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This is the alarm circuit for XK X250 quadcopter. This circuit will switch on the active buzzer when navigation LED's are off. It will help to know when battery is near empty and can help to spot a location of quadcopter, when it fell in tall grass on bushes by switching the transmitter off or by switching LED's off from better transmitter (X6 Tarantula, V686 etc.)
At X250 control board navigation LED's are controlled from HL004 MCU GPIO output. The best spot to get the signal - at pin 2 of Y1 Zenner diode near M4 motor socket. Signal level here is 0.7V when LED's are ON and 2.68V when they are OFF.
Alarm circuit contains the signal inverter (1st transistor) and high-side switch (2nd transistor) with active buzzer in collector circuit. Input resistor (at left) value may vary between 10 and 47 kOm. Transistors T1 and T2 are common low voltage bipolar transistors. Complementary NPN / PNP pair is preferable. For example:
NPN - BC548, 2N2222, КТ315
PNP - BC556, 2N2907, КТ361
Buzzer - low voltage active buzzer which must be loud enough to be spotted at least from 20 meters (Vmin 3V, >80dB). In circuit it is represented by lamp because I didn't found the matching element in EC toolbar. Circuit is powered from quadcopter 1S LiPo (4.2V to 3.4V).
I tried to minimize the size of circuit as small as possible. You are free to comment and fix the errors, if you're found those.
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