Can anyone shred some light onto my observation please?
And not a pun is sight
Some questions:
1. What is the power supply. 4.5V battery or 5V from mains plug pack?
2. What is the value of resistance in series with each LED
if you are operating with the typical 330 Ohm resistors you will have about 11.5 to 12mA into each LED so the PICAXE is being overloaded.
Typical/"normal" LED’s as opposed to the super bright types can handle around 20mA.
However you will find they are generally bright enough at around 10 to 12mA and some on this forum have operated as low as a few mA. I can vouch that at 0.3mA an LED is visible straight on but very dull and not visible an any real angle.
Maybe a slight generalization here but from past looking at some of the PICAXE datasheets, the PICAXE chips can sink (draw) a total of 95mA from the power supply into Vdd.
Each output can source up to 25 mA but the total load on a port (all 8 pins) AND the total load of all external devices is 90mA.
So, if you have absolutely nothing else connected you could operate all LED’s at say 10mA max better still a little less.
To directly drive all 8 LED’s directly form a PICAXE port I would suggest starting with something like 470 Ohm 0.25W resistors which would give about 8mA to each LED.
Next,, there is no internal means of accurately limiting the current from each PICAXE output so some variation will always occur.
A further question . . .
While recognizing that compactness might be your goal, why not consider either a 74HC595 shift register or one of the 74xx or 4xxx digital BCD to 7-seg display decoder /driver chips to take lighten the load on the PICAXE.
EDIT:
hmmm Dippy is in there with a response while I was typing.
A further thought since you have a 28X1, why not consier a PCF8574 i2c 8-bit expander IC. That whay you keep the max number of IO pints for other devices and the PCF8574 can also sink 25mA per pin on a LOW output. Mind you, even these have a total/max current into Vdd or Vss of 100mA - so again about 8 to 10mA per LED at best. I am running 8 discrete LEDs with a PCF8574 via interface transistors (to avoid logic inversion). Roughly 8mA into each LED and when all are on no discernable variation in brightness.