Pull-up on NPN transistor (during microcontroller power up)

Question

I am using an ESP8266-01 to control a 3.3 V relay via a 2N7000 NPN transistor. The ESP8266 GPIOs use a 3.3 V logic level.

I have it connected to TXD (GPIO 3) and I am trying to stop the relay from activating a couple times on boot as the pins are floating for a moment. Then they settle and the relay works fine after boot.

I tried adding a pull-up to 3.3 V thinking that would solve the issue, but it does not. There is mention of this exact problem here: Pull up on NPN transistor (during microcontroller power up) as well as here: ESP01 enable GPIO2 before boot

In the second link the person talks about using an inverter, however, I don't know what the part # may be, and I do not have enough reputation to comment on it, so I was forced to make a new question.

My idea is that the relay should be off until the ESP has booted fully, and then stays off unless I turn it on.

Here is my schematic:

Details of boot time data for each GPIO on the ESP8266:

Answer 1

You could add one part- an ADM809 to the circuit you have.

^{simulate this circuit – Schematic created using CircuitLab}

The open-drain output of the ADM809 will inhibit the relay operation for a minimum of 140ms after power is applied (typically 240ms). Pick one with a suitable threshold for 3.3V monitoring such as ADM809R (the higher voltage ones intended for monitoring 5V power will never release the /RESET).

Answer 2

Adding an answer based on the comments. The schematic below is essentially what you need. You need to select R2 to be a value such that when the MCU output is high, the gate voltage, \$V_g\$, is enough to fully turn on or saturate the transistor M1.

R1 and R2 form what is called a voltage divider. Assuming MCU Output is 0 or 3.3V, then when it is high (3.3V);

\$V_g \approx 3.3V * \frac{R2}{(R1+R2)} \$

That is, VG will be a fraction of 3.3V, based on the ratio of R2 vs the sum of R1 and R2.

In order to saturate M1, such that it turns on fully, you need the gate voltage to be above the threshold voltage, often denoted as VGS_Th in datasheets.

^{simulate this circuit – Schematic created using CircuitLab}

The reason we add R2 is so that when MCU output is floating or high-impedance, \$V_g\$ is well defined (0V).

Answer 3

I think the problem is that during boot that pin goes high so until the ESP is running and you can turn off the pin, a pull down resistor will not stop the relay from coming on during booting even though it's a good idea to have it.

You could add a capacitor to form an RC circuit so that the 2n7000 takes a while to trigger, stopping the relay from triggering during booting but this would of course affect the response time of the relay when switched on in code.