transistor. Always check
VBE(sat) listed on the
datasheet to verify.
Finally, we calculate
the voltage drop across RB
(VRB) by multiplying the
base current (1.5 mA) by
the resistance of RB
( 2.933K). Therefore, VRB =
Figure 4 shows the
finished LED circuit with
all the components and
clearly marked (Ohm’s
Law was also used to
calculate the resistance for R(LED) and
We now have the correct resistor
values in order to operate the LED
and transistor circuit in a safe
manner: Rc = 200 ohms; and RB =
I’m sure you’ve notice that our
2.933K resistor is not a standard size
you can actually purchase anywhere.
The rule of thumb in this case states
that you can use the next standard
resistor value below 2.933K ( 2.7K to
The lower resistance only helps
to decrease the chance of the
transistor from falling out of
saturation mode during temperature
and power supply variations by
increasing the base current (i.e.,
transistor goes even deeper into
Let’s review all the steps required
to use a transistor as a switch:
1. Download the datasheets for
the LED and transistor.
2. Determine the max current
(IC(MAX)) you want to go through the
LED and transistor, and verify that it
doesn’t exceed the maximum current
rating of the LED (IF) or
the transistor (Ic); refer to
3. Calculate the value
for resistor Rc. Make sure
to include the voltage
drops for the LED (V(LED))
and the transistor (VCE(sat))
in the Ohm’s Law formula.
4. Calculate the
transistor’s base current IB
using an ODF of 10.
5. Calculate the
resistance value for base
That’s it. Kind of easy
— well, maybe not.
On a personal note, when I
breadboard a circuit I only use
through-hole, red, 5 mm diameter,
ultra-bright, water-clear dome 640 nm
LEDs. I’ve tried other LEDs, but the
water-clear dome LEDs are the best.
They’re so bright, they hurt your eyes
— no kidding!
Order a bag of LEDs from Digi-Key, Jameco, or Mouser for your next
project — it makes life easier. NV
May 2015 49