Servo - June 2008

know how to bias these sensors, you need to read the datasheet. For the newcomer to the art of datasheet reading, this can be a daunting prospect because datasheets might just as well be written in Klingon for all the difficulty it can be to wrestle useful information from them. Fortunately for us, the Fairchild Semiconductor datasheet is very straightforward in telling us what we need to know.

The first graph to look at is the one that tells us where the device is the most sensitive. Figure 3 gives this chart. It shows us that 150 mils is optimal (0.15 inches).

The chart gives this data for a 20 mA I_F(the current through the LED) and V_CE(voltage used on the sensor) of

5V, which suits us, so we’ll use those values. This chart has normalized collector current as the Y axis — what is that? This is a fancy way of saying that if we call the maximum current 1, then every other point on the curve is a percentage of that value. We’ll need another chart to tell us what the typical current would be through the sensor to set our bias resistor.

Now, how much current should we allow through the sensor? As usual, that depends. We want to avoid the maximum values and pick something that is typical or recommended. We won’t get any direct answers from the tables here. All we have to work with are maximum values for certain chosen settings. However, in the Electrical/

Optical Characteristics table of the datasheet, we find the information shown in Figure 4.

This information shows us that if we put 40 mA through the LED and had no resistor limiting current, and used 5V to power the device that we should get a maximum of 0.60 mA (600 microamps) current through the device. Further, it tells us that the saturation voltage from the collector to the Emitter would be 0.4V. This means that whatever we use as a dropping resistor would have about

5V – 0.4V = 4.6V across it. That’s a pretty good logical ‘1.’ I like to connect the bias resistor from Vcc (5V to the collector of a transistor) so that when the sensor is fully on, I read a logic ‘0’ and when it is off, I read a logic ‘1.’ A voltage of 0.4V will read a logic ‘0’ very nicely on our microcontrollers so no ADC (Analog-to-Digital) reading is needed; we can just use a digital input line. We’re only going to be driving the IR

Figure 2. QRB-1134 reflection sensor.

Figure 3. How close should we be? (Used with permission from Fairchild Semiconductor.)

LED at 20 mA, so let’s estimate that our fully on current would be a minimum of 100 μA (half of that seen at 40 mA of LED current). Since I like to use worst case, let’s use 100 μA as the current for that worst case. If we were to be fully turned on with all of these settings, what resistor value would give us 100 μA if the sensor transistor was dropping 0.4V across it? The answer is:

(5V – 0.4V)/100 μA = 46000, or 46K ohms

That isn’t a very common value, so let’s choose 47K ohms from our junk box.

We’ve decided that we want an LED current of 20 mA. How do we choose the resistor value for that current? Again, we look at a chart. In this case, we will use the Electrical/Optical Characteristics table again, but the section shown in Figure 5. This shows us that at 20 mA, our LED will drop a maximum of 1.7V across it. So, our resistor value needs to be: