Tell-Tale lights

Many voyagers use sophisticated system monitors to keep track of their electrical system’s capacity. Another, less well-known, way to get a feel for the activity of an electrical system is via the use of low-cost annunciator lights. Sometimes called "idiot lights" when used on automobiles, these simple indicators can be put to good use in telling what your electrical system is doing.

The term idiot lights appears to have originated in autos when Detroit manufacturers, ever ready to reduce the cost of their cars, decided to replace analog oil-pressure gauges and ammeters with indicator lights. While hard-core car aficionados were distressed at the demise of gauges, many cars were undoubtedly driven for miles with gauges that unobtrusively indicated, for example, low oil pressure. However, this didn’t stop the motoring press and drivers from assigning a negative meaning to what were,in reality, quite useful annunciator lights. The depreciatory term carried over into other engine-driven apparatus, including boats. Annunciator lights remained honored only in aircraft cockpits, where their ability to command immediate attention makes a vital contribution to safety. The same ability to call attention to a system’s operating status, normal, intermittently abnormal or dangerously abnormal, can be of equal value on a boat.

Most marine engine installations include annunciator lights for low oil pressure and high coolant temperature powered from sensors that are separate from those that provide signals to the temperature and oil-pressure gauges. In addition, these annunciators are usually connected to an aural alarm. A third standard annunciator is an alternator charging-failure light, which may be backed up by an analog voltmeter. Use of redundant sensors for these functions can be very valuable, both in maximizing the probability that the boat operator will be alerted to a malfunction and by aiding an immediate assessment of the cause for the annunciator indication a real problem or a malfunctioning sensor.

While this typical, three-light engine-annunciator group is worthwhile, it is by no means as complete as it should be, especially on sailboats where visits to the cabin can be infrequent, allowing problems to go undetected for protracted periods. For example, a freshwater tap left partially open can drain even a large tank in a short period of time, and the constant cycling of the freshwater pump can also cut into the capacity of a battery. A normal drip from the stuffing box can become a serious leak and may go undetected if the automatic bilge pump is operating properly. Alternatively, failure of the automatic bilge pump or its switch may allow an undesirable amount of water to collect in the bilge. Low battery-voltage annunciator

To obtain reasonable life from the deep-cycle, lead-acid batteries used in the house battery bank of a boat, it is important to limit the depth of each discharge cycle to not more than approximately 50 percent of the batteries’ capacity. This depth of discharge can be determined by measuring the output voltage from the battery. The measurement’s validity will be best when the drain on the battery is limited to not more than about 10 percent of its amp-hour rating; for example, 10 amps for a 100 amp-hour battery. Fortunately, the output voltage of a lead-acid battery varies quite linearly with percent of charge. At 100 percent of charge the voltage will usually be 12.6 volts or slightly higher. Each 0.2-volt decrease corresponds to a 25 percent decrease in the batteries’ state of charge. A reading of 12.4 volts = 75 percent charged, 12.2 volts = 50 percent charged, 12.0 volts = 25 percent charged and 11.8 volts = full discharge (for the purpose of calculating the amount of energy remaining in a deep-cycle battery).

You can avoid discharging any battery too deeply by monitoring the output voltage of the battery in use and switching from a battery whose voltage has decreased to 12.2 volts to one with more available energy. When all batteries are down to 12.2 volts, they may be paralleled and recharged as a group. This technique will also minimize the time required to recharge the battery bank and make optimum use of most marine engine alternators, using their full output to carry the ship’s load and recharge the battery bank.

You can either buy a monitor or build one. Commercial voltage monitors are available at quite reasonable prices. For those who would like to put one together themselves, the total cost, including components and a small plastic box to house the finished device will likely be less than $20. Other uses

Here are some other cases where an annunciator light is useful. Once you start installing these useful lights, you will undoubtedly think up a few applications of your own. For example, a generator set, turned on to power the air-conditioning system to cool off the boat’s interior during the hour before reaching shore, can go off-line without anyone in the cockpit knowing of its failure. Or the temperature in the fridge or freezer may go above safe limits if something as simple as a door latch malfunctions. Some of these conditions are simply annoying; others can be troublesome or, in the event you run out of potable water three days into a 10-day trip, decidedly unpleasant. Fortunately, each one of these conditions lends itself to the simple addition of an annunciator light and, if desired, an aural alarm.

