Power outage alarm

Sounds like a school homework project. Here are some hints.

Firstly, you need to look at the pico power supply circuit.USB is 5v . Vin you are saying is 9v, but inside its converted to something else. There should a blocking diode provided so the USB volts doesn't backfeed the Vin if Vin gets lower than USB.

Use a voltage divider to attenuation the voltage of both USB and battery, and bring in as analog inputs. Use code to set limits and the levels for making low voltage alarms. You'll want to know if the 9v battery is OK.

Use a counter or real time clock for elasped time. The 2 second delay is easy code, proably an interrupt would be 

Use a piezoelectric buzzer for the alarm. There are integrated circuits that find the buzzer resonance and drive it there so they are really loud.

Thanks for your help. As power outages become more frequent due to storms, I need an alarm to save the contents of my freezer and refrigerator. I want to be alerted while I'm asleep after a two-hour power outage to start my generator and prevent these losses.

if you wanted to keep it simple and be successful, find a time delay after de-energization relay (TDAD) relay with the coil voltage and freq the same as the power supply feed for the fridge and freezer.  One example is ABB 1SVR730120R3300 Time Relay, OFF-Delay, CT-ARS.21S, 24-240V AC/DC, 0.05s - 10 min, 1 SPDT. It doesnt have the elegance of all the features you want, but after 10 minutes of power loss,  a N.C. contact can connect a 9V battery to a buzzer to wake you up and remind you to start the generator.

You could also use that relay closure to power up the Pico 2 , and then provide the additional features you want without summing power supplies and attenuating analog circuits to check the power levels.  

This is the control power circuit for the RPi Pico 2. SYS input isnt rated for a 9V battery input, maximum of 5v. You'll need a buck power supply to step down the 9V to 5V ish and a blocking diode so it doesnt backfeed the USB. 

robogary said:

You could also use that relay closure to power up the Pico 2 , and then provide the additional features you want without summing power supplies and attenuating analog circuits to check the power levels.

Could perhaps just use a Wallwart PSU into a relay to do similar. Power goes off, relay drops connecting battery power to the Pico, which then starts a 2h countdown timer to trigger a buzzer. If power comes back on before the 2h elapses, the relay closes, power is removed from the Pico. 

good idea. that suggestion works and is probably about $100 less expensive than a TDAD relay.It relies on the author simplifying his functional spec. I had also given some thought too for using a quad 555 timer IC especially since likes 9V.  one 555 for a 2 sec "blip beep" and another for a 2 hour "wake me up firehouse alarm".  Pico 2 is way overkill unless someone wants to go full IOT Pico W or start sending emails.  

good idea. that suggestion works and is probably about $100 less expensive than a TDAD relay.It relies on the author simplifying his functional spec. I had also given some thought too for using a quad 555 timer IC especially since likes 9V.  one 555 for a 2 sec "blip beep" and another for a 2 hour "wake me up firehouse alarm".  Pico 2 is way overkill unless someone wants to go full IOT Pico W or start sending emails.  

Could also perhaps use something like a PowerBoost to act as a mini UPS if you want 'always on' processing power for future features.

Monitor the state of the USB power to start/reset the 2h timer.

The likes of a Feather RP2040 or similar might be a neater all-in-one solution.

robogary The Pico 2 may be overkill, but at $5 each... they're quite affordable.   I think I still have 5 left from my initial impulse buy when they first came out.  And then there's still those "salvaged" original Pico's from the other projects I lost interest in.

ROFL - I got qty 16+ of RPI Picos  on a shopping cart to use for teaching a class at the library, and a Pico W.  I ended up donating at least 10 to our local VoTech School, and still have a bunch of Picos. I still have a bunch of Arduino Nano clones that I bought many years ago for less than $2 each. I just keep recycling them, very few have failed.   

I really really miss bi-monthly E14 project contests and Tariq shopping carts.