Application 248 - Basic Steps

Application Bulletin 248
datasheet will specify the range of forward current that can be
Connecting and Using Infrared
forward current will be set at 20 mA. The forward current can be
safely used without causing damage to the LED. Typically, the
controlled by placing a current limiting resistor in line with the
power supply and the LED as shown in Fig. 2.
This application note is intended to assist someone with little or
no experience in connecting basic optoelectronic assemblies. A
basic optoelectronic sensor assembly consists of two separate
components. The first component that must be considered
before making electrical connections is the LED, which is called
the emitter and which will typically emit infrared light. The
second is the phototransistor which is called the sensor or
detector and which responds to the infrared light. To ensure
good photo-coupling between the emitter and detector, it
is recommended the user consider using a standard Optek
assembly. The Optek OPB815WZ is a very good example of an
Infrared Slotted Switch and will be used in this example.
The first step when connecting the sensor assembly is to provide
appropriate power to the LED which is why it is important to
understand two key characteristics of the LED. These are the
Fig. 2
forward voltage (Vf) and forward current (If). The forward
voltage is the minimum voltage required to turn on the LED.
In our example of the OPB815WZ, the LED used inside is
This will change depending on the desired forward current
an OP240. We will plan to drive the LED at 20mA at room
and ambient temperature.
For gallium aluminum arsenide
temperature (20°
C). We can obtain the typical forward voltage
LEDs, VF typically will be 1.2 to 1.3 volts DC @ IF = 20mA, as
for this LED by looking at the VF curves shown in the OP240 data
seen in the forward current curves in Fig. 1. The forward current
sheet and at the end of this article. Alternatively, the user can
obtain the forward voltage from the datasheet of the OPB815WZ.
We see that we can expect to have a nominal ¬VF value of 1.3
V. Because the supply voltage is 5V, we must drop 3.7 V across
the resistor, Rf. Using Ohm s Law, E=I*R, we can calculate the
resistor to be 185 Ohms. The power being dissipated by the
resistor is I2*R or 74 mW. Therefore, a 1/8 Watt resistor or larger
will work in this application.
As seen in Fig. 2, the positive voltage source will be connected
Fig. 1 Typical curve shown for GAALAS LED
will determine the amount of power emitted by the LED. The
Application Bulletin 248
through the resistor to the anode (A) while the cathode (K) is
connected directly to ground. Alternatively, the voltage source
can be connected directly to the anode, and the cathode can
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Application Bulletin 248
be connected through the resistor to ground. The anode can
be easily identified on the LED as it is usually the longer of the
two leads. Or, in the case of an Optek assembly with wires such
• LED degradation over time.
Allow for a reduction of 20% @
10K hours of operation.
as the OPB815WZ, the anode will typically be the red wire and
• LED power output reduction at higher temperatures (when
the cathode will be the black wire. Before making connections,
applicable). Allow for a reduction of 25% @ TA = 60C (for
verify the pin outs by checking the data sheet of the particular
LED or assembly that you are using. To verify the connections,
• Minimum Ic(on) value on datasheet is measured at Vce =
measure the voltage at TP to ground and confirm it is 1.3 V.
10V, but we are operating at Vce = Vsat. Allow for a 20%
If it measures 5 V, the LED is open or backwards, or there is a
reduction @ Vce = Vsat
wiring problem. If it measures 0 V, the LED is shorted or there is
a wiring problem
Taking all of these factors into account gives us a new
recommended RL value:
Now that the LED is connected properly and emitting infrared
light, the phototransistor is to be connected so that it can
RL (recommended) = 1.3K / 0.8 / 0.75 / 0.8 = 2.7 Kohms
detect the light. The current conducted by the phototransistor
will be proportional to the incident light shining on it. The
The completed circuit for the OPB815WZ will look like the circuit
phototransistor functions like a standard transistor except that
diagram in Figure 3 below.
the base current is produced via an integral photodiode. Because
of this, most phototransistors only have two leads ‒ the collector
and the emitter. In our case we are looking at connecting the
phototransistor half of the OPB815WZ. The datasheet for this
assembly tells us that the minimum Ic(on) of the device will be
3.5mA @ Vce = 5V when the input LED is driven at If = 20mA. It
also tells us that the saturation voltage (VSAT) of the assembly
is 0.4 volts maximum. We now have the information needed to
calculate the load resistor RL value that will be required to give
a well-defined change in output voltage when an object passes
through the slot of the OPB815WZ. We can calculate this value
by the following formula:
Rc(min) = (Vcc ‒ Vsat) / Ic(on) min.
Fig. 3
For the OPB815WZ, the minimum IC(on) value is 3.5mA.
When connecting a phototransistor, you will see both wire leads
Substituting this into our formula gives:
are usually of equal length on a single component. The collector
and emitter are identified by the packaging around the sensor.
RL(min) = (5 ‒ 0.4) / .0035A = 1.3 Kohms
For axial leaded Optek components, the collector is typically the
lead nearest the flat on the package. On an Optek assembly
Because of several factors, we will want to use a higher value of
with wires such as the OPB815WZ, the collector is typically the
RL than the initially calculated value of 1.3 Kohms. These factors
white wire and the emitter is the green wire. Again, verify the
Application Bulletin 248
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Application Bulletin 248
pin outs by checking the data sheet of the particular detector or
sensor assembly that you are using. Using the RL value we have
calculated, the voltage measured at the Output will go from less
than 0.4 V with the LED on (and unblocked) to more than 4.6
volts with the LED off , or blocked by an opaque object.
There are many other factors that may be considered when
designing with infrared optoelectronics including aperture sizes,
amount of ambient light present, switching speed and more.
For a more in depth look, please refer to Optek Application
Bulletin 213.
TT electronics OPTEK Technology
1645 Wallace Drive
Carrollton, TX 75006 USA
Tel: +1 972 323 2200
Fax: +1 972 323 2396
[email protected]
Application Bulletin 248
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