30 reads the value whenever the counter 34 generates an interrupt again many other configurations are possible green light sources 12 14 are driven along green light source power driver lines 35 by drivers 36 although four green light source power driver lines 35 are shown for clarity in the alternative there need be only two such lines and they and the sources 12 14 are electrically connected such that only one source emits green light if one of the two driver lines is grounded and the other is at e g 5 vdc current limited and vice versa for example if the sources 12 14 are diodes 13 15 then the cathode of diode 13 is connected to the anode of diode 15 the anode of diode 13 is connected to the cathode of diode 15 and the two nodes are connected via green light source power driver lines 35 to current limited drivers 36 the drivers 36 drive the sources 12 14 at the required voltage and current in an alternating manner as known to those skilled in the art if sources 12 14 include leds 13 15 then several suitable driver configurations known to those skilled in the art are available to drive the leds 13 15 at the required voltage and current for example a 74 74h or 74s family buffer or inverter such as a 7400 can be used to directly drive leds with suitable current limiting resistors all not shown as another example it is common to drive leds from cmos nmos 74ls or 74hc family devices with an npn or pnp transistor such as a 2n2222 with suitable current limiting resistors.