LED plus LDO regulator => see what happens

One of my favorite circuit additions is putting a Light Emitting Diode (LED) in series with the positive input of an LDO (Low DropOut) voltage regulator:

An LDO regulator will keep the output voltage constant even though the input voltage may only be a few tenths of a volt higher.  In addition, these regulators often have very low idle (aka quiescent) currents sometimes as little as 1 microamp.  Many ham radio projects that use a microcontroller have the capability to sleep, essentially stopping the clock or "heartbeat" of the chip until awakened by something like a key press for a keyer.  While sleeping the current is so low it's hard to measure.  Behold the amazing lack of power of unclocked CMOS.

Adding the LED to the circuit allows the builder to take advantage of the LDO regulator's characteristics to provide a visual indication of current.  As shown in this video a keyer is "sleeping" at first, there is a barely visible crackle of light at the roughly 1.5 uA current.  Then a memory button is pressed to play:  

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and the LED comes up in brightness, slightly increasing during each mark, finally reverting to the sleep crackle:



Here is a long video I did for the PK-4 keyer which also shows the LED + LDO idea for both the keyer test fixture and the paddle (skip to 56 seconds to see the LED): 



Now if a regular regulator is used the LED would be on all the time and only increase in brightness slightly as the keyer comes out of sleep mode.  This is due to the current consumed by the voltage regulator chip itself.   For example in this video the idle current of the voltage regulator chip is 2.4 mA and the active current of the whole keyer is 3.1 mA, the LED only goes out when the power is switched off:



So why not always use an LDO regulator ?  They are more expensive and somewhat finicky compared to regular voltage regulators.  I'll go into more detail on a couple of my favorite LDO regulators in a future blog post but normally I include a somewhat expensive tantalum capacitor on the output to help with the stability of the regulator, this may not be needed with regular linear voltage regulators.  LDOs may be electrically noisy, too, that may account for the dim "crackling" seen on the LED during sleep ?

Why not use an LED in all regulated circuits then ?  Because LEDs do have considerable dropout voltages of their own, red LEDs something like 2.1 volts, white/blue LEDs over 3 volts.  Also, LEDs can be surprisingly tender, destruction can result with polarity reversal.  And their normal maximum current is something like 20 mA.

But for something like a keyer with an LDO, an LED can be the ideal way to "see" what is happening.

Best Regards,

Chuck, WB9KZY

http://wb9kzy.com/ham.htm