Float Switch - Circuit for Timed Relay Shutoff

[DustinB]

Hey guys, wondering if you might be able to help me out. I mainly use microcontrollers, but I'm wanting to do something with basic electronic components if possible.

I have a saltwater fish tank and you have to replace evaporated water. I use a float switch(reed switch) to activate a relay to turn on the water topoff pump.

Right now the float switch is activated when the water level drops, this applies 9v to the relay switching the water pump on. The problem is that I get a lot of relay chattering due to the switch floating on water and bouncing.

What I am hoping to do would be to switch the relay on once the switch is activated and turn the relay off after 10-20 seconds. The circuit would have to account for, or not be affected by the "pulsing" 9v signal from the float switch. The pulsing stops after the pump has been on for a couple seconds though.

Any ideas what the best way to go about this would be? It has to be fairly fool proof as the pump getting stuck on would be disastrous. I do have a second switch that is wired in series with the first, wired as normally closed. This way if the first switch gets stuck, the water level rising would eventually trigger the second switch breaking the 9v circuit. I suppose the second switch could still be used as a failsafe to break the 9v circuit directly at the relay control pin.

Currently using:
9v DC Power Supply
Float (Reed) Switch
120v AC Water Pump.

Sorry if this is really simple, I'm just a little lost on how to do this without a microcontroller, especially with the chattering/pulsing switch. Thanks!

 

[KJ6EAD]

A simple 555 monostable will do it.

 

[SABorn]

I would use a micro controller, a 08m2 picaxe would do well, as then use the float to trigger the picaxe to latch the relay, when the float closed the first time (or first pulse) take that as a shut off for the pump and unlatch the relay.

Then use the pause statement to create a delay for a minute or 2, the water would have settled enough and if the float was open it would latch the relay again for a second top up, and repeat the process.

You could also add a time out period for the pump runtime should a float get stuck then it would only run the pump for a given period, should a time out occure then stop the system, and bring on a warning light/led until the system was reset manually.

All done in a little 8 pin chip, allowing for some fail safes to be included.

Pete.