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RO Water Controller - Main Board <<< Previous Next >>>

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The main board is fairly complex after the other boards. The board receives inputs from float switches and then dependent on the state of these switches controls the solenoid valves. It also includes the timer circuit for the drain to waste solenoid.

I have used NAND chips (4011) as they are cheap and easy to obtain. They can be combined to produce other gates such as AND, NOT etc. click here for a overview on gates. You can obtain specific chips to cover these type of gates but it wouldn't reduce the number of chips in this circuit.

I am also using a number of the chips to produce what is called latches, click here for an overview of latches.

Main Circuit


4011 NAND Chip. Switches the relays off if a fault conditions occurs. A fault condition is when both the Top and Bottom float switches are on. This is detected in IC7B, IC7C, IC7D.


4011 NAND Chip. Forms a latch between the Top and Bottom float switches. Output of IC1D is not reset until it goes through a cycle i.e. reaches the top float switch. The top-up pump then drains water from the reservoir until the water level reaches the bottom float switch. On reaching this the controller then open the solenoid valves to fill the reservoir again.


4011 NAND Chip. When the fill part of the circuit is enabled, in other words when it needs to fill the reservoir. This part of the circuit detects whether or not it has already drained. If the circuit is draining (solenoid before DI) the fill relay is off. Once it has finished draining this part of the circuits switches on the fill solenoid.


4011 NAND Chip. Similar for IC2A,IC2B,IC2D except for the SW reservoir. Basically doesn't switch on the SW fill relay until the controller has finished draining.


4011 NAND Chip. I am not sure why I need these, but if you don't the circuit permanently enables the relays. I can't get my head around this one.


Switches on the fill circuit (timer) if we need to fill either the RO or SW reservoirs.


4011 NAND Chip. A problem with the latch (IC1C,IC1D) is that if the bottom float fails for any reason (i.e. sticks on) and the water level hits the top float. A Fault condition occurs and the circuit will ignore the top float. In these circumstances the controller will continue to fill the reservoir and you will get an overflow. These 3 gates detect that condition.


4011 NAND Chip. These are not used, I could have removed one of the chips but that would have made the PCB a lot harder with many more link wires.

Transistor Q1

BC108C. Switches on or off the fill circuit which incorporates the timer chip.


This the timer chip which determines the drain time. The time is determined by a rotary switch which is fed through varying values of resistors to 5,10,15,30,60,120,180,240,300,600,900,1800 seconds drain time. Basically the higher the resistance the longer the timer is activated. Pin 3 of NE555 switches on the drain relay through transistor Q3. I have recently learned since creating this circuit that this transistor is superfluous as the NE555 should be able to power the relay without the need of a transistor.


The final layout of the board had a large number of link wires, if you have some experience in producing boards it would probably be better to produce a two layer board. i.e. copper tracks top and bottom of the board.

The Eagle board schematic can be download by clicking here and the PCB board layout can be downloaded by clicking here.


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