Light source circuitry for controlling the color of emitted light

1. Light source circuitry comprising:

a first group of one or more light emitting diodes configured to emit light of a first color;

a second group of one or more light emitting diodes configured to emit light of a second color;

a third group of one or more light emitting diodes configured to emit light of a third color, wherein the first, second and third groups of light emitting diodes are configured such that, when a fixed current arrives at the light source circuitry, the fixed current is shared between the first, second and third groups;

a first transistor connected in series with the first group of light emitting diodes; and

a second transistor connected in series with the second group of light emitting diodes, wherein the first transistor and second transistor are configured such that a variation in a single received control signal causes a variation in a first proportion of the fixed current that flows through the first transistor and causes a variation in a second proportion of the fixed current that flows through the second transistor, such that the proportions of the fixed current that flow through each of the first and second groups is controllable in dependence on the single received control signal and such that the remainder of the fixed current flows through the third group;

wherein the light source circuitry is configured such that a first signal based on the control signal is provided to the control terminal of the first transistor and such that a second signal based on the control signal is provided to the control terminal of the second transistor;

wherein the light source circuitry is further configured such that, when the control signal is at a first level, the largest proportion of the fixed current flows through the third group of light emitting diodes and such that, as the control signal changes to a second level, the proportions of the fixed current that flow through the first and second groups of light emitting diodes increase.

2. The light source circuitry of claim 1 , wherein the light source circuitry is configured such that the first and second signals based on the control signal vary relative to one another in dependence on the control signal.

3. The light source circuitry of claim 1 , further comprising a third transistor connected in series with the third group of light emitting diodes.

4. The light source circuitry of claim 3 , wherein a third signal based on the control signal is received at the control terminal of the third transistor, wherein the third signal is always larger than the first and second signals.

5. The light source circuitry of claim 1 , wherein the combined forward voltage of the third group of light emitting diodes is larger than the combined forward voltage of the first group of light emitting diodes and the combined forward voltage of the second group of light emitting diodes.

6. The light source circuitry of claim 1 , wherein the light source circuitry is configured such that, as the control signal changes from the second level to a third level, the proportion of the fixed current that flows in the first group of light emitting diodes decreases, and the proportion of the fixed current that flows in the second group of light emitting diodes continues to increase.

7. The light source circuitry of claim 1 , wherein:

the second group is constituted by one or more light emitting diodes operable to emit red-colored light;

the third group is constituted by one or more light emitting diodes operable to emit white-colored light; and

the first group of light emitting diodes is constituted by one or more light emitting diodes the combined emission of which is lime-colored.

8. The light source circuitry of claim 1 , wherein the light source circuitry is configured such that, as the control signal gradually changes from a first level to a different level, the color of the combined emissions of the first, second and third groups of light emitting diodes changes generally in accordance with the Planckian locus.

9. A LED light source comprising light source circuitry according to claim 1 .

10. A method of manufacturing light source circuitry comprising:

providing a first group of one or more light emitting diodes configured to emit light of a first color;

providing a second group of one or more light emitting diodes configured to emit light of a second color;

providing a third group of one or more light emitting diodes configured to emit light of third color,

configuring the first, second and third groups of light emitting diodes such that, when a fixed current arrives at the light source circuitry, the fixed current is shared between the first, second and third groups;

providing a first transistor connected in series with the first group of light emitting diodes;

providing a second transistor connected in series with the second group of light emitting diodes; and

configuring the first transistor and the second transistor such that a variation in a single received control signal causes a variation in a first proportion of the fixed current that flows through the first transistor and causes a variation in a second proportion of the fixed current that flows through the second transistor, such that the proportions of the fixed current that flow through each of the first and second groups is controllable in dependence on the single received control signal and such that the remainder of the fixed current flows through the third group;

configuring the light source circuitry such that, when the control signal is at a first level, the largest proportion of the fixed current flows through the third group of light emitting diodes and such that, as the control signal changes to a second level, the proportions of the fixed current that flow through the first and second groups of light emitting diodes increase; and

providing a first signal based on the control signal to a control terminal of the first transistor and providing a second signal based on the control signal to a control terminal of the second transistor.

11. Light control circuitry comprising:

a controllable voltage supply configured to provide a single control signal;

a first transistor connected in series with a first group of light emitting diodes configured to emit light of a first color; and

a first control circuitry comprising at least one parallel circuit branch connected to the controllable voltage supply and the first transistor;

a second transistor connected in series with a second group of light emitting diodes configured to emit light of a second color, and in parallel with a third group of light emitting diodes configured to emit light of a third color;

a second control circuitry comprising at least one parallel circuit branch connected to the controllable voltage supply and to the second transistor;

wherein a fixed current is shared between the first, second and third groups, and

wherein the first and second control circuitry is configured such that a variation in the single control signal causes a variation in a first proportion of the fixed current that flows through the first transistor and causes a variation in a second proportion of the fixed current that flows through the second transistor,

wherein the first and second control circuitry is configured such that a first signal based on the control signal is provided to the control terminal of the first transistor and such that a second signal based on the control signal is provided to the control terminal of the second transistor.

12. The light control circuitry of claim 11 , wherein the first and second control circuitry is configured such that, when the control signal is at a first level, the largest proportion of the fixed current flows through the third group of light emitting diodes and such that, as the control signal changes to second level, the proportions of the fixed current that flow through the first and second groups of light emitting diodes increase.

13. The light control circuitry of claim 11 , wherein the controllable voltage supply is connected to the second transistor via the first control circuitry and the controllable voltage supply is connected to the first transistor via the second control circuitry.

14. The light control circuitry of claim 11 , wherein the first and second control circuitry is configured such that, as the control signal gradually changes from a first level to a different level, the color of the combined emissions of the first, second and third groups of light emitting diodes changes generally in accordance with the Planckian locus.