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Figure 21.1 The model train three-traf c-light control circuit.
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least bit unsure of what you are doing or you are not fully comfortable with working with ac voltages. The circuit shown in this application is designed only for the 18 V ac that is available from model train transformers for driving accessories. Using 18 V results in very little danger of electrical shock but short circuits can result in high currents that can burn you or cause re if you are not careful with the circuit. The bill of materials for the application is listed in Table 21.1. The circuit has a 5.1-V, 50-mA power supply that also provides half-wave-recti ed power from the 18-V ac source. This sounds like a pretty fancy speci cation, but it actually uses the simply regulator circuit shown in Fig. 21.3. The Zener diode limits the voltage across it to 5.1 V, and this voltage is used for powering the PIC microcontroller that controls the circuit. The silicon diode allows current to pass in only one direction. These two diodes have a combined voltage drop of 5.8 V. Since I want to
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GP0 6 GP1 5 GP2 GP3 4
TRIAC2 TRIAC3
220 Ohm 1 Watt
AC Common Train Sensor Input
Figure 21.2 The model train light-controller circuit is actually quite simple.
PROJECTS
TABLE 21.1
TRAIN CONTROLLER BILL OF MATERIALS DESCRIPTION
REFERENCE DESIGNATOR
PIC12C508 5.1 Zener diode 1N914 silicon diode Gate current TRIAC1 TRIAC3 220 330- F 0.1- F Train sensor Miscellaneous
PIC12C508 JW 5.1 V, 1/2 W 1N914 silicon diode Forward current 6 A, 25 mA 220 , 1 W resistor
330 F, 35 V electrolytic capacitor 0.1 F capacitor, any type Hall-effect sensor (see text) Prototyping PCB, wiring, screw terminals for electrical connections
provide 50 mA to the PIC microcontroller, I use this information to calculate the resistor value needed. Using Kirchoff s and Ohm s laws, Vac 18 V Vr R VZener 5.1 V 18 Vdiode 0.7 V 5.8 V Vr Vr
12.2 V@50mA V/I 12.2 V/50 mA I used a 220244 resistor in my circuit because it is a standard value.
Vcc - Logic Power 0.1 uF
330 uF 30 Volts 18 Volts AC In
5.1 V Zener 1N914
Logic Ground 220 Ohm 1 Watt
Figure 21.3 supply.
Model train 18-V ac to 5-V dc power
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With 50 mA of current going through the 220dissipated is P V I 12.2 V 0.61 W
resistor, the maximum power
50 mA
This is actually a reasonable amount of power, and this is why I speci ed the 1-W resistor. The maximum power going through the diode is P V I 0.7 V 50 mA 0.035 W
which is actually quite low a small signal diode such as the 1N914 can be used safely. The large (330-mF) capacitor is used to maintain an even voltage to the PIC microcontroller even when the diode isn t conducting. This capacitor is very effective because I measured a 10-mV ripple on the PIC microcontroller s Vdd relative to Vss (or what I call logic Gnd). The 0.1- F capacitor is used for decoupling the PIC microcontroller and should be as close to the Vdd and Vss pins as possible. TRIACs are interesting devices and come under the heading of thyristors, which can be used to switch ac signals on and off. TRIACs do not rectify the ac voltage because they consist of two silicon-controlled recti ers (SCRs), which allow the ac current to pass without any clipping. A typical circuit for TRIACs is shown in Fig. 21.4. TRIACs do not allow ac current to pass unless their gates are biased relative to the two ac contacts. To do this, I pull the gates to logic Gnd by the PIC microcontroller. As noted in the bill of materials (Table 21.1), the current required to close the TRIAC is 25 mA, which can be sunk by most PIC microcontrollers. The load in this project are colored grain of wheat bulbs that are designed for model train applications and can handle the 18-V ac accessory drive provided by the
TRIAC AC Power Input Control Logic Signal AC Output to Load
AC Common
Figure 21.4 The TRIAC acts like a switch, controlling the ow of ac power to a load.
PROJECTS
Magnet Deflected Current Sensor Non-Deflected Current Sensor Current Path Inside Magnetic Field
Silicon Current Source Current Path if No Magnetic Field
Figure 21.5 The Hall-effect switch detects the change in ow of current through a semiconductor.
model train transformer. For this application s circuit, I have put three of these circuits in parallel to control three separate lights in a traf c light con guration. To detect the train, I used a Hall-effect switch with magnets glued to the bottom of the train and its cars. A Hall-effect switch is a clever device in which if a current passing through a piece of silicon is de ected by a magnetic eld, the output changes state, as I ve shown in Fig. 21.5. The Hall-effect switch that I used is an open-collector device that requires a pull-up on its output, which can be provided by either an external resistor or the internal port pull-ups of the PIC microcontroller. You ll nd that you will have to play around with which magnet pole works best. For the parts I used, I found that the North pole and top edge of the Hall-effect sensor worked best. Other sensors could be tried in this circuit which is why I left it somewhat vague. Along with Hall-effect sensors, light-dependent resistors or even simple switches could be used for the application. The application code is very simple, barely over 100 instructions, and it turns the green light on until the sensor detects the passage of a train. When this happens, the green is turned off, and the amber light is turned on for a second. After the second, the amber light is turned off, and the red light is turned on, and the Hall-effect sensor is polled. The red light will stay on until the Hall-effect sensor has not returned on for 10 seconds. This code, while being very simple, very nicely simulates the behavior of traf c lights with a traf c sensor. The TrainCtl application, which is found in the code\trainctl folder, can be described using the following pseudocode:
main() { red = on; dlay(5 seconds); while (1 = = 1) { red = off; green = on; // Green light on // // Train Control Application Initially Stop Traf c
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