I am going out of town for a week so I am designing an auto topoff for my fish tank. Some of you have already helped me with the first part of this which was using a 555 timer to filter out the rapid cycling of the pump motor associated with the float switch bobbing up and down on the surface of the water.

http://forum.allaboutcircuits.com/showthread.php?t=55428

The next part is to add a voltage monitor to the circuit to shut the top off down if my 12 volt battery gets too low. I want to avoid completely discharging the battery as it is an SLA and I believe discharging it completely can ruin it. I have a few TC54 voltage monitors laying around:

http://ww1.microchip.com/downloads/en/DeviceDoc/21434g.pdf

which I figured I could wire the input up to a voltage divider to shut everything down if the voltage across the battery terminals falls below....I don't know.....say 11 volts but it could be anything. My concern is that when the pump first turns on there is going to be a current surge which could cause the battery voltage to fall below 11 volts momentarily. I could see a situation where I would be back to the rapid cycling that I am trying to avoid. So I guess I am just looking for ideas on how I should approach this problem.

 

I think all you'd need to do is put an RC tank at the input to the detector, so that spikes get filtered to ground. Choose R and C to give you the time delay you need, which I think could be several seconds. If voltage is below x for 2 seconds, shutdown.

 

That hysteresis stuff is where having a microcontroller comes in really handy. Those problems are easy to solve with a little code doing a running average on the voltage level. Check out the PICAxe or Arduino if you're new to uC's.

 

How about a series inductor to input a high impedance on startup. Or a soft start circuit if the load is too big...

 

I appreciate everyone's ideas! I originally was going to do the whole thing with a microcontroller but decided that the 555 timer was a much simpler option. I have an arduino and an STK500 development board currently and a bunch of ATTiny85's on their way from Mouser for another project but for something as simple as this I thought the 555 would be a good option. I like the idea of the RC filter and I will have to read up on using a series inductor as I am unfamiliar with what that will accomplish (I am very much an amateur). I was thinking maybe that I could just hook an analog multimeter up to a battery with a low charge and then see what the voltage drop across the terminals is when the motor starts up. Then perhaps design a circuit that shuts itself down completely (and stays off rather than turning back on once the load is removed and battery voltage comes back up) as soon as the voltage monitor detects a voltage drop significant enough to indicate a low battery.

Maybe something like a second 555 (or one 556) in bistable mode with an output that is triggered high on startup but which switches low when the voltage regulator goes low. The output of this one will be wired up to the base of a transistor which supplies power to both 555's so that when it switches low, power to both 555's is switched off permanently until I provide an external pulse to the second 555 to turn the whole system back on. This is hard to explain in words. I will draw it up tonight and post it on here to get some feedback.

 

Quick question about the 555. If it is operated in bi-stable mode, what is the initial state of the output when power is first applied?