I have a small device that has a TTL-level UART interface, the signal is fed into a Schmitt trigger, which is then used in a small IR-LED. An IR Sensor is also attached to a Schmitt Trigger, via a FET and that signal is then put on the TxD pin.
Now, the entire device consists of a 74HC14 (Hex Schmitt Trigger of which only two are used), the LED and the IR Sensor, two MOSFETs, four resistors, and three caps.
The device I'm controlling this from has only RxD, TxD, and Ground exposed. I can't easily use a 3.3V or 5V rail from the controller. I'd have to use an external PSU, like a USB charger just to drive this one little device, that isn't gonna draw much more than 10mA maximum.
Now, I'm wondering, if I can make a parasitic power supply from the RxD and/or TxD pins of the UART interface. Voltage levels aren't significant, as the device has a very wide Vdd input range: 2V - 6V.
My idea is to use perhaps a Schottky diode from RxD through a resistor to a capacitor, perhaps 100nF or maybe a little larger. The idea is to run the Schmitt trigger and the LED as well as the sensor from just the power the controller is able to provide. The specifics, such as the correct capacitance, resistance, etc. is to be determined, but I'm just generally asking if that idea is sensible, and if my approach is not wrong.
Data rate is rather low, max. 9600 baud. Data is just around one to five bytes every second. At all other times, there's no data transmission in either direction.
As suggested by some comments, I shall clarify that the controller talking to that device, is a Raspberry CM4 module. So the CM4 is the UART controller that I'd take to talk to that 74HC14 IC. I don't know how much current I can draw from those outputs, but the CM4 normally is normally pretty robust like that, in my experience.