R/C Aircraft backup power, 9-volt

9V Backup Power for R/C Aircraft by Tony van Roon

Backup for large aircraft

A Couple Notes:
You will need a 1N4007 Diode, a (General Purpose) PNP transistor, two ceramic capacitors, switch harness, buzzer, and a small amount of Vero Board, or you can make a pcb! Its up to you. Replacement values for the BC558 are NTE159, ECG159, etc.
CAUTION: If you use the replacements, watch the orientation of collector/emitter, since it may be reversed from the european format (example: BC558=CBE, NTE159=EBC).

Diode(s) Dx is the place where you insert as many diodes as needed (even schottky) to obtain the required voltage drop. For a 9-volt alkaline battery that is about 5 diodes. 6 x 0.7 = 4.2V. 9 - 4.2 = 4.8V which is the voltage drop when the backup is switched on and so more than adequate.
D1 must be a silicon type only because the voltage drop of a schottky type is too low for Q1.

To test the backup battery, you simply switch the backup battery on, and the Led should be lit. You should have power to the receiver via the diode. Then switch on the main battery, and the Led goes off. This indicates that the main battery is ok. Should you land, and the red Led is on, then you are running on the backup battery, and it's time to go home.
The two capacitors absorb possible micro-spikes; C1 is extra protection and may not be needed depending on the quality of your receiver. However, it cost almost nothing and does no harm.\

TEST THE CIRCUIT before hooking it up to your (expensive) receiver!