Even if you are designing a microcontroller-based LED display system, often it may calls for a manual option to control brightness of the LED display unit. On the otherhand, if it is based on discrete components, such an option is a crucial requirement. Here is an economical solution for such situations. The visible brightness of an LED display can be continuously varied by applying a pulsed-signal and varying its duty cycle. Usually, almost all 7-segment decoder ICs have a blanking input (RBI) connection to imply this type of brightness control. Because, the frequency in picture is above 50 Hz, unwanted flickering effect will not be noticeable.
In the practical world, CD4033 is a very popular CMOS IC used with 7-segment LED displays. CD4033 consists of a 5-stage Johnson decade counter and an output decoder which converts the Johnson code to a 7-segment decoded output for driving one stage in a numerical display. This IC is particularly advantageous in display applications where low power dissipation and/or low package count is important. Another one is the TTL IC 74LS47 which is a BCD decoder/driver for seven segment common anode (CA) displays. Note that, the IC 74LS48 also makes the same work, but designed for seven segment common cathode (CC) displays. CD4511 is the CMOS version BCD decoder/driver for seven segment common cathode displays, with latch ability.
The solution described here is nothing but a simple add-on, built around the 555 chip (IC1) wired in ‘freerunning’ mode. Modified configuration using the 1N4148 diode (D1) provides a control over duty cycle without changing the output pulse frequency. As a consequence, the output frequency of 555 remains unaffected while its duty cycle is varied over a wide range by the 100K brightness control potetntiometer (P1). Output of the 555 IC (pin 3) can be applied to the decoder’s blanking input through the 2N2222 driver transistor (T1).
According to datasheets, in CD4511, when the input of the light test (LT) is activated in LOW, regardless the values of the rest of the inputs, all the segments of the display are lit up. When the blackening input (BI) gets activated in LOW with the light test input HIGH and regardless of the values of the rest of the inputs all the segments of the display are turned off (blackened). Similarly, in 74LS4X, when the light test input (LT) is activated in LOW, all the segments (a-g) of the display are lit up. The terminal BI/RBO functions as an input or an output. It becomes an output when the input RBI is activated in LOW. When the blackening input BI is activated (LOW), the display is blackened, meaning that the outputs become OFF. By activating the RBI input (LOW), the terminal BI/RBO changes into a blackening output RBO and is turned into LOW. Note that, ‘blanking’ means none of the segments of the LED display is turned on.
Note: This is an unorthodox attempt to draw your attention to the working of 7-segment LED display driver ICs, especially their ripple blanking input/output (RBI/RBO) features. You can learn more about these less-known features by referring related articles/ tutorials/datasheets published elsewhere in this website/internet. As a bonus, you can build your own LED display brightness controller circuit using the ubiquitous 555 chip!