I would like to modulate a UART output with a PWM signal. I think the following schematics should work. Can you see any mistake?

The result I'm expecting:

PWM = GND and UART = VCC -> LED is on

PWM = GND and UART = GND -> LED is off

PWM = VCC and UART = VCC -> LED is off

PWM = VCC and UART = GND -> LED is off

Is this correct?


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ The logic of the circuit is correct. But you are probably driving the LED with too much current. \$\endgroup\$
    – rioraxe
    Mar 16, 2016 at 22:52
  • \$\begingroup\$ I changed your truth table (you had two lines that were identical, and I'm guessing you wanted a conventional truth table). You can fix it if not. Also, is that the actual LED you are using? Otherwise, what is the current rating of your LED? \$\endgroup\$
    – uint128_t
    Mar 16, 2016 at 23:06
  • \$\begingroup\$ The truth table is correct now. Thanks for the fix. The LED is an infrared and it requires 100 mA. So the resistor R4 should be around 25 ohm (with 1,5V for the led + 2 * 0,6 in the transistors) \$\endgroup\$
    – FEYE007
    Mar 16, 2016 at 23:52
  • 2
    \$\begingroup\$ Besides the fact that you want to validate the cicuit. Is there any specific question? I'm sure you have done your math so I won't challenge resistor values. \$\endgroup\$
    – Krauss
    Sep 26, 2016 at 17:49


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