Wednesday, December 28, 2011

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.