GPIO - Raspberry Pi Hardware Reference - page 154

Current flow into the base of Q 1 creates a voltage drop of V BE = 0 . 7 V , from the input
base lead to ground. So to calculate the resistor value R 1 we take the V R 1 divided by the
current. The highest voltage coming from GPIO is going to be slightly less than the 3.3 V
power supply rail. It is safe to assume that GPIO HIGH = 3 V . The voltage appearing across R 1
is thus GPIO HIGH -V BE .

GPIO - V

R =

HIGH BE

1

I

B

307
0 002
1 150

- .

=

.

= , W

The nearest 10% standard resistor value is R 1 = 1 . 2 k Ω. Using this resistor value as a
check, let's compute backward what our actual drive capability is from Q 1 . First we need
to recompute I B now that we know R 1 :

GPIO - V

I =

HIGH BE

B

R

1

307
1200
19

- .

=

= . mA

This tells us that the GPIO output pin will not have to source more than 1.9 mA of
current, using R 1 = 1 . 2 k Ω. Now let's calculate the maximum drive we can reliably expect
in the collector circuit of Q 1 :

IIH =

=
=

´
0 0019 50
95

CB FE

. .

mA

■ Note this discussion glibly avoids the effects of components being within ±10%

tolerance.

This computes that the designed 2N2222A driver circuit is capable of driving up
to 95 mA.