Application Bulletin 248 BASIC STEPS datasheet will specify the range of forward current that can be Connecting and Using Infrared Optoelectronics 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 Page 1 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 are: • 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 example). 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 RD RL 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 Page 2 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] optekinc.com Application Bulletin 248 Page 3