Saturday 8 July 2017

What transistor do I need to drive many LEDs and how should I hook it up?


I know that a transistor amplifies a signal, but I've never used one. I have this design here and I want to add 15 LEDs connected to each pin, instead of 1.


How I could incorporate a transistor to help me achieve a strong enough current to power the LEDs ?




Answer



The simplest solution would be to place the 15 LEDs in series and use a high enough power supply. The "high enough" is the problem here. You would need at least 30V and that's too much for the LM3915. Placing the LEDs in 3 parallel strings of 5 LEDs each isn't a solution either; the LM3915 can't sink enough current to drive the 3 strings.
So we'll need external transistors. The driver's outputs are most likely open collector NPNs. Then this schematic should work:


enter image description here


If the LM3915's output is low (LEDs must be on) T1 will be switched on via R2 and supply current to the LEDs. Those could have been placed in a single string, but then you would need a too high voltage for V+. Now 12V will be sufficient for common red LEDs.
If the LM3915's output is switched off R1 will pull the base of T1 to V+, and T1 will be switched off.
The components' values will depend on V+ and the current required for the LEDs. For V+ = 12V you could make R1 = 1k\$\Omega\$ and R2 = 4.7k\$\Omega\$. This will give a base current of 2mA, which is enough to saturate most transistors, so T1 can be a PNP type like a BC556.


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