I bought a very cheap, $3 door/window alarm that will be triggered when the magnet is moved away from the reed switch. I want to somehow delay the alarm for about 20 minutes.
I want to install this in a remote controlled model airplane and replace the reed switch with an on/off switch and flip the switch before the plane is taking of. Now the alarm was triggered by flipping the switch, but delayed for 20 minutes. An average flight takes about 10 minutes. After landing, I can flip the switch back, so the alarm will not sound, but in case of the plane getting lost in the forest, in the bushes, or in the corn field, I want the alarm to start by itself 20 minutes after takeoff (20 minutes after flipping the switch)... so we can increase the chances of finding the plane.
Any help is appreciated.