CS wrote in
:

I am trying to devise a simple circuit to monitor when a 24v nav lamp
has blown. Found this
http://www.gammatron.com.au/datashee...0Datasheet.pdf but at
AU$54 it is pretty pricey. Should not be to difficult to make up, but
I need to know he values of the components. I already have the
switch, LED and a resistor to monitor when lamp is on. TVMIA.


Instead of putting the LED/resistor ACROSS the bulb, let's install a
little shunt in SERIES with the hot lead to the bulb that will rob 3
volts from the bulb's voltage when it's operating. We'll then use the
LED and a smaller current limit resistor in series with the LED to
protect the LED from overcurrent if the bulb shorts. INstead of the LED
operating on voltage provided by the switch, the LED will now light ONLY
if there is CURRENT going through a GOOD bulb, remaining dark if the
bulb blows....which it will do LOTS less operating from 21V, not 24V, at
only a tiny difference in brightness. If you're worried about the
voltage, notice how the CHARGING voltage is 28V when the engine is
running, which is what's blowing the bulbs in the first place.

switched shunt
+24VDC----|-/\/\/\/\/\--|-----to light
| |
|-LED--22ohms-|
A K

A=anode
K=cathode

Measure the current the bulb draws by leaving the switch OFF and putting
an ammeter (VOM on 10A scale) across the open switch terminals which
will make the bulb current go through the ammeter. Use the formula:

Ohms = 3V divided by AMPS to figure out the value of the shunt.

Use the formula AMPS X 3 VOLTS = WATTS to find out how many watts the
resistor gets then use a resistor spec that's about double what you
calculate.

The only way for the LED to light, now is when the lamp draws current
through the shunt, not just because there's voltage to ground. The
brightness of the LED is adjusted by varying the value of the 22 ohm
series resistor. You could easily make that a 100 ohm variable resistor
so you can set it like you like to see it. Use any 100 ohm thumbwheel
pot and set the pot to maximum resistance. Then, the first time you
light off the light with this circuit, just reduce the resistance to
increase the brightness but don't go to zero because 3V will destroy the
LED with overcurrent. If the LED gets warm, it's too much.

This circuit works on 12V too, but the bulbs never blow operating on 12V
(14.2V charging minus 3V shunt = 11.2V on 12V bulb.) We haven't
replaced a light bulb in years....

Sorry there's no fancy computer circuitry, LCD screen, big keyboard....
(c;]

Shhh....don't tell anyone, but his same circuit monitors the tower
lights blinking on the 2000' tower at the TV station to make sure it's
lit up so it doesn't attract airplanes.

By the way, the current through the LED is only about 20-80 milliamps
and you cna use the tiniest wire you got to remotely mount the LED far
away from the shunt, which needs to be right were the light's wiring is
now. Our LED panel is over the hatch to the forward compartment and
head so you can easily notice it every time you have to pee, which is
quite often, considering our beer consumption...(c;]