I have the circuit below. GPIO15 goes to a ESP32 and lights up an LED. If 12v is completely connected, LED is on. When 12v is completely disconnected LED is off. This is the correct behavior I expect.

When only 12v- (GND) is connected (no 12v+ connection) LED is off. BUT, when only 12v+ is connected (no 12v ground) LED is flashing on and off.

Why is it flashing?

Am I missing something on how transistors work when positive only source is connected?

12V+	12V-	LED
CONNECTED	CONNECTED	ON
DISCONNECTED	DISCONNECTED	OFF
DISCONNECTED	CONNECTED	OFF
CONNECTED	DISCONNECTED	Flashing (Why?)

 

What are you trying to do? I can make no sense of your circuit.

is the GPIO an input or output? What is its purpose?

What kind of LED? Vf an If?

10K is way too large a current limiting resistor.

Bob

 

Am I missing something?

Yes. You are missing showing us where GPIO15 goes, the ESP32 and the LED.

 

What are you trying to do? I can make no sense of your circuit.

is the GPIO an input or output? What is its purpose?

What kind of LED? Vf an If?

10K is way too large a current limiting resistor.

Bob

I’m trying to capture when a voltage source is provided. 12v is a pump that activates every few minutes so I want to count how many times it actually activated. The LED is just for testing purposes. GPIO 15 is set as input and the microcontroller reads and write to the LED GPIO. LED is not shown in this picture.

This is for a comercial product so I suspect that either the positive side or negative side can remain connected even when the pump is off. In this case I want to read off.

 

Yes. You are missing showing us where GPIO15 goes, the ESP32 and the LED.

Apologies I oversimplified the circuit and omitted the LED. LED is connected to another GPIO set as output and I read GPIO15 and write it on or off for testing purposes. In the real world this will be sent wireless via MPTT. GPIO15 goes to ESP32 microcontroller.

 

The base resistor is probably getting enough residual voltage through the pump circuit or the base maybe free floating when the pump is off. Connect a 100K resistor from the 12 volt input to ground , the right side of R4
What is the voltage on the GPIO 15 pin when the pump is on?

 

What is the voltage on the GPIO 15 pin when the pump is on?

Reads 3.3v

 

I assume the two supplies are isolated.

I would suspect some ground connection...maybe earth?

 

OMG! I did not even notice there was no LED in the circuit! My brain put it in between the 10K resistor and ground.

So, without any indication of what the program on the ESP is doing, you want us to tell you why it is blinking the LED? Sorry, but my crystal ball is down tonight.

Bob

 

Reads 3.3v

OK that's what I expected. Have you tried the 100k resistor ?

 

Might I add that the title is totally misleading. The transistor switch is not blinking the LED.

Bob

 

I assume the two supplies are isolated.

I would suspect some ground connection...maybe earth?

Correct. The two supplies are isolated and share the same ground in my test.

 

OMG! I did not even notice there was no LED in the circuit! My brain put it in between the 10K resistor and ground.

So, without any indication of what the program on the ESP is doing, you want us to tell you why it is blinking the LED? Sorry, but my crystal ball is down tonight.

Bob

Here is the source code...

const int buttonPin = 15;   
const int led = 13;
int buttonState = 0;

void setup() {
  Serial.begin(115200);
  while(!Serial) continue;
 
  pinMode(buttonPin, INPUT);
  pinMode(led, OUTPUT);
}

void loop() {

  buttonState = digitalRead(buttonPin);

  if (buttonState)
    digitalWrite(led, HIGH);
  else
    digitalWrite(led, LOW);

  Serial.println(buttonState);
}

 

If they isolated, they do not share the same ground. Which is it?

Bob

 

What you have is too complex.
Remove the ESP32.
Remove the connection to GPIO15.

Now focus on the behaviour of the circuit you posted using a DVM.

 

OK that's what I expected. Have you tried the 100k resistor ?

Adding a 100k resistor SOLVED the blinking issue! Thanks @sghioto

Now when only one of either 12v leads are connected, the LED is completely off. When both 12v are connected the LED is on.

 

Adding a 100k resistor SOLVED the blinking issue! Thanks @sghioto

Meh. ‘Too many notes, dear Mozart, too many notes’ as Emperor Joe said to Mozart.

All you need do is constrain the 3V signal to an acceptable range for the ESP32, being 0 to 3.3V. Also, when the 12V is active you want to present a signal large enough to represent a HIGH to the input.

I'm not sure what the range for a high is, but >2.0V should suffice. I would recommend going to 3V for a high. To achieve this all you need is s simple voltage divider. I'd toss in a diode (1N4148 but not critical) too for safety in case the 12V is larger than expected.





Note I had to move *my* ESP32 board aside to type this

 

I still don't know what the TS is trying to do.

 

I still don't know what the TS is trying to do.

He is using an EPS32 micro controller to sense a 12V signal.

 

He is using an EPS32 micro controller to sense a 12V signal.

Your voltage divider and diode clamp will do it.