I decided to preemptively replace my Plane Power AL12-EI60 alternator at 513 hours. It whines like a coyote whereas my backup B&C BC410-H is as quiet as a mouse. Late last year, B&C released the SF601 internally regulated alternator which is what I went with.
That new B&C alternator doesn't have an indicator output so I redesigned my indicator circuit board to make the ALT indicator a low voltage light and also made each flight flash when they are enabled (other than the pitot heat light, because that would be annoying) to help grab my attention. The new board also uses quick-connect tabs to make servicing it easier and it has less exposed trace area. And since the ALT light is now a low voltage light, I won't need that tiny circuit board I made to ensure the ALT light always indicated the status of which of the two alternators was selected.
Here's what the new board looks like in CAD (easier to make out the details with the CAD rendering than with an actual image of the board). The other side of the board attaches to the LEDs and indicator test push button. After I soldered it up, I sealed it with lacquer.
The following schematic outlines what I designed. A 555 timer is used for a TOn of ~0.7 s and a TOff of ~0.3 s. Since the 555 can't source much current, it controls a high side PFET which connects and disconnects +12 V from the LEDs in concert with the 555's Q output. The Canopy Open and Oil Pressure LEDs will thus blink when enabled (active low).
Q2's base is pulled high by the PFET output which normally enables the ALT LED. But a 13 V Zener diode with a few resistors will break down above about 13.7 V, causing Q1 to turn on which turns off Q2, which shuts off the ALT light. Thus the ALT light will be off when the main bus voltage is about 13.7 V and higher and blinking when it's lower than 13.7 V, indicating a low voltage situation on the main bus.
A 1 μF bypass cap on the power input was necessary to clean up the 555's output. And a 3.3 μF cap was necessary across the ALT LED since when the PFET turns off, the LEDs' power line drops below 13.7 V, causing a dim and momentary flash on the ALT LED even when the bus voltage is higher than 13.7 V. That 3.3 μF cap across the ALT LED holds the voltage high enough for long enough during the TOff phase that the LED won't briefly try to turn on. Since it's only 3.3 μF, it doesn't cause the ALT LED to fade off when being illuminated. Capacitors across the Canopy Open and Oil Pressure LEDs aren't necessary since when they are not illuminated, their cathodes are floating so there is no current path available.
As before, an "indicator test" input will momentarily turn on all LEDs to verify they work. This would cause the Oil Pressure, Canopy Open and ALT LEDs to flash and the Pitot Heat light to turn on solid.
Video showing canopy being locked/unlocked with lights flashing.
Circuit board parts list:
- 555 timer: Texas Instruments SE555DR
- Capacitor 3.3 μF: Nichicon UVR2A3R3MDD
- Capacitor 1 μF: Nichicon UPX1V010MPD
- Diodes: Kyocera AVX SD1206S040S2R0
- PFET: Infineon Technologies: IPP120P04P4L03AKSA2
- Quick connect tab, 0.25": Keystone Electronics 4902
- Resistor 4.7 k: Panasonic Electronic Components ERJ-8GEYJ472V
- Resistor 10 k: Panasonic Electronic Components ERJ-8GEYJ103V
- Resistor 270: Panasonic Electronic Components ERJ-8GEYJ271V
- Transistor: onsemi MMBT2222ALT1G
- Zener 13 V: Vishay General Semiconductor BZX55B13-TAPGICT-ND
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