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Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Apparently, you didn't try hard enough. Googled this one quickly.
Here's a 10 minute timer. It comes with a schematic and source code. Surely, you can convert it to a 20 minute timer.
To quote from the originating page:
02/08/2000 10minf84.zip - (59K) - Source code and schematic for a PIC 16F84 based 10 minute ID timer / beacon. It uses the PIC along a piezo speaker and a keying transistor to accurately time 10 minutes for ID purposes. The delay can be programmed for any length from 0 to 99 minutes, 59 seconds. At the end of the delay a Morse code message of up to 60 bytes is sent. During the delay time, an LED is "winked" and optionally, the piezo speaker is "clicked" once a second.
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