Skip to main content

Power Off Delay Circuit

Recently I have been kicking myself for forgetting to turn off the motor that pumps water from a sump to an overhead tank at home. Every time I turn on the motor, I try to remember to turn it off in 30 minutes (about the time it takes to fill up the overhead tank from empty to full), but I keep forgetting to turn it off most of the time, wasting a lot of water before I realize it. Then I got an idea. Why not just make a simple, cost effective timer circuit that will turn off the motor after say 30 minutes?

So I started thinking about it. All I need is a 555 timer and the correct resistors and capacitors that will delay the timer for the exact time I want the motor to stay on. Next I need a relay to control the motor.

First the timer has to run in monostable mode. We just need a one-shot pulse. The pulse should be long enough to keep running the motor. Next we use t = 1.1RC to get the approximate time (in seconds) the output of 555 timer will be high based on the R (in ohms) and C (in Farads) values. I wanted the motor to be on for 30 minutes. And since the time is so long I would need a huge capacitor. The biggest I could quickly find was a 1000uF capacitor. Plugging in those values:
t = RC
30 * 60 = R *  1000 x 10^-6
R = 30 * 60 / 1000 MOhms = 1.8 MOhms

However, the best I could find was a 2M4 resistor. Which would mean the motor will be on for more than 30 minutes. I could mix a bunch of resistors to get 1M8 resistance, but I decided to keep it simple. So put 2 2M4 resistors in parallel and let the time come down to 22 minutes. That is fine for my purpose. Next I proceeded with the circuit diagram.


Pretty straight forward circuit for running the timer in monostable mode. The trigger pin (pin 2) has to be low (the voltage on the pin should be below 1/3 the supply voltage) for a small period of time. Normally a 100K resistor in series with 1uF capacitor is used. I was too lazy to find those and just used what I had (1K in series with 100uF). Once the trigger goes high the relay is ON for 22 minutes.

Next I made the circuit on a breadboard to make sure it works and then proceeded to built the circuit board. Been a long time since I used the soldering iron. However, I did not plan on a proper design and so the wires crisscrossed each other.


All done. Now to actually test it. The plan is to connect the motor to the NO (normally open) of the relay. I have not yet tested the circuit, but hopefully I will be able to do it this weekend.

Comments

harryalby said…
As these kinds of notary services are less popular, public awareness is also important. Common people from different places have started understanding the important of these Apostille San Francisco services. Hope you benefit from it.
harryalby said…
It is through these Notary Public Bay Area services that you will realize that these professionals are efficient in managing things.

Popular posts from this blog

Attesting General Power of Attorney in SF

Recently I had to go through the motions of getting a General Power of Attorney (GPA) document attested in San Francisco. I am an Indian by birth. My parents were trying to buy a house back in India for me. Since I did not want to travel to India they needed a GPA so that they can act on my behalf to sign all the documents required to buy the house. The problem however is that they needed it urgently because the seller lives in UK and wants to get all the things done quickly so he can go back. My parents send me a GPA document that they obtained from a lawyer. This is a document that will give the power to my parents to buy the said property in the document on my behalf. The lawyer said that I will have to get the document attested at an Indian Consulate in USA. The closest one for me is in SF and I can drive there in about an hour from where I live. So I though it will be like a day's work to get all the things done. I looked up at their  website  for the procedure to att

XBMC / Boxee remote control android app

I have been writing a few android apps over weekends at home and during 20% time at Google. However I never actually released any of them in the android market mainly because they were quick and dirty apps that fit my needs but perhaps would not be appealing to the general public. One such app that I quickly wrote over a couple of weekends is a XBMC remote. The media center that I use at home is XBMC and I have always wanted to have more control and faster access to my media. Using my remote to navigate through the menus is not as fast. Especially when I wanted to queue a lot of music it is very slow. So I wrote this nice little app called "XBMC remote" for my android phone to control XBMC from anywhere :). Give it a try. Search for "xbmc" in android market and install it if you use XBMC as your media center. When you first launch the app you will start with this screen. You will have to setup your web server address, username and password (if required) by

gtkdocize not found

If you are ever configuring an app and see the message "gtkdocize not found" in Gentoo, then you need to emerge gtk-doc. I had some hard time figuring this out so I am writing it in my blog for the next time. When I saw that error message I did an "emerge -s gtkdocize". Usually it is that simple in Gentoo. But not this time. The emerge command returned no results at all. Then I searched for gtkdoc and still no luck. After searching in Google, I still did not have a solution. After thinking for a while I decided to try to search for gtk-doc. Bingo! That worked! Interestingly, this is my first post from my Virtual machine :-)