Transistors as Switches

Switching a device to ground from a microprocessor output pin (I/O pin)


to find the value of R1 use the magic formula:    R1 = switching voltage(I/O pin voltage) * Hfe / (1.3 * load current)

R2 should be about 100 * R1 value.


2N3904 (NPN) can be substituted by BC547, 2N3906 (PNP) by BC557.





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