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  Low Oil Pressure Indicator Light


Here is a write up from Beech Lister/Mechanic Bob B., which outlines how to retrofit a low oil pressure sending unit into your existing Hobbs oil pressure activation circuit.




The Parts List:


T1-3/4 UltraBright Red LED available as DigiKey 4308H1-ND


4372 Clear Lens for T-1 3/4 LED available as DigiKey 4372-ND


1N4004 Diode available from DigiKey


1500 Ohm 1/2W resistor from DigiKey. Use two of these in series in a 28V aircraft



Here is the schematic:



Another Beech Lister, Lance F., adds the following thoughts:


An easy alternative (fewer parts) is to use a SPDT pressure switch, but I see nothing wrong with the LED/diode/resistor approach except that you're looking at about 8ma of LED current, which won't be very bright unless the LED has very high efficiency.


You could drop the resistance value and increase the resistor wattage to get that up to 15-20 ma for more light.


680 ohms 1/2 watt for 14v = 18ma


1500 ohms 1 watt for 28v = 17.5ma


Here are some sources of oil pressure sending units/switches, courtesy of Beech Lister Eric Toler:


And, if one wants to get really fancy, one could use this adjustable pressure switch:




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