low battery alarm

[jackorocko]

Because Q3 is off, the +ve end of C1 is effectively connected to ground via the 33ohm resistor. As soon as Q1 turns off, C1 begins to charge through the 330K resistor and the voltage at the base of Q2 rises toward 0.6V

Please explain this too me. The schematic shows an electrolytic capacitor. I don't understand how the capacitor can charge when the polarity is backwards???

 

[wingers]

I did fit the 100uF capacitor in the end but it exploded as soon as I powered up the circuit. Then I tested it without that cap and found that the LED stayed off for all voltages below 12.8. It then turned on dimmly and got brighter until about 14V.

Something is clearly not right with this but I have no idea. I'm going away for the weekend but might think again about this next week. The odd thing is that that's twice I've made the circuit now and both times it did the same thing which is completely different to the intended use.

Unless I can think of something obvious then I think it's time to call this a day and stick to timing flights like everyone else.

Wingers