![]() Again as before, a series resistor, RS is required to limit the LED current.īrightness of the light emitting diode cannot be controlled by varying the current simply flowing through it. ![]() ![]() If more than one LED requires driving at the same time, such as in large LED arrays, or the load current is to high for the integrated circuit or we may just want to use discrete components instead of ICs, then an alternative way of driving the LEDs using either bipolar NPN or PNP transistors as switches is given below. Under are some examples of driving the light emitting diodes using inverting ICs but the idea is same for any type of integrated circuit output whether combinational or sequential. In either way the LED current should be limited to a safe value using a series resistor as we have seen already. The normal integrated circuits (ICs) have output drive current of up to 50mA in sink mode configuration, but have an internally restricted output current of around 30mA in the source mode configuration. ![]() The output stages of TTL and CMOS logic gates can both source and sink the useful quantity of current thus can be used to drive an LED. As now we know that what is LED, we need some method of controlling it by switching it "ON" and "OFF". ![]()
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