Two-Eyed LED Lamp

We have featured numerous torch circuits using white LEDs. This simple design adds a couple of other types of LED to extend light emissions either side of the visible spectrum.LEDs are usually driven from a constant current source so that the LED brightness is not dependent on a varying supply voltage. The circuit here in Figure 1 shows a constant-current source produced by transistor T1. To explore its workings in a bit more detail assume that at switch on the voltage at the base of T1 (VB) is greater than the base conduction voltage (VBE = 0.65 V) of T1. Current flows into the base and T1 starts to conduct. This will make current flow through its collector-emitter junction and resistor R2, generating a voltage across R2.