Well, only because it's christmas:
Circuit is very simple - a relay, a beeper or warning device of your choice, and 2 switches. 4 parts....OK make that 5 if you get a relay that needs a diode as shown in the schematic. Although you can omit the disable switch of you don't want it and bring it back down to 4 parts... :-)
The idea is that the magnet in or on your bag is close to the reed switch when the bag is on the bike.
When the ignition is on, the beeper will go off if the bag is not in place.
If you want to disable it you press the pushbutton, and it will be disabled until the next time you switch the ignition on and off, or put the bag on the bike. That's the important bit - the fact that you cannot forget to turn the circuit on.
As a hint - play with the placement of the magnet or put a couple on different areas of the bottom of the bag so that you don't have to faff around getting the bag in just the right spot when you put it on the bike.
If you don't want the disable switch as a seperate switch, just leave it out. Another way to disable is to stick a magnet to the bike next to the switch when your bag is not in place, but this is dangerous because you can forget the magnet is there and put your bag on, and then the circuit will not do it's job if you leave the bag off.
Also the beeper doesn't have to be particularly loud, if you just want a reminder that you are about to leave home/work/petrol station without your bag - it goes off when you switch the ignition on, before the engine is running. If you want something to warn you if the bag falls off in transit, you might need a screamer!
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