Guru
Joined: 11/02/2018
Location: AustraliaPosts: 2247
| Posted: 04:25am 17 Oct 2022 |
4k7 may be too low, try values from 10k to 33k. 33k works with a PN200.
Also the BC557 might not have enough reverse gain, try swapping C and E. The Pico pin capacitance needs to be rapidly charged and discharged requiring some gain in both directions.
To test the reverse gain of the BC557 connect the collector to 3.3V with emitter to ground via 1k and base to ground via 56k. That will give a base current of about 50uA and the millivolts across the 1k will give the C/E current in uA. Divide that by 50.
The PN200 had a reverse current gain of about 30.
If it's gain is too low and you can't get a PN200 try two BC557s in reverse parallel (C to E, E to C). Each with its own 33k base resistor to LCD_CS
Edit.
Doh,
The light just went on!
MMBasic uses 16 bits for LCD screens, not 24 bits. You will never get 255 back for any colour.
Your transistor is probably working just fine.
Edited 2022-10-17 17:26 by phil99