It is not wise to operate a solenoid for a long period of time off a battery, small pulses are OK if you mitigate the other issues Batteries have an internal resistance, for short duration pulse loads you can treat them like a power supply with a resistor between the supply and your load, this means drawing a large amount of current will make the voltage on your load decrease, and generate some heat in the ESR / resistor, The value of this ESR tends to get worse as the battery depletes, and changes with temperature, This is making some assumptions, but lets say your solenoid resistance is about 22 ohm (9V / 0.4A), a 9V battery is normally 1.5-5 ohm ESR for the first 50% of its capacity For your solenoid, you could on a proper supply work out what its 'Actuation Voltage' is, e.g. How can a 9V survive long like that? How is it that a 9V operated electronic safe is common place when they use a solenoid? Thanks FBOARC

It appears that the solenoid is drawing over 400ma when activated. triggers the solenoid for a short period when the combination is entered. Stays awake if a button is pressed within a short period. The MCU circuit to control the solenoid was taken from The code puts the ship to sleep. When the solenoid is activated it pulls the pin out of the way and the safe handle is allowed to be turned and open the door. The safe locking solenoid does not need to capable of much force at all but how do I control it with a 9v battery and not drain the battery very rapidly? The solenoid at rest has a very weak spring that keeps the pin out and blocks a mechanism keeping the safe locked. The short version of this story is success.almost. I have an adafruit ItsyBitsy 32u4 chip and I wired up the 4x3 keypad. I thought it would be a good beginner micro controller project to replace the electronics.

I guess its a piece of junk as the electronics failed with in days of getting it. I have a gun safe with an electronic lock.

Sentry safe open circuit solenoid