Last week I mentioned relays. We are going to talk about those this week, along with switches, light bulbs, LEDs, diodes, resistors, and turnout motors.I'll sart with the simpler stuff and work my way to the more complicated. Light bulbs and LEDs give off light. We all know this. Last week I showed you how to use them as headlights. They can also be used as marker lights, interior lights, structure lights, signals, or indicator lights for panel boards using similar circuits that I used yesterday. They all have a voltage and current rating. If the voltage rating is below the voltage of your power source, you need a resistor, which I showed you how to do last week. I also showed you how to put two LEDs or bulbs in parallel on the same resistor. You can also put them in series with each other. For example, the 2 volt LEDs that I use can be wired without a resistor at all if I put 6 of them in series on a 12 volt power source, because each of them uses 2 volts so the whole string uses the full 12 volts.
Diodes let current pass in one direction but not the other. An LED is actually a diode that emits light (light emitting diode) but we don't often use them as diodes. Diodes can be used in locomotives with DC power to establish directional headlights. Reversing the locomotive in DC mode means reversing the polarity, so putting a diode on the front headlight means it only lights when the locomotive is going forward. The same can be done with the reverse headlights. Diodes have many uses in electronics, but not a lot in model railroading.
Resistors are just that...they use up electricity and do nothing. But they are very helpful. We use them to drop voltages to useable levels. They are not polarity or voltage sensitive, which makes them easy to use. Simply put them in the circuit in series with whatever needs a lower voltage, and you're good to go. We covered the calculations involved two weeks ago.
We all know what a switch is, but let me introduce you to a few different kinds. All switches have a common terminal and one or two travelers, or switched leads. Some switches have multiple poles, which means they have multiple sets of what I just described. Switches are used on panelboards to route power to different places. They can turn interior lights in structure on or off, they can throw powered turnouts, they can change signal indications, they have many uses. Rotary switches are switches with more than two possible positions. They can be used for complex yard ladders to select which track you want to go on and throw the turnouts accordingly. That is a more complicated circuit that I have not tried, so I will not talk about it. There are two good articles about it in recent Model Railroader issues if you want to look it up.
Turnout motors are used to automatically move the points of a turnout with the throw of a switch on the panelboard. They work by reversing the polarity on the motor, which moves a stiff wire, which moves the points. Reversing the polarity can be done in two different ways. The easy way is to take a double pole, double throw switch (a toggle switch with six contacts) and connect the two motor leads from the turnout motor to the two common leads on the switch. Next, connect the power source to two of the traveler leads, both on the same throw but opposite poles. Next, connect those two travelers diagonally across the switch to the other two travelers, so the polarity is reversed at that point. That way, when you throw the switch, the polarity to the switch motor changes.
A second way to accomplish this is by using a 2 phase power source. This is easier to do than it sounds. Simply take two identical 12 volt DC power sources and connect the negative on one of them to the positive on the other. This creates a common wire, and the other negative and positive left over create the negative and positive wires. Now you get 12 volts DC at opposite polarities across either the negative and common wires or the positive and common wires, and you get 24 volts DC across the negative and positive wires. A similar setup, but with AC, is what gets you both 120 volts and 240 volts in your house. Take the common wire and connect it to one lead on the turnout motor. Take the negative and positive wires and connect them to the two travelers on a single pole, double throw switch. Connect the common lead on the switch to the other motor lead on the turnout motor. Now you can change the polarity to the turnout motor.
Turnout motors have another useful feature built into them. They have a relay, which is nothing but an electrically or mechanically activated switch. They have a common and two travelers, or two commons and four travelers, like any toggle switch. These can be used to change signals, they can throw other turnouts on complex yard ladders, and on DC powered layouts they can be used to not allow power to the track that has the turnout set against it. However, turnout motors aren't the only place you can find relays. If you need more then the two sets of contacts on the turnout motor, you can buy relays on websites such as allelectronics.com. I've used some relays from there and they work great. Just make sure the rated voltage and current is what you need.
That brings us back to where I left off last week. I wanted to run ditch lights on both ends of my locomotive with only 2 available functions. To do this, I got a double pole, double throw miniature relay. I connected the wires for the reverse headlight function to the reverse headlights and to the relay coil. I connected the wires for the ditch light functions to the commons on the relay, and then the travelers went both forward and backward from the relay to light the ditch lights. Now, when the forward headlights are on, the relay coil is not energized, and the power for the ditch lights is routed to the front of the locomotive. When I change the direction of the locomotive, the reverse headlights come on, the relay coil is energized, and power for the ditch lights is routed to the back of the engine. This is all done through the magic of DCC, which I will talk about next week in the last installment of this series.
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