LED for TX and RX of MAX232N

[sean87]

Hi all,

I have made a PC serial to CMOS level circuit (if this kind of circuit all called this!) using MAX232. It works fine, but I want to add 2 LEDs so I can see data going out or coming in to the chip. I want this to the CMOS level side of chip (T2IN and R2OUT pins).

Can I just hook up to LED's to this pins and ground? If not, what should I do? I looked in datasheet but didn't find anything on this regard.

[digsys]

Assuming a 5V system, use 2K2-2K5 resistors (~2mA) and "good" leds. 50-100mcd (or high efficiency). Similar price to "cheap" leds.
That way you won't load the signals at all. I wouldn't try to run 20mA on a standard led.
OR IF you are not using the other 2 channels on the MAX232, link the 2 pairs together and use the 2nd pair just for leds.

[sean87]

Thanks, got the idea.

[aluck]

Wouldn't it be better to put a small BJT to drive those LEDs?

[Smokey]

Logic level FETs are great for this kind of stuff.  2N7002LT1 or something like that. 

[hlavac]

If you want a nice visible indication even for a single bit you could build two pulse stretchers
from single 74hct00 chip.
Look at the right side of my logic probe schematic to see what I mean...

[sean87]

Thanks for ideas!

I made the circuit. For TX/RX indicator LEDs I am facing a small or big problem! The CMOS level side of chip is always at 5v when idle. I connected a normal 5mm led to the GND and RX of cmos side and it is always on...when I send some data to chip, it slightly dims for a mili sec (I think it tries to show low bits !).

Any idea how to invert this process?

[kxenos]

Connect the cathode of the LED to the signal and it's anode to Vcc.