LED is always on on Extended Keyboard Dongle

[nharrer]

LED is always on on Extended Keyboard Dongle

[svenpetersen1965]

Good, that it works, now. Issue closed.

[svenpetersen1965]

IC1B is used as an inverter. A High in one Ctrl-Input will switch the "output" Pin 4 to GND on Pin 5. If the on-Resistance of the 4066 is too high, the voltage on Pin4 will not get low enough to switch off the feedbacks and the LED. My mistake in Rev. 0 was connecting the ctrl input directly to the base of the transistor. The voltage could not get higher tan about 0.7V, which is still low. the ctrl needs to be 3.5V for a valid high signal.

The on resistance is typically 125 Ohm (I just read, that is for 15V supply voltage is can be 470ohms). And the valid LOW level is 1V. Hmm... so my dongle works, but the values are a bit delicate. Which IC brand did you install?

I have played a bit with the values. R2 should be good with 3k3 and R4 with 10k. Are you able to check these values? I cannot find my prototype right now.

[nharrer]

Thanks for the explanation. I didn't realize that R_on of the 4066 is quite high.

I have no idea what brand I have. Probably bought on Aliexpress:

I measured I_A and V_A at Pin 4 when IC1B is on. Which is 1.32mA and 0.7V. That results in about 260Ω. Which is quite higher than 125.

And I tried the values R2=3k3 and R4=10k. That works! That ctrl signal high state is 3.8V in that case.
I stuck it onto three different C64s and it looks good so far.