LED indicators using Dark Cockpit philosophy

First post : 23 August 2018

Flashing LEDs: More or less?

Unfortunately the answer to this question, as with many, depends on who it is for. Paying customers are usually happier with more flashing lights. Technicians that are expected to use or maintain the equipment, are usually better off with less green LEDs, and red LEDs that are on or flashing only when there is a fault.

Dark Cockpit

LED functionality is to be guided by the Dark Cockpit Concept. Accordingly, we attract attention to faults (red LEDs), not to where everything is in order. So, for example, if a communications link is up, we don’t indicate this with a green LED, rather we use a red LED to indicate when a link is down. Red LEDs for where there is a problem, Green LEDs where everything is good.

All in order

With all systems functioning as expected, the ideal system only has one flashing green cpu LED. This simplifies identifying a functioning system from non functioning system. An additional green power can also be on all the time.


A green flashing LED is used as a CPU running indicator. Flashing cycle time should be at least once a second and can be of a short duty cycle (this reduces energy consumption and heat). At start up, the CPU running LED is continuously ON for 1 to 2 seconds. This allows some indication of a a rebooting CPU. The booting LED indication prevents failures such as a CPU watchdog reboot cycle to appear as normal operational flashes. The main firmware should not wait for the LED boot indication sequence to complete, but should operate consecutively.

A quick glance by a field technician should show up any red LEDs. Seeing a green flashing LED and no Red LEDs should mean than everything is functioning as designed. This is much quicker, and more accurate than checking that all the green LEDs for all the functions are on.

Power LED

Where used, a green power LED does not indicate anything about the board’s functionality, it says something about the power supply and associated wiring feeding to the board. The LED indicates the board has power, not that the on board fuse is blown or that the board’s regulator or CPU is working. The power LED only indicates that the supply is on and connected. The information is mostly what a field technician can get by putting a voltmeter across the incoming supply terminals.

Blue is the new Green

A significant percentage of the male population (11% of caucasian males) is colour blind. Consideration should be given to using blue LEDs wherever green LEDs are specified above.

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