Dragging up an old post!
Greetings everyone, I have been playing with the transistor switch circuit in Uncle $crooge's post for exactly the same application - switching segments of a 7 segment display (multiplexed) - from a 74HC595. The only difference between this circuit and mine is that I have put a 'pull down' resistor from the output of the 595 to ground to prevent the base of the transistor from floating.
I have a couple of questions;
Why do we need to try and pull almost max current from the 595? Surely this will eventually damage the chip.
I have experimented with various values for the resistances and found that what seems to work best is R(base) = 5.1k , with R(pull down) = 1K, R(load) = 510. I am using 3V to simulate the signal from the 595 and a different 3V source for load. I have found that using any R(base) value less than 5.1 K, causes the LED to light without the 3v for the load being connected (must be getting a leak through the transistor). Obviously I will have to change these values for the 5v that will be eventually used. I am just thinking that the 100 and 10 ohm resistors will cause to great a current, or does the transistor require this base current to switch properly.
Last one.......
It does not seem to matter if I use a BC337 (NPN) or a BC327 (PNP). I get exactly the same outputs. Why would that be?
Sorry for the 'noob' style questions, but that is what I am Well not really, its just been over 20 years since I played with electronics!
Regards Jon
Greetings everyone, I have been playing with the transistor switch circuit in Uncle $crooge's post for exactly the same application - switching segments of a 7 segment display (multiplexed) - from a 74HC595. The only difference between this circuit and mine is that I have put a 'pull down' resistor from the output of the 595 to ground to prevent the base of the transistor from floating.
I have a couple of questions;
Why do we need to try and pull almost max current from the 595? Surely this will eventually damage the chip.
I have experimented with various values for the resistances and found that what seems to work best is R(base) = 5.1k , with R(pull down) = 1K, R(load) = 510. I am using 3V to simulate the signal from the 595 and a different 3V source for load. I have found that using any R(base) value less than 5.1 K, causes the LED to light without the 3v for the load being connected (must be getting a leak through the transistor). Obviously I will have to change these values for the 5v that will be eventually used. I am just thinking that the 100 and 10 ohm resistors will cause to great a current, or does the transistor require this base current to switch properly.
Last one.......
It does not seem to matter if I use a BC337 (NPN) or a BC327 (PNP). I get exactly the same outputs. Why would that be?
Sorry for the 'noob' style questions, but that is what I am Well not really, its just been over 20 years since I played with electronics!
Regards Jon