The parts you will need are a 10 amp silicon diode, a 10 kilohm quarter watt
resistor, a small PNP transistor, a LED of the appropriate color, and a 470
ohm quarter watt resistor. All these parts are available from Mouser
Electronics, DigiKey Electronics, and the rest of the usual suspects.

The silicon (NOT schottky, plain silicon) diode goes in series with the wire
going to the pitot, anode to switch, cathode to pitot. THe diode will
probably require a heat sink.

THe emitter of the PNP transistor goes directly to the anode of the diode.

One lead of the 10K resistor goes to the cathode of the diode. THe other
lead of the resistor goes to the base of the transistor.

One lead of the 470 ohm resistor goes to the collector of the PNP
transistor. The other lead of the resistor goes to the anode of the LED.
The cathode of the LED goes to airframe ground.

HOW IT WORKS: When the pitot switch is turned on, 7 amps through the diode
will result in about 1 volt drop across the diode. A PNP transistor turns on
with 0.5 volts from base to emitter, so the transistor will turn on through
the 10K current limiting resistor. When the transistor turns on, it flows
current through the 470 ohm current limiting resistor to the LED.

If the pitot heater fails there will not be any significant voltage drop
across the diode and the PNP transistor will not turn on and the LED will
not light.

If you can't find a suitable 10 amp diode, a 0.1 ohm 10 watt resistor will
do the same job and will probably be cheaper. In either case, I suspect
that the pitot heater will be more reliable than the diode OR the resistor,
so a false heater-out alarm will be the predominant failure mode.

Be careful where you mount the diode or the resistor. Under normal
operating conditions, they will get hotter than hell.

Jim







"iflycozy" wrote in message
ups.com...
Question to you electrical designer gurus:

Here is what I would like to do and Jim Wier suggested that I post this
here. I want an indicator on my Cozy homebuilt airplane to show me
that my heated pitot is actually working when I turn on the switch on
the instrument panel. So, the light will go out (with the switch still
in the on position) if the heated pitot stops working for some reasons
(but not because it tripped the circuit breaker). So, how can I build
such a device or circuit? I would appreciate specific parts or
identification of parts as I am an amateur. This is a 12 volt DC
system and the heating element draws 7 amps. I know I can not use a
LED in series because it would blow the milli-second I turned the unit
on. I know I can't use a light or lamp in parallel because it would
not indicate if the heated pitot was on or off. So that is my dilemma.
Please help.

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