This is related to the earlier post. This was done to save battery power. The output of some equipment may be in the form of LED Annunciators In the Security Guard Monitor, an Array of 24 LEDs would show the Status of alertness along with the day, this was scrollable.
When the “Display Now” is pressed it triggers the Monostable Multivibrator made of a 555. The output “EN” goes high for the time duration defined by C27 and R71.
The 555 output as you know can drive more than 200mA for quite some time without much heating up. Many LEDs can be driven with the current limit resistors. I used CMOS chips to drive the LEDs, this circuit was used for the Logic only.
Hobby Projects – LED Circuits and Small Test Tools.
This circuit was a part of the user interface like the previous one. You could use it for voltmeters or even backlight for LCD.
Example – When you press the switch the Time of a clock can be seen in darkness for a few seconds.