Fast LED Output Indication

colin_m

Member
Hi,

I am using a Teensy 4.1 to control 10 different stepper motors. The outputs from the Teensy are connected to a 74LCX245 bidirectional transceiver for level shifting the 3.3 V outputs to 5 V for the stepper motor drivers. That all works fine.

However, I thought it would be nice to add LED indication to the outputs to see the pulse and direction signals. For this I added an LED and a 330 Ohm pull-up resistor for each output between the Teensy and the transceiver. Obviously with very high pulse speeds, the pulse LED should look solid, but what I'm seeing is that even with low pulse speeds the LED seems to be on constantly. If I use the built-in LED pin as an output, I can see it pulsing, but my LEDs seem to be solid.

I looked at the Teensy 4.1 schematic and saw that the built-in LED uses a 470 Ohm pull-down resistor, so I'm wondering if that might be the difference? I opted for the pull-up resistor with the LED because of the limited current output on the Teensy pins. My other thought was that the input channels of the transceiver were affecting the LED operation?

If someone has had experience with this kind of arrangement or can offer me more insight to the problem, I would really appreciate hearing from you.

Thanks.
 
Human senses are quite non-linear. They also tend to be extremely slow relative to the speed of electronics. The ways our minds interpret sensations can vary quite a lot.
 
I agree 100%, but in this case the LEDs I added to the Teensy output pins are definitely not working like I expected them to. I tested it by changing the stepper speed to 10 steps/second. You can clearly see the internal LED pulsing at 10 Hz, but the external LEDs I added stay solid.
 
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