Numerals are produced by arranging LEDs in a seven-segment arrangement as indicated below. A LED may be thought of as a 1.5 volt globe for design purposes. If voltages of more than this are involved, a 'dropping' resistor is needed, as indicated below. The maximum current flowing through a LED should be limited to around 20 milli-amps. The table below shows how to calculate the resistance required to use LEDs with different voltages.
E is the supply voltage.
I is the maximum current in amps. As with all diodes, orientation of LEDs is critical. If you connect the legs the wrong way around it will not conduct. The following diagram should provide a useful guide. If a diode is built into the power section the rest of the circuit will be protected in the event of somebody connecting the power supply the wrong way around.