RS-232 Tx / Rx monitoring LEDs?

[Brent Hilpert]

On 2015-Aug-23, at 9:06 AM, Chuck Guzis wrote:
> On 08/23/2015 07:10 AM, dwight wrote:
>> I've used the capacitor method to provide most of the drop in the
>> past. I don't usually max out the LEDs at 20ma. I find there is
>> little difference between 10 and 20ma. Yes, the 10 ( or 20ma ) is
>> current flow through the capacitor. It is necessary to have some
>> resistor in series as well to suppress line spikes.
> 
> Another similar dirty trick back in the day was to run a 6SL7 dual-triode form the line using a 1.0 uF nonpolar capacitor in series with the line to provide a supply for the 500 ma 6.3 v heater and then use one of the triode sections as a half-wave rectifier.  You thus had the other triode section for whatever stupid purpose.  Of course, this was horrible abuse of the tube, particularly in the area of heater-cathode voltage ratings.  It probably wouldn't work as well in 220VAC countries, but it worked well enough in the 120VAC ones.

Those ubiquitous motion detectors for outdoor lights use capacitive dropping to supply the low voltage for the ICs and electronics. Cap and small R in series with the AC input to a bridge rectifier.
Works out well in that the bridge rectifier permits current flow in both directions, which is necessary to get current flow through the cap (the cap has to charge and discharge).