A simple logic control yet it does not work

Question

I have two separate LEDs. Both LEDs have 500 ohm resistor and are powered with 5V. These LEDs are controlled by an MCU. LED anode is connected to VCC and the diode is connected through the resistor directly to a MCU GPIO port capable to drive the LED.

So I have two separate signals that controls LEDs and the VCC. Those are the only things that I can access. I have no access to GND. What I would like to do is a kind of logic that will ensure that only one of those two LEDs can glow. If MCU tries to turn on both LEDs none should glow.

In fact, this is not just a LED but a SSR module (G3MB-202P) with internal photodiode. the SSR controls a 230V AC motor. One SSR spins motor in one direction and second in the other. I just want to protect in case when the MCU will try to spin the motor in both directions as it would damage the motor.

My current approach is depicted below, but it does not seem to work. Could anyone please guide me how to do the logic properly?

Answer 1

Your problem was that the transistor was inverting from the Base and non-inverting from the Emitter, so your XOR logic failed.

Using any CMOS logic level input, a simpler XOR solution works.

Your R values will yield an 8 mA LED current with 5V logic <~50 ohms.

A phototransistor was used to simulate the phototriac logic, so also disregard the 3.3V.

I changed the interface from high Enable input to low (!) Enable input which is safer during a power On Reset with all I/O's floating as inputs.

H/L inputs in box are interactive.

Answer 2

What I would like to do is a kind of logic that will ensure that only one of those two LEDs can glow. If MCU tries to turn on both LEDs none should glow.

This sounds like an XOR behaviour:

^{simulate this circuit – Schematic created using CircuitLab}

The above is a crude, two-transistor XNOR gate (NOT of XOR) and, of course, is for two inputs and one output yet you have two inputs and outputs. But I think you can make use of it somehow.

Answer 3

Your circuit really should work, but only if the Input0 and Input1 pins can source current. I suspect that they can only sink current — i.e., they are open-collector or open-drain outputs — therefore neither transistor can turn on.

If this is the case, just modify your resistor position slightly to use them as pull-ups:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

You could just connect both SSR inputs to both GPIO0 and GPIO1 if those and VCC are only available signals.

Then you can't light up both LEDs simultaneously.

However, this offers no safety. You can blink both LEDs on and off so fast that the motor or SSRs still burn, if something goes wrong.

Of course you can make software whigh delays the correct amount with both off for some time before activating one SSR.