Re: LED Strobe Circuit
The transistor type depends upon the amount of current the LEDs will be carrying in total. Although the original message says 20mA it also asks how to add more LEDs which means more current. For example, if there were two chains of LEDs the current would be 40mA, if there were 10 it would be 200mA and so on.
I use BD682 in similar applications, they are rated at 4A absolute maximum so in theory they could be used to switch 200 LED chains, each at 20mA.
The reason for using a darlington transistor is that the base current in Q1 is calculated by dividing the collector current by its gain (hfe) and the values of R2 and R3 used on the schematic are going to restrict the base current enough to limit the number of LEDs that can be used. Darlington transistors have much higher gains so they will switch the same LED current without having to increase the base current.
For example:
BC327, max gain = typically 250 at 0.1A, max load 0.1A
BD682, max gain = typically 750 at 2A load, max load 4A
Generally, the gain drops as the current increases.
Brian.