Not everyone is going to be interested in the safety circuit.  I have mixed feelings about that, but there isn’t a way to force the issue.  Table saw manufacturers can’t keep people from removing the safety guard on the saw, this isn’t any different.  I’ve removed the circuit from the main PLL board and put it on it’s own board.  This also allows a lot more real estate on the main board, but I must stress the safety… splitting water is a hazardous business.


I’ve worked out the design and the board, I’m working through the logic to make sure that it’s viable.  It’s an ‘active live’ design, any failure causes a ‘fault’.  A fault should occur if:

* wires are broken
* the optoisolators get burned out

And the more normal faults:

* low or high water level
* low or high pressure

Low water causes a fault, and also switches a relay for a pump or valve solenoid.  Very low or high water, wow, that shouldn’t happen, especially high water.  High pressure, yeah, definitely turn it off for a while.  Low pressure: normal when the system is first starting, but what happens when there’s a blowout?  You wouldn’t want this to just keep on running with a hole in the plumbing, so low pressure causes a fault.  To get the system running, there’s a “pilot” momentary switch which defeats the low pressure fault, this is just like the pilot switch on a gas appliance, you hold it down for a minute while the system starts, then you can let it go as soon as there’s enough pressure to keep it running.

Parts list

Qty Value Device Comment/Digikey Parts
1 0.1uF poly capacitor, 5mm lead-in voltage regulation C2
1 1K 1/4 watt resistor, 10mm lead-in R3
1 1N4003 regulator diode or 1N400X
relay coil clipper
1 1N4148 small signal glass diode or 1N914 D1
1 2N2222A NPN transistor
TO-92 ( or TO-18 )
relay driver Q2
1 2N2907A PNP transistor
TO-92 ( or TO-18 )
relay driver Q1
5 4.7K 1/4 watt resistor, 10mm lead-in 5V pull-up resistors R1, R4, R5, R7, R10
1 74AC02 IC: Quad 2-input NOR 14-DIP 74AC02PC-ND IC1
1 100uF electrolytic capacitor
8mm with 3.5mm radial leads
voltage regulation C1
4 330 1/4 watt resistor, 10mm lead-in LED current limiters R2, R6, R8, R9
1 G5V-1 Omron G5V-1 RELAY SPDT
water pump switch
1 LTV-847 Quad Optoisolator 16-DIP 160-1370-5-ND OPTO1
1 SPST Normally open SPST
momentary contact switch
for “pilot” S1
4 Fuse clips for ATO fuses F067-ND F1, F2
1 1A ATO Fuse for logic circuit F1
1 10A ATO Fuse for cell circuit F2
1 MF-R040 PTC Resettable Fuse
400mA hold, 800mA trip
logic circuit protection

28-SEP-2008 Added fuses and PTC.  Board will fit 90mm spaced holes.  Schematic, files and pictures updated.

01-OCT-2008 I got the relays, tested that part of the circuit, and it’s good.  The ATO fuse clips look really good, I’m happy with the way that turned out.  Files and schematic updated, this is the version I’m submitting for manufacture to Sierra Proto Express .  It’s been accepted and is “in progress”.  6 boards should be here 09-OCT-2008.

Creative Commons License

Copyright (c) 2008 MDVE.NET. Some rights reserved.
This design is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.

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