Circuit to convert voltages from 3.3 V and 1.5 V to 3V3 logic levels

Question

I have a PCB which I am trying to connect to a Raspberry Pi. The PCB has some LEDs which indicate some status. Each LED anode is connected to 3.3 V while its cathode goes through a resistor to ground.

I have soldered some wires to the negative side of the LEDs, as that was the only accessible point I could safely solder something to. However, this means that on that point, when the LED is emitting, I get 1.5 V more or less, and when its not emitting, I get 3.3 V.

I have tried to read those values directly in the Raspberry, hoping it was low enough to consider it a logic LOW but it's not.

How could I convert an LED voltage of LIT=3.3 V and OFF=1.5 V to corresponding logic voltages of 3.3 V and 0 V?

My circuit will be similar to this. The control can be 0 V or 3.3 V, depending on some logic or an IC on the PCB I am trying to connect to the Pi.

Answer 1

Here is an easy way to do this, though not exactly what you are asking for:

^{simulate this circuit – Schematic created using CircuitLab}

You should enable the pull-down in the Raspberry Pi input or add a resistor to ground such as 10kΩ

The input will be high when the LED is on (or power is off in the remote device) and low when the LED is off and power is on in the remote device.

Just about any small-signal PNP transistor can be used.

It's possible you may have to add a pull-up resistor across D1 depending on what exactly is driving it.