Low-Voltage Unidirectional Ground-Level Shifter

Question

I am trying to interface the status LEDs of a third-party device to the GPIO of my ESP32. I am using internal pull-ups on the ESP32 inputs and trying to trigger on the voltage drop that occurs when the LEDs switch on.

For blue and green LEDs, this voltage drop is from +3.3 V to +0.8 - 1.0 V, and the ESP32 triggers fine. The voltage drop of the red LED is much smaller, from +3.3 V to +2.16 V, which is insufficient for my ESP32.

Within the device, both anode and cathode of the LEDs disappear into active circuitry that I don't control, and I mustn't load this device too much.

I know I might be able to solve this problem by programming my ESP32 differently, but my preferred solution would be to pull +2.16 V closer to 0 V, to get a nice logic level. I don't need bidirectionality, and inverting the logic would be OK if this is a major simplifier. There is no particular bandwidth requirement, although the LEDs happen to be PWM-driven at 4kHz.

Answer 1

Given the vagueness of the rest of the circuit, I would strongly recommend you use a high sensitivity, logic-level opto-coupler such as an H11L1.

(Logic symbol taken from the onsemi datasheet for the H11L1)

On the input side, you are effectively putting another LED and a resistor in parallel with the existing LED and resistor. If you select a high sensitivity coupler, you only need to draw ~1mA, which is probably a fraction of what the existing LED is drawing, and is almost certainly covered by the design margin of the existing setup.

On the receiver side you get a direct logic level output referenced to your ESP32 rails.

Job done.

(Optocouplers are a much overlooked technology, but are used widely in everything from high-side FET drives, to galvanic isolation of comms lines. "Go optocouplers!")

Answer 2

If the anode of the red LED is connected to +3V3, then you can use something like that. The transistor will turn on if there is more than about 0.6V on the LED, and that will pull the ESP input high.

If the anode isn't connected to +3V3, for example if the LEDs are multiplexed, then...

When the LED turns on, there will be ~1.8V across it, so Q3 acts as a current source of about 1.8V-Vbe = 540 µA. That turns on Q5, and the ESP input is pulled low.