ESP8266-01 GPIO flicker during boot

Question

I have been experimenting with a ESP8266-01 module (8-pin).

I would like to use the ESP-01 module as an MQTT client and control two relays.

For this I bought an ESP-01 module and a 2-CH relay module (HW-383) and my schematic is as below (GPIO-2 has the same circuit as GPIO-0):

The EPS-01 and the optocoupler share +3.3 V which is isolated from the +5 V used for relay and transistor. I pulled GPIO-0 and GPIO-2 to 3.3 V using 10 kΩ as it is necessary for ESP to boot properly.

Everything is working perfectly except the flicker (relay turn ON and then OFF) during ESP-01 boot or reset (in my code I have set GPIOs to HIGH and then set them as O/P).

I read that this is due to the internal process of the ESP module. A few people suggest using GPIO-2 to avoid flicker and a few suggest adding a 470 μF capacitor across the optocoupler input terminals (only few people had success with a capacitor).

Is it a good idea to add a capacitor across the optocoupler? Can I add a capacitor across the GPIO pin and 3.3 V_GND? Both ways cause the capacitor to discharge into the GPIO pins at LOW signal on GPIO, is this safe for the ESP-01? If so, what is the optimum value for the capacitor?

Or do I need to add an RC network between the GPIO and the optocoupler to delay the switch ON of optocoupler (schematic would be helpful)?

I know there are lots of discussions on this topic but I could not find a reliable solution.

Update:
Here's a link to detailed GPIO behavior during boot.

The last post in the form says "GPIO0 will output the clock frequency (e.g 26 MHz) of the external crystal by default".

I think this clock might be causing the relay to turn ON and OFF during boot. How to avoid relay turn ON during this clock output?

Answer 1

Don't add a cap between GPIO pins and ground, don't add caps between GPIO pins and 3.3v. You can draw too much current doing that and burn a pin.

GPIO 0 is needed for setting boot conditions (must be pulled high as your 10k resistor does.

Your problem comes from the two LEDs in series. Your optocoupler likely has an IR-LED with a 1.0 to 1.3v drop and your indicator LED is at least 2v drop (assuming red color). This means that driving GPIO 0 LOW to turn on the optocoupler will be very low current and any voltage line fluctuations (like the load on your relay or even the ESP checking wifi) my cause brownout conditions.

First, Try removing your indicator LED. If that fixes your problem, you can move the indicator to a position parallel to the 10k pull-up resistor (add a current-limiting series resistor to the Indicator LED).