I have a circuit that draws little power (under 5mA), it accepts 2 MIDI signals and does some switching. I'm not using the standard optically isolated circuit as there is only signal manipulation done by the circuit - not connected to anything "audio".
As written above, the device accepts two MIDI signals. Each has a voltage ref. pin as part of the current loop. That voltage might be 3V, 3.3V or 5V.
My circuit will work fine with a voltage going low as 2.8V and as high as 3.3V.
When VCC = 5 V, then R = 220 Ω. When VCC = 3.3 V, then R = 33 Ω.
In order to power my circuit I need to tap into one of the voltage lines of the input/s. At any current time there could be a single input or two inputs with the voltage lines being 3V, 3.3V or 5V with the two devices not necessarily have the same voltage. How can I select the line which is 3V or 3.3V in case it is present and if not step-down the 5V to 3.3V (LDO regulator). But how does the selection or bypass being done?
The logic signal are referenced to the voltage line (same voltage for logic 0 or 0V for a logic 1). I need to translate all the input signals to the voltage the circuit is operating in. So if the circuit is powered from 3V I need to have 3V signals going through without alternation, however a 5V signal would require stepping down. How can I achieve this?
Simple power OR-ing using MOSFETS (as diodes), although this does not work as should in the simulation (any ideas why). This is one part of the solution.