Automatic switch from battery and DC Power Supply. Main switch at top is actually DPDT where one leg goes to the DC jack and the other to the battery supply. This circuit shuts off (mostly) the battery path if the battery is switched on and you plug the DC plug in.
The left power source is a cell phone battery hooked up to a "1A 5V Micro USB TP4056 Lithium Battery Power Charger Board Module TE420" and "MT3608 Step-Up Adjustable DC-DC Switching Boost Converter Power Supply Module 2-24V to 5V-28V 2A" and I'm too lazy to draw those in lol. The switch is between the charger and converter to keep the battery from discharging while not in use.
The right switch is just a representation of the 9v adapter where open is not plugged in.
I created this circuit as a addition modification to the DSO 150 Shell Oscilloscope. All of the other battery addition modifications I have seen make the 9V jack unusable and that seemed like a waste, what if you did not have a micro USB charger and cord handy and the battery died. Well now you can use the battery, the micro USB plug, or the 9V jack for power.
The main issue is that when you have the 9v plugged in and the battery switched to the off position you have 8.5ish volts going from the base to emitter. The max on the spec sheet for 2N3906 is 5v.
Added some bleed off resistors to get rid of the charges in each line after switched off.
