Just an NPN small signal transistor that can handle the voltage you need to power 4 LEDs in series. Don't forget to include a series current-limiting resistor.qty 4, 5mm LEDs
The ULN2803 is far from obsolete; it is commonly available and is listed in Texas Instruments site as being "active". ST also list the chip as "active" and available in four different packages. Neither manufacturer appears to list the device as "end-of-life" or "not recommended for new designs" and supplies appear plentiful (RS can let you have 1900 next day!).The ULN2803 drivers are obsolete now and much better stuff is available.
There are still suppliers of ULN2803A and UDN2981A 8 channel Darlington driver and the ULN2803 is still frequently recommended on websites, etc.The ULN2803 drivers are obsolete now and much better stuff is available.
I doubt a PICAXE will threaten the speed capability of a 2N2222, 2N3904 or 2N7000. There is no real need to get very fancy here.Good evening,
For trying various models on my CNC drivers, which of course on MosFET, the use of a transistor faces the problem of switching frequency.