

by way of a light source, and may comprise a single device. In a preferred embodiment each visual indication means comprises a light emitting diode.

The reference voltage source conveniently comprises a threshold device such as a reference diode connected between the said junction and a reference point. for example earth.

In preferred embodiments each means for deriving comprises a potentiometer connected between the reference point and a terminal to receive the voltage to be monitored, the mid-point of the potentiometer being connected to the said circuit node.

The voltage to be monitored may then be applied to a resistance connected to the junction of the said visual indication means with the reference voltage source.

Embodiments of the invention serve to indicate the level of the voltage to be monitored with respect to two predetermined voltages which define the extremes of a range. When the monitored voltage is higher than one extreme, one of the light sources is illuminated alone. When the monitored voltage is lower than the other extreme the other light source is illuminated alone. When the monitored voltage has a value within the range. both light sources are illuminated simultaneously and their respective intensities are determined by the differences between the monitored voltage and the mean of the high and low predetermined voltages.

Comparison of such intensity of illumination is easier if the light sources are of different colours. preferably displayed close together.

A specific embodiment will now be described by way of example and with reference to the accompanying drawings in which: Figure I shows a circuit for visual monitoring of voltage.

Figure 2 shows a graph of brightness against voltage for indicating means of that circuit.

In Figure 1. first visual indication means, for providing unidirectional current flow and indicating such flow. comprises a lightemitting diode 1() which emits yellow light when conducting. Second such visual indicating means comprises a second light emitting diode 11 which emits red light when conducting. The two light emitting diodes are connected in series aiding between circuit nodes 12 and 13. the junction 14 between the L.E.D.s being connected to the cathode 15 of a reference diode 16. The anode 17 of the reference diode is connected to a reference point. line 18 which is shown earthed at terminal 19. Junction 14 is connected by a resistor 20 to a terminal 21 to which will be applied the voltage to be monitored.

Circuit nodes 12 and 13 are connected bv resistors 22 and 23 respectively to reference line 18 and by resistors 24 and 25 respectively to terminal 21, these resistors 23. 24 and 25 all being of similar low value. whilst resistor 22 has a much higher value.

In a particular application to monitoring car battery voltage, resistors 23. 24 and 25 were 120 ohms, resistor 2() was 100 ohms.

resistor 22 was 3,300 ohms and the reference diode voltage 9.1 volts.

These values provide for illumination of diode 10 when the monitored voltage is above VL of 11.4 volts. alone when the voltage at terminal 21 is greater than a vH of 14.4 volts. and illumination of diode 11 when the monitored voltage is below sH, alone when the voltage at terminal 21 is less than vL. Thus illumination of both diodes.

with varying intensity, occurs for voltages between VL and VH.

When the voltage to be monitored has a value above VH. up to a supply limit 'S in Figure 2. reference diode 16 maintains junction 14 at a fixed reference voltage of 9.1 volts. The voltage at node 13. determined bv the divider formed bv resistors 25 and 23. of approximately equal value. is so high that the potential difference between junction 14 and node 13 is insufficient to illuminate light-emitting diode 11. On the other hand, because of the high value of resistor 22. the voltage at node 12 is virtually equal to that at terminal 21 and so the potential difference between node 12 and junction 14 is sufficient to illuminate lightemitting diode 10.

Thus. above a voltage of 14.4 volts L.E.D. 10 alone is illuminated. When the voltage to be monitored falls below 14.4 volts. the voltage at node 13. determined bv the resistors 23 and 25. falls so low that there is sufficient potential difference between the fixed reference voltage at junction 14 and node 13 to cause light-emitting diode 11 to light. Both diodes are then illuminated simultaneously.

As the voltage monitored falls still further the voltage at junction 14 remains constant and the potential difference across diode 10 decreases. the brightness of this diode decreasing accordingly. At the same time the potential difference across diode 11 increases as the voltage at node 13 falls. so the brightness of this diode increases accordinglv.

When the monitored voltage falls to 11.4 volts. the potential difference across diode 10 becomes so low that it is insufficient to maintain illumination of this diode which is consequently extinguished. Below 11.4 volts diode 11 alone is illuminated, and remains so down to a useful lower voltage limit VA.

It will be seen from Figure 2 that. within the operating limits of the circuit components. there is defined a range of voltage.

the limits of which are Vl and VH.

For voltages below V1. only one light emitting diode is illuminated. and for voltages above VH. the other light emitting diode is illuminated. For voltages between Vl and VH. both diodes are lit. their relative intensities being in proporion to the variation of the monitored voltage from the mean of the preset values VZ and V. The levels of Vl and VH can be adjusted by suitable choice of the reference diode. the light emitting diodes and values for the resistors.

