HOW TO – Alarm clock of the short now

ClockA beginner’s PIC project, the Maker writes “I am unemployed at the moment, but I’m very busy. I tend to work late nights and wander over to bed when I run completely out of steam. The problem is that this happens at a different time every day. I end up being too lazy and stupid to re-set the alarm clock, so I don’t bother. I oversleep. Recently I realized that I need a special alarm clock that instead of going off at a particular time goes off after a set duration. Like an egg timer, but grand!. So, I made one, and I realized that it would be a great beginner’s project. It includes a very basic C program and a simple PIC schematic.” Thanks Zantor! Link.

8 thoughts on “HOW TO – Alarm clock of the short now

  1. I like the idea, A pressure sensor in the bed to turn it on would be an enhancement to the project! For your on/off switch you might want to remove the connection to ground. Every time you turn it off you are shorting out your 100uF cap. It isn’t a huge cap and may never cause a problem but still isn’t a good idea.

  2. thanks abbtech! And thanks for bringing up that point. There is a reason for shorting the cap out when the device is turned off: the power draw is so low on this circuit (when not driving the motor) that the capacitor takes hours or even days to get drained enough to allow the pic’s internal reset circuit to work properly. Without a proper reset, the clock thinks it is still the end of the 8 hour span.

    so, why not use a smaller cap? I would havem but the motor draws so much current when it first goes on that there is a risk of resetting the system. So, I opted for a slightly odd power switch in order to make a much more reliable alarm clock. There might be a way to fix this in software, but you know the whole thing is kind of crazy anyway…

    I know it might seem strange to short out a cap, but it’s really only dangerous with much higher voltages or with capacitors with extremely low internal resistance. 100uF charged up to 5 volts just isn’t anywhere near that. The capacitors in a recently operational television on the other hand can kill you, or melt a chink into a small screwdriver. But that’s a whole different ball o’ wax.

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