Guru
Joined: 11/02/2018
Location: AustraliaPosts: 2247
| Posted: 07:15am 18 Oct 2022 |
That is all useful information.
That the BC557 needs such a low value base resistor suggests that it is marginal in this application, but if it works it will do.
I suspect that it isn't acting as a transistor at all in the reverse direction, but the two junctions are making a "Double Diode Switch".
Before decent high frequency transistors were available a pair of diodes could be used to switch video and VHF signals. When the diodes are reverse biased it is off and pass the signal when they have sufficient forward bias.
Edit 27/10
Tested a pair of diodes and and SD card, Blit, Save Image and Pixel() all work properly.
Have tested with resistor values from 1k8 to 10k.
A pullup from MCU MISO to 3V3 could be added if needed. 5 to 10 times the CS resistor value.
Counting the colours on an ILI9488. It is indeed RGB 666
> p=0 :b=0 :for n=0 to 255 : pixel 9, 9, n :p= pixel(9,9):if p=n then :inc b :endif :next : ? b;" Blues"
64 Blues
> p=0 :g=0 :for n=0 to 255 : pixel 9, 9, n<<8 :p= pixel(9,9):if p=n<<8 then :inc g :endif :next : ? g;" Greens"
64 Greens
> p=0 :r=0 :for n=0 to 255 : pixel 9, 9, n<<16 :p= pixel(9,9):if p=n<<16 then :inc r :endif :next : ? r;" Reds"
64 Reds
> ? 2^6
64
Edited 2022-10-27 22:01 by phil99