Tech Forum

The input to the circuit has an RC filter so the water valve doesn't turn on immediately when the switch closes.  When the water level falls and the switch on time exceeds the off time, the opamp will turn on Q1 which powers the solonoid valve.  As the water rises and the switch off time exceed the on time, Q1 turns off.

You asked for a circuit that would provide about 15 to 30 seconds delay before sending 12 volts to your water relay, because the float switch bounces which keeps the water constantly turning on and off. The on delay timer circuit in figure 1 will give you that delay.

25 to 30 seconds after the float switch closes (and stops bouncing), the output on pin 3 of the 555 will go positive and the MJE3055 NPN power transistor will conduct to turn on the existing water relay. The 555 will keep the MJE3055 conducting until the float switch opens.

However, I think there is a better approach to your problem. Even with the delay you wanted, the water will still turn on and off constantly as the water level raises and lowers at the set point, closing and opening the float switch. It is better to use two level switches, set to transfer at an upper limit to turn the water off and a lower limit switch set to turn the water on. When the lower switch transfers, water will flow and continue until the upper switch transfers. The float switch circuit in figure 2 will do that. The vertical distance between the switches will determine how often the water runs and how long it takes to raise the water in the pool from the low level to the high.

I solved this problem many times over the years by using 2 floats in a simple start/stop pushbutton configuration. This comes from the common push button controls used by major motor control systems. One float needs a N.C. contact and the other needs a N.O. contact. Many floats have both. In your application, the top level float should operate N.C. contacts as the Stop button. The lower level float, which starts the pump, would use the N.O. contacts as the Start button.

The power relay needs an extra N.O. contact to serve as a holding contact and is wired in parallel with the N.O. contact of the Start float.

In operation, with a dropping water level, the Stop button contacts would be closed. As the water level continues to fall, the Start float N.O. contact would close and start the pump. The holding contact in parallel then holds the motor drive relay in the energized condition with no regard for any bouncing by the Start float. The pump would continue to run until the level rose sufficiently to trip the Stop float and stop the pump motor. Falling water level will then reset the Stop float.

The water level will continue to fall again until the Start float again closes and the cycle repeats. Run time of the pump is determined by the vertical distance between the Stop and Start floats and the flow rate of the pump.

The easiest solution is to use a time delay relay. If the relay is on for 30 seconds straight, then the contact will close for the relay. So wire the float switch in series with the time delay relay COIL. Then wire power thru the relay contact to the water valve. So the water valve will come on when the float is low for 30 continuous seconds - until the float switch opens up. They have a 30 sec and a 60 sec time delay relay with socket at www.mpja.com for $9.95. You should probly put a reverse diode across the time delay relay - so you don't burn out the contacts on the water float switch. If you need a schematic - email me at jimsATruralhostDOTcom and I should be able to come up with one for ya......

Debouncing might fix the problem. Adding hysteresis into the circuit might also work. But you might consider fixing the problem the way the problem of float switches in a fluid is usually fixed: use two switches (which both debounces and adds hysteresis). A lower float switch turns on the water supply when the level falls below the lower limit. The supply then runs until the water level rises above the upper float switch, where it turns off. Look up the circuitry used for septic tank and sump pump controllers that use two float switches.

The pool-filler setup can use a low-pass filter, but not the electronic kind.

Take a piece of large-diameter PVC pipe (see figure) and glue a cap on one end (the bottom). Drill a small hole near the bottom below the level at which you want to start the pump. Drill a large hole above the level at which the the pump or valve should stop the flow of water.

This arrangement provides an aquatic low-pass filter. The small hole does not let waves or disturbances in the pool affect the level in the tube. The large failsafe hole lets water get into the pipe quickly in case the the small hole gets plugged and cannot let water fill the tube.

So if the water reaches the failsafe hole it immediately floods the tube and causes the level switch to turn off immediately. Think of the small hole as a high resistance connected in series with a capacitor — the large tube. Attach the tube to the side of the pool with a bracket or a pair of suction cups.

Gary has described what is known as a "switch debouncer." The MC14490 contains six switch debounce circuits which may be cascaded in order to create a longer time delay than a single one will provide. The IC also contains an oscillator circuit whose frequency is controlled via a single external capacitor.  This IC followed by a transistor can be used to power his relay.

From your description it looks like the wave motion is exceeding the movement differential or differential gap of your switch. I have a couple of solutions.

#1 Mechanical Solution (easy):  Insert the float in a piece of PVC pipe which is large enough to allow free movement of the float. In industrial applications this is called a stilling well and it works by shielding the float form the wave action of the pool surface.

#2 Electrical Solution (more complicated): By using two Normally Closed (NC) float operated switches and a 12V relay with an extra set of NO contacts, you can build a circuit to turn the solenoid valve on at a "low" level and off at a "high" level. Wire the relay coil, Low float switch and High float switch in series between the +12V Source and Ground. Wire the "extra" relay contact in parallel with the Low float switch and the other relay contact in series with the solenoid valve. This arrangement works as follows:

(a) The two float switches are Open when the float is high, when the level drops to the "Low" level the Low float switch closes and activates the relay which closes the relay contacts.

(b) as water runs into the pool the "Low" float switch opens up but the "Hold In" contact on the relay keeps the relay (and thus the solenoid valve) activated allowing the pool to continue filling.

(c) When the pool level reaches the "High" float, the High float switch activates (opens) and the relay and solenoid valve deactivate stopping the flow of water. You can experiment with the "low level" position to obtain the operation you want.

CAUTION: Water and electricity do not mix. Any electrical devices used around water where humans can contact the electrified water should be fed through a ground fault interrupt device.

An effective method to prevent short-term fluctuations from exercising the float switch is to dampen the frequent rise and fall of the water level by a non-electronic method. Just enclose the float switch in a vessel (such as a one-gallon milk container with the top cut off) and make a very small hole in the bottom of the container. This is the equivalent of adding a large capacitor to a varying dc voltage to smooth out the variations. The float switch will respond to an average level over time. Depending on the size of hole, it will take some time for the water level in the container to rise or fall. I believe this is essentially what lake level monitors use to tune out temporary wave action.