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RO Water Controller - Battery Backup for Counter Board <<< Previous Next >>>

Image of board

I introduced a counter circuit to record the number of times the RO reservoir fills The counter itself was fairly easy. But it had a major problem. If the power is cut for any reason, it would lose the count. This led to the creation of a 9V battery backup circuit that powers the counter circuit until the mains power is restored. The circuit switches off the rest of controller to utilize as little power as necessary. I still have some work to do on this circuit as the circuit worked fine on the pro typing boards. But since I have made the PCB and installed into the box. It isn't working! the circuit does not maintain the count when the power is switched off.

Also one of issues you face with using switches,relays is a condition called contact bounce. When a contact opens or closes the contacts bounce around (opening and closing) for a short time (milliseconds). Your light switch in your home does it as well, but as it is happening so fast, you cannot detect it. Digital circuits work at this speed and detect the condition. In a circuit, if you are counting the number of times a switch is opening and closing, you need to deal with this condition.

Also when using NAND gates for latches, the gates themselves initially bounce around before settling in their final position. The counter chip detects this, again leading to false counting.

This board attempts to deal with this problems.

Battery backup circuit

IC1

This chip is a Intersil ICL7673 which acts as an automatic battery backup switch. This switches between two power supplies, main and battery backup.

IC2A, IC2B

4011 NAND Chip. This part of the circuit latches when there is a power loss and illuminates the power loss LED which is attached to X3-1.

IC2C, IC2D, IC3A, IC3B, IC3C, IC3D

4011 NAND Chips. Clocked SR latch so that if an fault condition occurs (i.e. both Top and Bottom floats are set) doesn't increment the counter. Additionally only increments the counter when there is a full cycle. In other words when the bottom float is reaches, fills to the top float and then back down to the bottom float.

IC4A, IC4B, IC4C

4011 NAND Chip. NAND gates equivalent of OR gate. Detects if there is a fault condition i.e. both the top float and bottom float set. If this is the case switches off the counting circuit.

NE555 Timer Chip

This is used to deal with any contact bounce, or settling of the NAND latches. Output of Pin 3 gives a signal for the counter chip.

The Eagle board schematic can be download by clicking here and the PCB board layout can be downloaded by clicking here. (Please note that you will need the Eagle software to read these files)

 

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