NodeMCU with a 5V latching relay - LOW triggers relay

Question

I am going to connect NodeMCU to a latching relay board like tis one (4 channel version): LINK

Both MCU board and relay board will be powered from the same 5V voltage regulator.

Now I have two problems to solve:

Problem 1 - logic levels

NodeMCU logic level is 3.3V and relays board works in 5V levels. I can either use logic level converters or simple transoptors. Because I am going to attach NodeMCU to a custom PCB, I think transoptors will be simpler to solder.

Problem 2 - relay triggers with low pulse on the control pin

This relay boards works like this: every time the channel pin (T1 - T4) is connected to ground for few miliseconds - the relay flips to other state. Here is the question: when the MCU board resets/reboots on power cycle, how do I ensure that the control pins are HIGH from the beginning? If they are not, the relay can flip randomly before my program starts and I will not know the initial state of relay.

What would be the best approch here ?

Answer 1

Since ports commonly are configured as inputs at reset, you will need to add a pull-up resistor.

Then write your setup so that the pin is high before switching to output mode.

EDIT:

Some microcontrollers have internal pull-ups, that are even activated at reset, see the specific data sheet. (Thanks, Juraj. I don't know NodeMCU enough to go in details. Anyway, my answer might apply to other controllers and other loads, too.)

The value of the pull-up can be determined by the maximum input current for the relay module (or your adapting circuitry) and the respective voltages. To be sure, make it a bit smaller, for example to the next E-series value. Just an example: minimum supply voltage is 3.15V (3V minus 5%), the maximum input voltage to detect "high" is 2.4V and maximum input current is 10 µA. Then the resistor will need to be at most Rmax = (Vdd_min - Vin_max) / Iin_max = (3.15V - 2.4V) / 10 µA = 75kOhm. This value is part of the E24 series, but is can be 68kOhm that is in the E6 series.

However, it seems as if the circuit of the modules contains a simple PNP BJT, a pull-up will not work here, because the input needs to be pulled up to the supply voltage of the module. You need a level converter, this could be a simple transistor. How to design that circuit, is another issue. There are lots of information in the web how to do this.