Hey everyone, I have a latch circuit designed for powering an ESP32. Here's how the circuit operates: when the SW6 button is pressed, it activates transistor Q6 NPN, which, in turn, opens the U1 MOSFET to power the MCU. Once the MCU boots up, it sets pin D8 to HIGH, which activates transistor Q4, keeping the U1 MOSFET open. D11 is also used to read the button state, enabling power cut-off after a long press.

Here's the circuit:



This circuit works well with a 7.2V power source. However, I need to switch to a 4.8V and ensure it operates effectively at voltages down to 3.3V. Currently, this circuit does not function at lower voltages.

When the button is pressed, the MOSFET activates, and the MCU begins to boot up. However, there are a couple of problems:

• The MCU does not fully boot, as the display remains non-functional; only the LED on the development board lights up.
• Although the MOSFET opens, it fails to maintain its position. As mentioned earlier, everything works fine when the system is powered by 7.2V. Here is the video: youtu.be/CUsgkXfuH38
• ESP32 doesn't set D8 to HIGH (the voltage is below 1V)

I believe some components need to be modified. Unfortunately, my limited knowledge of electronics doesn't allow me to determine the necessary changes. Could anyone suggest any directions I should explore?
Thank you in advance!

Update: The issue was caused by a significant voltage drop in the voltage regulator on the ESP32 Devkit. The solution is adding an LDO regulator, I used MCP1702T-3302E/MB.

 

Lower R29 and R31 to something like 10k. Not that it is a big deal, but R30 more of a standard value like 47k if a 51k is hard to find

 

Lower R29 and R31 to something like 10k. Not that it is a big deal, but R30 more of a standard value like 47k if a 51k is hard to find

Just tried that, but unfortunately, it didn’t help. I changed R29 and R31 to 10K, and R30 to 43K.

 

Q4 can be eliminated using this modified version.
D8 switches HIGH to latch U1 ON.

 

Q4 can be eliminated using this modified version.
D8 switches HIGH to latch U1 ON.
View attachment 336807

Thanks for the answer, I'll take this modification into consideration. However, it doesn't address the main issue—the MCU still isn't booting. I tried to change all resistors to 10k and it didn't help.

 

Are you sure the transistor and mosfet are connected correctly

 

Are you sure the transistor and mosfet are connected correctly

Yes, as the circuit works well with 7.2V power supply.

 

Do you a volt meter to get some measurements?

 

Do you a volt meter to get some measurements?

Yes, I do. By the way, the voltage on MOSFET drain when the button is pressed is 4.4V (with 4.8V power supply).

 

4.4v seems low for that mosfet.
What is the voltage on the collector of Q6 when the button is pressed and held
I believe the ESP module requires a minimum of 5 volts to operate correctly.

 

4.4v seems low for that mosfet.
What is the voltage on the collector of Q6 when the button is pressed and held
I believe the ESP module requires a minimum of 5 volts to operate correctly.

The ESP32 can be powered by 3.3V. However, after your comment, I realized that in my setup, the power goes through an AMS1117 3.3V voltage regulator. With a dropout voltage of the AMS1117, 4.4V input is likely insufficient. I’ll need a low-dropout voltage regulator instead. I've already ordered a few to test.

 

Still feel that 4.4 volt output is low did you get voltage reading mentioned in post #10?

 

Still feel that 4.4 volt output is low did you get voltage reading mentioned in post #10?

I just checked the drain voltage again, and it's 4.57V. There was likely a bad connection. When the button is pressed, the voltage at the Q6 collector is 0.06V.

 

Almost 4.6 volts and Q6 is good.
Read the voltage on the output of the ams1117 regulator.

 

Almost 4.6 volts and Q6 is good.
Read the voltage on the output of the ams1117 regulator.

3.3V, which is good as well...

 

So what is the problem again?

 

So what is the problem again?

MCU isn't booting and not setting D8 to HIGH, here's the demo: youtu.be/CUsgkXfuH38

 

Is it a program issue?

 

Is it a program issue?

Did you watch the video? It works fine with 7.2V power supply.

 

I did watch. When the button is held down what is the voltage reading at D8?