Hardware Reference
In-Depth Information
see that at low voltages there is little or no current through them. As the voltage rises there
is little change in current until a sort of threshold is reached, and then a small increase in
voltage gives a large increase in current. his threshold, or
breakpoint,
is known as the
forward
voltage drop,
and you need to run an LED at this voltage. his is not done by supplying a volt-
age of “just the right value” for various reasons - the two main ones being that producing
this right voltage takes some complex electronics and the value that is “right” changes with
the temperature and the age of the device.
Figure 8-1:
he voltage
against current
relationship for
an LED.
What you have to do is to arrange for a constant current through the device. For low-current
LEDs, which is what you will be using here, it is simplest to use a resistor to simply limit the
current. Most LEDs work with currents of up to a maximum of 20mA, so that is normally the
target to aim for, although the more current you have, the brighter the LED will shine. Figure
8-2 shows a resistor and LED connected to a 5V power source.
