Short story;
DIY Fluke IR189USB clone (USB<->IR transceiver):
-extremely sensitive to correct value of pull-up resistors on IR phototransistor and LM393 output.
-low power on emitter IR LED.
Long story;
I've just finished to assemble a clone of the Fluke IR189USB (USB to IR module) based on an existing design (FT232RL and LM393). I built it for updating the firmware of my Fluke 289. It didn't work (updater couldn't establish link). I then tried to manually send commands (ID, RI, DS, etc) while I tweaked all resistors, and after a while my Fluke 289 responded correctly (factory resets the meter, default settings, etc). Except the received IR message was garbage. I narrowed it down to being very sensitive to pull-up resistor values of IR phototransistor (R6) and RX line (R3) between LM393 and FT232RL. In the DIY schematics, they use R6=1k (phototransistor) and R3=12k (LM393 and FT232RL) pull-ups, and many users have reported success out of the box. But with these values, I only receive garbage (K„ÔÛ˚nœ√Ûœ√˚Ô√√€Ôˇ). I connected two potentiometers and started to tweak them while constantly sending 'ID' until I received the readable response "FLUKE 289,V1.10,1069xxxx". I achieved this with R6=2.2k(ish) and R3=31.21k(ish) but it drifted off without even touching the potentiometer and I would receive something like "ê..LUKE 209,V1.1 (1 69 029". I should probably add a fixed resistor in series with a very small value potentiometer (or a multi-turn pot), to form a center around 2.2k and 31k, so I could properly fine-tune, but could this sensitivity be a symptom of something else? Junk LM393 (I only had 10 Aliexpress chips lying around, and none from reliable sources). Or could some filtering help?
PCB is quite neatly routed, so I don't suspect this as a noise source.
Also, how do I deal with the low LED output? If I set my power supply to 5V/20mA, the IR LED is quite bright in the webcam and draws 18mA@1.26V.
In the current design the LM393 is only able to provide the LED 0.95V@0.2mA. While it actually works, I'd like to increase range... External transistor seems excessive as LM393 should be able to sink at least 4mA?
I have no datasheet for the IR LED nor the IR phototransistor.