LED as a voltage dropper and troubleshooting indicator?

Hi- haven't seen this posted despite searches both here and on Google, so there's probably an alarmingly simple reason this isn't done, but could I use just a regular old LED (1.7vf) to drop my 5v MCU output to ESP8266-safe 3.3v by putting it in series with the signal (and a resistor)? Obviously the resistor would limit the current, but according to info I found, the max sink current for a digital pin is 20ma, which is perfect for an old-school T1. Add in the fact that I can confirm things are working well by seeing the LED status real time...seems perfect! So please tell me what I'm missing!?

Hi,
Could work… How much current does the 8266 draw? IT will limit the current.

If you want to power the ESP8266 up and down, you could parallel two Arduino outputs for more rated current. I do that to run Bluetooth modules like in THIS example

Yes, it should work quite well - just check that the particular diode doesn't have too small a Vforward
by using a 10k load or similar as pull-down.

However note that this approach has issues - if there are over-voltage spikes on the 5V rail, these will
sail straight through to the 3.3V line without attentuation or current limiting should the voltage
exceed 3.3V + a protection diode's forward drop.

A resistive divider is more forgiving as it will limit current.

However overvoltage spikes shouldn't be present on the 5V anyway - that's a bad place to start from!

Tune the pull-down resistor to provide the brightness you want for indication purposes, 1k to 10k range
is probably reasonable.

I suspect it would be possible to "get away with" using an LED as a series voltage regulator dropping
5V to 3.3V for low-powered loads too! [though that does feel a bit rough and ready]

MarkT:
Yes, it should work quite well - just check that the particular diode doesn't have too small a Vforward
by using a 10k load or similar as pull-down.

However note that this approach has issues - if there are over-voltage spikes on the 5V rail, these will
sail straight through to the 3.3V line without attentuation or current limiting should the voltage
exceed 3.3V + a protection diode's forward drop.

A resistive divider is more forgiving as it will limit current.

However overvoltage spikes shouldn't be present on the 5V anyway - that's a bad place to start from!

Tune the pull-down resistor to provide the brightness you want for indication purposes, 1k to 10k range
is probably reasonable.

I suspect it would be possible to "get away with" using an LED as a series voltage regulator dropping
5V to 3.3V for low-powered loads too! [though that does feel a bit rough and ready]

I think I've reached the limit of my meager knowledge...if I put a 10k in series with the LED, I'll only be passing uA through. Are you suggesting a pull down to ground between the current limiting resistor (was going for 330R to give me 10mA)?

Do you want to power the ESP8266 through a LED used as zener diode?
The ESP8266-12 draws ~80mA when idle, and 170mA peak during comms.
A 3.3volt regulator would be wiser.

It it’s signal level shifting (5>3.3), then a 1k:2k divider is safer.
According to the ESP8266-12 datasheet, switch points are 0,25VCC and 0.75VCC.
Nothing stops you from also adding an indicator LED with CL resistor to the same 5volt output pin.
Leo…

soopirV900:
Hi- haven’t seen this posted despite searches both here and on Google, so there’s probably an alarmingly simple reason this isn’t done, but could I use just a regular old LED (1.7vf) to drop my 5v MCU output to ESP8266-safe 3.3v by putting it in series with the signal (and a resistor)? Obviously the resistor would limit the current, but according to info I found, the max sink current for a digital pin is 20ma, which is perfect for an old-school T1. Add in the fact that I can confirm things are working well by seeing the LED status real time…seems perfect! So please tell me what I’m missing!?

I tried that exact thing to change 5v to 3.3v (approximately) to power a digital FM radio breakout board.

Although the LED maintained almost a constant brightness, the voltage getting to the FM board was not at all stable (it varied a lot with minor changes in current).

I even tried using a large (1000 uF) cap on the 3.3v side to “average out” the 3.3v supply.

Never could get it to work well (although it SEEMS like it should).

It’s worth it for you to try, but I wouldn’t get my hopes up. It simply never worked for me.

I’ve seen that before in published circuits. The thing is, the voltage drop of the LED is slightly dependent on current so using it as a power supply for something that has a variable draw won’t work, as Krupski pointed out.

If it’s just to provide a reference voltage or something that doesn’t vary, then the LED dropper is a neat idea.

MorganS:
I’ve seen that before in published circuits. The thing is, the voltage drop of the LED is slightly dependent on current so using it as a power supply for something that has a variable draw won’t work, as Krupski pointed out.

If it’s just to provide a reference voltage or something that doesn’t vary, then the LED dropper is a neat idea.

Thanks all for the valuable input- to clarify, I’m not looking at using this as a power-supply, only to convert a 5V output from an arduino Uno digital pin to 3.3v that can be input by an ESP8266 digital pin. I get that the ESP can probably handle 5v, but I don’t want to push it, and would appreciate the visual indication during debugging. I’m going to give it a try.

You need a pull-down resistor as well as no current flows into an input pin.

That seems do-able. Just set up the LED with a resistor that gives the appropriate current which gives the appropriate voltage drop. Then attach the 3.3V input pin to the junction of the resistor and LED. The load doesn't change so the voltage drop will be consistent.