In particular. resistors 24 and 25 adjust the sensitivitv of the circuit. resistor 22 determines Vl and resistor 23 determines VH. For particular light emitting diodes the accuracy of adjustment will be dependent primarily upon the resistance. Although the voltages across the reference diode and the light-emitting diodes varv during operation.

typically from 9.0 to 9.5 volts for the reference diode and 1.8 to 2.3 volts for the light emitting diodes. for a particular combination of light emitting diodes and reference diodes. the voltage at which illumination and extinction occur will be substantially constant. The accurate adjustment of V, and VH can then be made by means of the resistors 22 and 23 respectively. Very good accuracy can be achieved by fabricating the circuit using film circuit techniques and trimming the resistances whilst the circuit is active.

An advantage of such fabrication is that the entire circuit can be made small and light. facilitating addition to existing systems.

Alternatively. desired switching voltages Vl and VH can be obtained. with a desired accuracv. bv careful selection of the diodes and resistors.

In the particular application for monitoring the state of a car battery. illumination of the red light-emitting diode 11 alone indicates a poor state of charge. battery voltage being below about 11.4 volts. illumination of the yellow light-emitting diode alone indicates that there is a fault such as the battery being overcharged. or high impedence or other fault resulting in the system voltage exceeding about 14.4 volts.

Illumination of both lights indicates that the battery condition is fair. and the relative intensities of the lights may be compared to determine the state of charge and. over a period of time. whether the battery is responding to charging. The circuit may be used on positive earth systems by merely connecting the terminals 19 and 21 to positive and earth respectively.

It will be appreciated that the circuit described hereinbefore has application wider than merely battery condition monitoring and could be used, for example for monitoring temperature, pressure, logic condition, circuit timing, humidity and in general to monitor any voltage, derived by transducer or otherwise, for variation with respect to predetermined lower and high values.

It will be apparent that the circuit will operate without one terminal grounded as described in the particular embodiment and will function differentially to compare different voltages applied between its terminals, provided that the potential difference between the terminals is such that the correct polarity of voltage is applied to the diodes.

WHAT I CLAIM IS: 1. A circuit for the visual monitoring of voltage comprising means for deriving at first and second circuit nodes respectively different proportions of a voltage to be monitored, first visual indication means connected between a reference voltage source and the first node for indicating visually when the monitored voltage exceeds a first predetermined level and second visual indication means connected between the second node and the reference voltage source for indicating when the monitored voltage is below a second predetermined level.

2. A circuit as claimed in claim 1, wherein one or more of said visual indication means comprises a single device for providing unidirectional current flow and visually indicating such flow.

3. A circuit as claimed in claim 2, wherein said device comprises a light emitting diode.

4. A circuit as claimed in any preceding claim, wherein the means for deriving includes an impedance connected between each node and a reference point.

5. A circuit as claimed in claim 4.

wherein the means for deriving includes a further resistor connected between each node and a terminal to which is applicable the voltage to be monitored to form a potentiometer.

6. A circuit as claimed in any preceding claim, wherein the reference voltage source includes a threshold device connected between the junction of the first and second visual indication means and a reference point.

7. A circuit as claimed in claim 6, wherein the threshold device is a reference diode.

8. A circuit as claimed in claim 6 or 7.

including an impedance connected between the junction and a terminal to which is applicable the voltage to be monitored.

9. A circuit as claimed in any preceding claim, wherein each different visual indication means indicates by emitting a different colour of light.

10. A circuit for monitoring voltage arranged and adapted to operate substantially as described herein with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. used on positive earth systems by merely connecting the terminals 19 and 21 to positive and earth respectively. It will be appreciated that the circuit described hereinbefore has application wider than merely battery condition monitoring and could be used, for example for monitoring temperature, pressure, logic condition, circuit timing, humidity and in general to monitor any voltage, derived by transducer or otherwise, for variation with respect to predetermined lower and high values. It will be apparent that the circuit will operate without one terminal grounded as described in the particular embodiment and will function differentially to compare different voltages applied between its terminals, provided that the potential difference between the terminals is such that the correct polarity of voltage is applied to the diodes. WHAT I CLAIM IS:

1. A circuit for the visual monitoring of voltage comprising means for deriving at first and second circuit nodes respectively different proportions of a voltage to be monitored, first visual indication means connected between a reference voltage source and the first node for indicating visually when the monitored voltage exceeds a first predetermined level and second visual indication means connected between the second node and the reference voltage source for indicating when the monitored voltage is below a second predetermined level.

5. A circuit as claimed in claim 4.

8. A circuit as claimed in claim 6 or 7.