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Simple Timer

I wanted to build a simple timer circuit for water pump in my home. It was having simple switch which we need to manually turn off once over-head tank start to overflow.

The idea was to create simple timer which will run motor for certain amount of pre-set time period and turn off itself.

I built it and it is in service for last two year (as of this writing in May 2020).

Schematic

I used an old Nokia mobile phone charger to provide DC supply to the current.

It does have separate two relays for phase and neutral lines to the water pump. That is not necessary, actually relay on phase line is sufficient.

Working

Start button closes when we press it, which will cause phase line of power adapter go live. It will supply voltage to 555 timer.

555 timer is configured in monostable mode. Capacitor C1 will be get charged which will trigger the timer, causing output on pin 3 to go high. It will switch ON MOSFET Q1, then 1117 5V regulator start to give output. All three relays will get triggered. First relay will ensure supply to power adapter after we release start button. Other two relays will connect water pump to main.

We can see set of resistors at bottom of 555 timer IC. They are connected to supply through a rotary switch. So, timer will run depending on the resistor set we chose with rotary switch.

Timer will run until capacitor C3 is charged 2/3 of supply voltage. After that, timer output will go down, causing output of 1117 voltage regulator to go down. All three relays will disconnected. It cause water pump to stop as well as whole circuit itself to turn OFF, because it is powered through first relay.

Possible Improvements

  • It required a stop button to manually turn it off sometimes. I missed it include it.
  • 1117 voltage regulator can be avoided if we use 5V power adapter. Regulator was used to protect relays. Removing regulator will cause to remove the MOSFET as well.
  • Optocoupler based relay driving is not required is. They are not isolated anyway. I originally designed it because that is how I learned about driving relays.

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