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The best way to learn about comparators is to use one in a circuit. The first thing you notice in Fig. 5.1 is that the comparator looks much like an operational amplifier (op-amp). This is true; comparators are specialized op-amps. The comparator used in our first example is the LM339 quad comparator. This integrated circuit contains four comparators in a 14-pin dip package. Like op-amps the comparator has an inverting and noninverting input. In this particular circuit the reference voltage is placed on the inverting input ( ).
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Out B 1 Out A 2 Vcc URef UIN
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14 13 12 GND A + D + 11 10
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LM 339 Quad Comparator Vcc 2 36 Vdc or 18 Vdc
5.1 Comparator and LM 339 quad comparator IC
Vcc R1 Vref R2
Vcc Vref Vref
5.2 Voltage dividers A, B, and C
Voltage divider
The voltage divider is a simple but important concept. When using it, you will be able to connect most resistance-type sensors to a comparator. The reference voltage is derived from a voltage divider made up of two 10K-ohm resistors (see Fig. 5.2A). The Vref in this case will be half of the supply voltage (Vcc) of 5 V, or 2.5 V (see Table 5.1). We can make Vref any voltage we require between ground and Vcc by adjusting the two resistance values of the voltage divider.
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R2 R1 R2
where Vcc
5 V.
To make an adjustable voltage divider, use a potentiometer as shown in Fig. 5.2B and C. I chose to use the Vref style of Fig. 5.2A because it is simple. The schematic for our test circuit is shown in Fig. 5.3. In place of a sensor we will use two 1K-ohm resistors and one 5K-ohm potentiometer. By varying the potentiometer we can adjust the voltage going to the noninverting input (Vin). The output of each comparator is an uncommitted open collector of an NPN transistor. The transistor can sink more than enough current to light a
I Table 5.1 Two-Resistor Voltage Divider
1K 2.2K 3.3K 4.7K 5.6K 6.8K 10K
Vref, V
4.5 4.1 3.7 3.4 3.2 2.9 2.5
'' '' '' '' '' ''
Vcc R3 10K A R4 B
5 + 12
1K Vcc Vcc
4 3
R2 R5 1K
VRef = 1 2 Vcc
5.3 Comparator test circuit schematic
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light-emitting diode (LED), which we can use as an indicator. In addition, the output may be used as a simple single-pole, singlethrow (SPST) switch to ground. This feature will be useful when we later need to trigger a 555 timer. With the circuit wired, let s see what happens. When the input voltage (Vin) is less than the reference voltage (Vref), the output is 0 V (ground) and the LED is forward-biased and lit. If we adjust the potentiometer so that the voltage is greater than Vref, the output of the comparator goes high, turning off the LED. You can verify the operation of the comparator by using a voltmeter to measure the voltages at points A (Vref) and B (Vin). Many people (myself included) feel this circuit is counterintuitive. I would like the LED to be lit when the sensor voltage is higher than the reference voltage. This can be accomplished by reversing the input leads and connecting the inverting input ( ) Vin and the noninverting input to Vref. The output function reverses also. When one doesn t need too many comparators, you may consider using a complementary metal oxide semiconductor (CMOS) op-amp configured as a comparator. The reason I like to use an op-amp is that it can source (supply) sufficient current to drive an LED or circuit directly (see Fig. 5.4).
Vcc 15K
Vcc 1K
3 + 4
CMOS Op-amp Comparator
15K 1K
* Sub miniature LEDop-amp test circuit schematic 5.4 Comparator
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Light sensors (sight)
There are a large variety of light sensors: photoresistive, photovoltaic, photodiodes, and phototransistors. Light sensors can be used for navigation and tracking. Some robots use an infrared light source and detector to navigate around obstacles and avoid crashing into walls. The infrared source and detector are placed in front of the robot facing in the same direction. When the robot encounters an obstacle or wall, the infrared light is reflected off the surface causing an increase in the infrared light detected. The robot s CPU interprets this increased radiation as an obstacle and steers the robot around it. Filters can be placed in front of light sensors to inhibit their response to some wavelengths while enhancing their response to others. One example of the use of filters is as flame detectors used in fire-fighting robots. One would try to enhance the response to light from a fire while inhibiting the response to light from other sources. Another example is the use of colored gels as filters to promote color response. One could imagine a robot that separates or picks ripened fruit based on the fruit s skin color.
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