Strange inverter behaviour; what am I missing?

[aeberbach]

I'm trying to use a dual inverter https://www.ti.com/lit/ds/symlink/sn74lvc2gu04.pdf?ts=1722469526037&ref_url=https%253A%252F%252Fwww.google.com%252F to power one of two LEDs based on the output of a caps lock LED from an old Amiga computer. Attachment shows the circuit.



The existing NPN transistor drives the caps lock LED in the original keyboard. Its base is the output of the 6507 keyboard controller (a variant of 6502) reflecting the state of caps lock. Low is lit, high is unlit, as the LED in the original keyboard has VCC on the other side.

What I want to do is not use the original LED, instead use that state as the input to the inverter/LED arrangement shown in "New circuit". When low, the blue LED is off and the red LED on. When high, the blue LED is on and the red LED off.

I don't see anything in the datasheet of the inverter to suggest I am doing anything wrong with it? What happens is that the LED state transitions after a brief pause, when it transitions at all. It isn't reliable. Is it as simple as needing a 10k pullup to pin 1 of the inverter?

[Benta]

Remove R3 and add a pullup.

[Terry Bites]

The 650x has a Vdd of +5V. Add a pull up to +5V as shown. If the pull up is >>100 ohms then you can leave R3 where it is.
An Rpu 1k to 10k will work fine. It is that simple.

As shown in your schematic, the inverter input is charging up to an on state via a leakage current.
Q1 etc can discharge it but provides no path to charge it again.

For this particular converter you can drive it directly from a 5V CMOS logic output even if the inverter Vcc is less than 5V.
The SN74LVC2GU04 data sheet says the inveter can be used as a "Down Translator". ie 5V down to the inveter's Vcc.
Swap the LED colours. If Vcc is less than 5V, the blue LED might fail to light up. Blue LEDS need at least 3.3V.

[brucehoult]

Also can't both LEDs can be driven by one inverter? It can source and sink the same amount of current.