A cockpit annunciator module, using 12-volt incandescent lamps or high-brightness LEDs, can be built into a small, plastic enclosure and located close to the helm station. The required circuitry is not complicated. Annunciator lights for the bilge and freshwater-pump functions require only a wire from each pump’s positive terminal to the associated indicator lamp. A flooded-bilge alarm can be created with the addition of a bilge-pump switch, mounted well above the normal, high water level in the bilge. Powering this annunciator from a nine-volt lithium battery will make its safety function even more reliable by making it independent of the boat’s electrical system. The addition of a relay, connected in parallel with the annunciator lamp, allows the flooded-bilge alarm to be connected to one of the boat’s normal, deep-cycle batteries, providing the power to sound a loud bell and activate a strobe light. Placing the bell and strobe light in the cockpit when the boat is unoccupied can alert marina personnel to a dangerous flooding situation.

Adding an annunciator to indicate normal, genset AC-output power is a bit more complicated but still quite simple to accomplish, especially if the annunciator light is to be illuminated whenever the genset is producing normal output voltage. Small, economical, battery-powered, digital temperature displays can be added to the boat’s refrigerator and freezer. These devices include alarm set points that can be used to trigger cockpit annunciators if the temperatures become dangerously high. All of these annunciators can be added at little cost and with a reasonable expenditure of time. And you don’t have to be an electrical engineer to plan and accomplish the work.

The circuits for the bilge and freshwater pump "on" indicators are shown in figure 1. Since these two systems operate whenever power is applied to the positive terminal of their motors, all that is required is a single wire from each motor terminal to the respective annunciator light in the cockpit.

The flooded-bilge alarm shown in figure 2a requires an additional bilge-pump switch wired to the positive terminal of a nine-volt battery (a lithium battery is recommended for its long shelf life, generally in excess of five years). The negative terminal of the nine-volt battery can be connected to the boat’s negative battery bus, if desired, or the circuit can be made completely separate from all other wiring. A modified circuit that can also power an on-deck gong and strobe light is shown in figure 2b. The relay is powered from the nine-volt battery when the bilge-level switch closes in the presence of high water. The relay contacts are used to power a 12-volt bell or gong and a strobe light. A sonalert or other small acoustic device can be wired in parallel with the annunciator lamp in either figure 2a or 2b.

The genset annunciator circuit is shown in figure 3. In this circuit the 120-volt coil AC relay senses the presence of normal AC power from the genset. The relay makes contact, completing the 12-volt DC circuit, and lighting the annunciator lamp in the cockpit. Using the relay eliminates the need to run 120-volt AC power into the cockpit. This annunciator could be implemented using a solid-state opto-isolator; however, the relay is simple, easy to understand and insensitive to stray currents.

Sensing temperature in the fridge or freezer, or anywhere else for that matter, is accomplished by installing one or more of the small, AA, cell-powered, LCD-readout temperature monitors such as those sold by RadioShack (item # 910-4911, $19.99 in 2001 catalog). These temperature sensors have programmable alarm circuits that can power a small lamp or DC relay when the temperature goes out of range, either above or below the set point. The digital readout can then be located in the galley so that the actual temperature of the food storage compartment can be checked.

Wiring the various annunciator lamps to an aural alarm is not difficult. The currents involved are very low, allowing use of small-gauge, multi-conductor wire. A solid-state acoustic alarm, a sonalert, (RadioShack catalog # 273-057, 108 db) can be wired across the flooded-bilge alarm where it will be powered by that system’s independent power source. If desired, separate sonalerts, each emitting a different frequency, can be connected across each of the other annunciators. So if everything goes wrong at once, the result will be an energizing musical event.

Contributing Editor Chuck Husick is a man of many talents, including sailing, flying, photography and the design and repair of all things electrical and mechanical. He lives in Tierra Verde, Fla.

By Ocean Navigator