Hi Ian,
Found this post below from 2001, and I am looking to monitor my 24v nav lights. I assume that I just need to alter the 1k resistor to suit 24v vice 12v. Or if there is a better way of doing it???
"Now, for each circuit monitored, take 2 small silicon diodes (signal diodes, 100 mA rating is plenty but 1A 1N4001 series will also do fine) ,a 1K 1/4W resistor, a tricolour led (3 leg, one common, one red, one green (both yellow)) and a reed switch. 1. check if common leg of LED is anode or cathode. 2. connect both diodes in series with RED led of LED. 2. connect reed contact in series with GREEN leg of LED. 3. join remaining reed contact to free end of series diodes on red leg. 4. wire whole assembly to lamp feed through the resistor and ground applied across joint in step 3 and common leg with polarity in accordance with step 1. 5. make sure the lamp feed has enough turns on a bobbin round the reed switch to reliably meet the pullin ampere turns rating. 6. DONT install next to the compass and make sure the coils around the reeds are vertical if within a few metres of the compass. Additionally, it would be worth winding the port and starboard nav light coils in opposite directions and the same for the stern light and steaming light so most stray fields cancel out if the panel is anywhere near the compass. Try it on the bench before putting it on the boat to make sure you have enough turns round the reed switch for reliable operation if the battery voltage is low. DO NOT wind the return from the lamp round the reed as well, it would cancel out the field.
Even if you dont like this idea, I would NOT add resistors in series with the nav light circuits on reliability grounds. 30-60 turns of normal marine wire rated for the circuit current round the reed will not affect reliability.
Action, the 2 diodes make the red LED drop more voltage than the green one so it doesn't come on. Open the green side circuit and the red one now lights. Additional suggestion, if the lamps are fed from fuses, not circuit breakers, wire a 12V flashing LED across each fuse so if the fuse is blown, it flashes when you switch the light on. (dont use with breakers, you can see if they are tripped and it stops the circuit from being totally voltage free with the breaker off and therefore increases the risk of corrosion.) "
Colin Stone