arrow said:
(With the emitter at +7.5V.) What I see happen is the entire matrix lights up (quite brightly). If I put the Emitter back to 5V then the right sequence of LED's are on.
The emitter is at +7.5V. When the 74HC595 is supposed to turn off the LEDs then its output to R1 is its supply voltage of +5V. The transistor has current through R1 so turns on the LEDs continuously. Actually, the current in the base resistor is trying to pull the output of the 74HC595
above its supply voltage, that's very bad for it.
When I measure the V at the Base with the 5V supply and the 120Ohm R1 I see that V = 4.1V and 5V depending what the 595 is driving. Why is the 4.1V when 595 is showing 0 (actually its showing 0.6V or so)?
If the display is multiplexed then each voltage is present only for a moment. A 'scope will show the transistor's base voltage at +5V when it is off, and at about +4.1V when it is on. A multimeter might average the voltage at 0.6V across the transistor's base-emitter.
The 32mA from the 74HC595 causes a voltage drop of about 0.6V across its output resistance. Its output voltage is 0V for a low without any current.
The 32mA in R1 is created by the remainder of the voltage across it.
Actually, with R1 at 120 ohms and the base at 4.1V and the output of the 74HC595 at 0.6V, the 3.5V across it limits the base current to 29.2mA.
The 2N4403 is nearly the same as a BC327, but has a different pinout.
They work very well up to about 100mA then above that their current gain drops and their saturation voltage rises.