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14.4 Sample Output Circuits
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The most simple output circuit is the LED driver (Fig. 14-3) in which one of the output lines of the robot controller applies either a high or low voltage to turn on or off an LED. The 470-ohm resistor is used to limit the amount of current passed through the LED to 5 to 6 mA. Outputs typically drive heavy loads: motors, solenoids, pumps, and even high-volume sound demand lots of current. The typical robotic control computer cannot provide more than 15 to 22 mA (milliamps) of current on any output. That s enough to power one or two LEDs, but not much else. To use an output to drive a load, you need to add a power element that provides adequate current. This can be as simple as one transistor, or it can be a ready-made power driver circuit capable of running large, multi-horsepower motors. One common power driver is the H-bridge, so called because the transistors used inside it are in an H pattern around the motor or other load. The H-bridge can connect directly to the control computer of the robot and provide adequate voltage and current to the load.
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COMPUTER PERIPHERALS
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Robot Controller
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When Output is High LED is On When Output is Low, LED is Off
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FIGURE 14-3 Robot controller connection to a simple LED.
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10K 1K Input b e c Output
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4011 (1/4) g 3
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d Q1 s
Load
1K Input b
14 1 Control 2 7
4011 (1/4) g 3
d Q1 s
e Output 10K
Load
FIGURE 14-4 NPN bipolar transistor drivers.
FIGURE 14-5 Power MOSFET drivers.
14.4 SAMPLE OUTPUT CIRCUITS
+V +12V 14 1 2 7 4011 (1/4) g 3 s d Q1 D1 C1 0.1 D3 d Q3 s g
Direction control
Forward 1 Reverse 0
4011 (1/4)
d g s Q2 D2 D4 Q4
d g s
D1-D4: 1N4002 Q1-Q4: n-channel MOSFET
FIGURE 14-6 Discrete component H-bridge motor driver.
Figs. 14-4 through 14-8 show various approaches for doing this, including NPN transistor, power MOSFET, discrete component H-bridge, single-package H-bridge, and buffer circuits. All have their advantages and disadvantages, and they are described in context throughout this book. See especially 20, Working with DC Motors, and 21, Working with Stepper Motors, for more information on these power drive techniques.
V+ V+
Enable/PWM 2 Direction Brake
7 741 6 Output
Input
FIGURE 14-7 Packaged H-bridge motor driver.
FIGURE 14-8 Non-inverting buffer follower interface.
COMPUTER PERIPHERALS
Microprocessor/ Microcontroller Input
FIGURE 14-9 Switch sensor connected directly to a computer input.
14.5 Digital Inputs
Switches and other strictly digital (on/off) sensors can be readily connected to control electronics. Figs. 14-9 through 14-12 show a variety of techniques, including direct connection of a switch sensor, an LED high/low voltage input and indicator, interface via a switch debouncer, and interface via a buffer. The buffer is recommended to isolate the source of the input from the control electronics.
14.5.1 INTERFACING FROM DIFFERENT VOLTAGE LEVELS
Some digital input devices may operate a voltage that differs from the control electronics. Erratic behavior and even damage to the input device or control electronics could result if you connected components with disparate voltage sources together. So-called logic transla-
Vcc 470 When Vcc is +5 Volts: 1 is 3.4 to 3.6 Volts 0 is 0.0 to 0.2 Volts
SPST Switch
FIGURE 14-10 LED used to indicate the logic level (high or low) or a switch input.
14.5 DIGITAL INPUTS
120K 6 7 555 2
Microprocessor/ Microcontroller Input
S1 0.1
FIGURE 14-11 Switch debouncer input.
tion circuits are needed for these kinds of interfaces. Several integrated circuits provide these functions in off-the-shelf solutions. You can create most of the interfaces you need using standard CMOS and TTL logic chips. Fig. 14-13 shows how to interface TTL (5 V) to CMOS circuits that use different power sources (use this circuit even if both circuits run under +5 vdc). Fig. 14-14 shows the same concept, but for translating CMOS circuits to TTL circuits that use different power sources.
14.5.2 USING OPTO-ISOLATORS
Note that in both Figs. 14-13 and 14-14, the ground connection is shared. You may wish to keep the power supplies of the inputs and control electronics totally separate. This is most easily done using opto-isolators, which are readily available in IC-like packages. Fig. 14-15 shows the basic concept of the opto-isolator: the source controls a light-emitting diode. The input of the control electronics is connected to a photodetector of the optoisolator. Note that since each side of the opto-isolator is governed by its own power supply, you can use these devices for simple level shifting, for example, changing a +5 V signal to +12 V, or vice versa.
Buffer or Inverter (Schmitt Trigger Shown) +V or Gnd S1 Microprocessor/ Microcontroller Input
FIGURE 14-12 Buffered input.
COMPUTER PERIPHERALS
+5vdc
+12vdc (or higher than TTL supply)
10K Input TTL (Any Gate) Output 1K b 2N2222 c e CMOS (Any Gate)
FIGURE 14-13 TTL-to-CMOS translation interface.
+12vdc (or higher than TTL supply)
+5vdc
10K CMOS (Any Gate) Output 1K b 2N2222 c e
Input
TTL (Any Gate)
FIGURE 14-14 CMOS-to-TTL translation interface.
FIGURE 14-15 Opto-isolator.
14.6 INTERFACING ANALOG INPUT
Limiting Resistor (as needed) Input Zener Output
Zener
FIGURE 14-16 Zener diode shunt.
14.5.3 ZENER DIODE INPUT PROTECTION
If a signal source may exceed the operating voltage level of the control electronics, you can use a zener diode to clamp the voltage to the input. Zener diodes act like valves that turn on only when a certain voltage level is applied. As shown in Fig. 14-16, by putting a zener diode across the +V and ground of an input, you can basically shunt any excess voltage and prevent it from reaching the control electronics. Zener diodes are available in different voltages; the 4.7- or 5.1-V zeners are ideal for interfacing to inputs. Use the resistor to limit the current through the zener. The wattage rating of the zener diode you use depends on the maximum voltage presented to the input as well as the current drawn by the input. For most applications where the source signal is no more than 12 to 15 V, a quarter-watt zener should easily suffice. Use a higher wattage resistor for higher current draws.
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