New Low Fuel Alarm Sensor

[Eric Page]

My apologies to anyone who was following this little side project. I posted something about this a month ago, immediately realized I had made an error in the schematic, took it down and immediately got distracted building an airplane! The schematic error has been corrected, so I'll try again.

---

First, here's the interface for using the DS1603NF ultrasonic liquid sensor with an EFIS alarm input:

Schematic: [Obsolete file removed; please see post #36, below, for final version.]

Circuit board:
EFIS PCB.jpg

As you can see, it's dead simple: just a couple of capacitors to clean up the supply voltage to the sensor, a 4.7kΩ pull-up resistor on the active low output, and a transistor to provide a self-test of the system.

The 4-pin connector (J1) on the end of the board mates with the one that comes installed on the sensor pigtail. This circuit will pull the EFIS alarm pin to ground when fuel is present and apply 12V when fuel is absent.

Since it takes some time for an EFIS to boot and it won't show an alarm until that's done, I incorporated a press-to-test function. This is simply a normally open pushbutton switch that, when pressed, turns off a transistor to interrupt power to the fuel sensor, causing it to restart. If you do this after the EFIS is booted up you should see the low fuel alarm while the button is pressed, and for 2.5 seconds after you release it. If the alarm then goes away, the sensor is working properly.

---

Next, here's the interface for use in non-EFIS equipped airplanes:

Schematic: [Obsolete file removed; please see post #36, below, for final version.]

Circuit board:
Flasher PCB.jpg

This circuit flashes an indicator LED when the sensor does not detect fuel. It's a simple 555 timer based oscillator running at ~2Hz, with the output driving a 20mA LED driver IC (Microchip CL520N3-G). The LED driver may seem like overkill, but it has a few advantages:

• It simplifies installation when users choose their own indicator by automatically providing 20mA to drive any LED, regardless of the compliance voltage required, up to the supply.
  
• It protects the output of the 555 timer from overload if the installer tries to use an incandescent bulb or accidentally shorts the output to ground.
  
• It costs $0.45.

I found an illuminated switch that I think would make a nice indicator for the sensor, and would allow the pilot to shut off the flashing to reduce distraction, or test the system by briefly interrupting power. It has a bright red LED inside, and you can install your choice of diffusers and actuator caps in various colors. The cap is meant to have a legend printed on transparency film behind it, which is visible when the LED is turned on. The face of the switch is 0.622" x 0.866" and it will mount in panels from 0.039" to 0.157" thick (my Series 5 panel is 0.070" thick). I'll order a switch, as well as diffusers and caps in white, clear and red to experiment with, to see which combination looks best. The image below shows a narrow-end view of the switch with the diffuser and cap removed.


Switch.jpg

This is my quick first-pass attempt at how an indicator legend might look:

Switch Legend.jpg

---

I'm asking that anyone with electronics experience please take a look at the schematics and feel free to critique my work. I'm not an engineer, so I'm looking for feedback on basic function, robustness and any ideas for simplification.

You could probably reduce parts count by using a microcontroller (and likely the cost, given how cheap they are), but I think that would represent a high barrier for builders who don't have the capability to flash firmware. You could likewise reduce board size by using surface-mount components, but again, it would be a barrier to builders who lack the equipment or skill to solder SMD.

I'll wait a few days to see if anyone has any suggestions. If there are, we can hash that out, then I'll order boards and parts. It'll take about two weeks to get everything, then I'll stuff some boards and give them a test. If they work, I'll wrap this up with instructions on where to get everything and how to assemble the boards.