Construction
The first step is to carefully remove the glass from the old lamp. A safe way to do this is to wrap the old lamp in a tissue or paper towel and crunch the glass with a pair of pliers. The object here is to totally remove the glass and dispose of it without getting any of it in your eyes, nose, mouth or fingers.

Next, with a small screwdriver scrape out enough of the insulating material and wire supports from the exposed metal base to enable the conductors for the filament to be unsoldered and removed. You are now ready to do a component replacement.

Light emitting diodes operate on a very low voltage, around three volts. To use them on higher voltages a dropping resistor is usually added in series with the LED to reduce current. While LED characteristics vary, a dropping resistor of around 680 Ohms to 820 Ohms (readily available industry standards) will do for 12-volt use. The lower the resistance, the more light is produced by the LED, but more strain is placed on the LEDs as well.

Go to your local electronics store and purchase one red LED, one green LED and one or more dropping resistors of around the above value. A couple of cheap diodes may also be required, as noted below. It may be wise to temporarily “breadboard” your circuit together on a tabletop before construction.

The LEDs will have a flat surface below the barrel on an otherwise circular plastic ring. This flat surface identifies the wire lead on that side as being the cathode, the lead that must be connected to the negative side of a direct current supply. Also, the positive lead is slightly longer than the negative lead. Test each LED in turn by connecting it in series with the resistor and putting the two series components across a fused 12-volt supply, with the resistor on either side of the LED.

When the two LEDs and the resistor have been proven to work, connected as in Figure 1, wire them into the metal lamp base. One end of the resistor is soldered into the tip of the base of the old lamp and the other end is soldered to the anode of the green LED and the cathode of the red LED. These may be reversed if it suits your preference, but reverse both LEDs if you do this. The other side of each LED is soldered to the metal lamp base, as seen in Figure 2.

The LEDs must be capable of withstanding up to 15 volts of reverse voltage. Your supplier can tell you if this is the case. If not, or if in doubt, purchase a couple of inexpensive diodes to protect the LEDs, which are inserted in series with each LED. One end of the diode will have a mark or ring on it and this end is connected to the LED terminal. Like the dropping resistor, the (blocking) diode can be located on either side of the LED, but polarity is important. If you think in terms of conventional current, which comes out of the positive terminal of a battery, the current will flow through a diode if it comes out the end with the stripe, and will flow through an LED if it comes out the end with the shorter wire on the side with the “flat.”

Testing the tester
Now when the tester is connected across a 12-volt source, one, and only one, of the LEDs will illuminate to indicate not only that voltage is present, but also its polarity. The green LED will illuminate when the alligator clip is connected to the negative side of the supply and the tester tip is connected to the positive side. Conversely, the opposite polarity will cause the red LED to illuminate. Should you choose to have the opposite indication simply reverse the lamp connections. On your tester the tip of the globe may be connected to the wire and alligator clip, rather than to the tip of the probe, and that also will change which LED lights with a particular polarity. Also, if a strong alternating current voltage is present, both LEDs will light.

When you are satisfied with the unit place some silicon sealant around the resistor, filling the old lamp base to prevent mechanical shocks from disturbing the wiring.

This tester allows you to test polarity of a supply before connecting it to loads that might be damaged by reverse polarity. Many high-quality instruments and radios have reverse polarity protection built in, but many do not, and connecting improperly can blow transistors.

If you are looking for an open circuit with a load connected, knowing the polarity on both conductors will tell you whether the open circuit is on the positive or negative supply upstream of your location. One side of the tester can usually be clipped to a ground point somewhere, which is convenient on DC systems with a grounded negative (or positive). Of course a voltmeter can be used to determine this, but in a dark location an LED is easier to see than a voltmeter. And a replacement is much cheaper if you drop it in the bilge.

Now you have a better tester for less than $1 worth of components, and it won’t burn out—unless you connect it to a much higher voltage. If your system operates at 24 volts, increase the resistor value to around 1500 Ohms, which will still light the LEDs, although a bit dimmer, when used on 12 volts.