Quicky overview then:
Power Supply
Mains in, isolation transformer to 15V AC, rectified and smoothed using 1000uF to about 20V D.C with loads of ripple, linear regulated down to a nice smooth 12V DC using standard LM7812, decoupled using 10uF – probably expect to get up to 1 Amp from this supply depending largely on the transformer used and heatsinking on the Linear Reg. Indication of system live when ‘On / Off’ switch is on via Red LED ‘Power On’.
Low water detect monostable
The first stage centres around NE555 Timer IC1: The timer is arranged as a mono-stable whose output at pin 3 goes high for a period of time determined by ‘C’ and ‘R’ when a low going trigger pulse is seen at pin 2. I have now made the R variable using a 1 Mohm pot, VR1 and the time the pump is on for can be adjusted from 1.1 secs to 110 secs. I was not sure of the state of the float switch, so if I have it wrong, cut the wire ‘X’d and use the inverter configured IC2B which should now be connected to pin 2 instead (if float switch OK, do not connect it).
When the output at pin 3 goes high, the mains relay comes on as will the pump. The two diodes are used to protect the circuitry from back EMF and erratic triggering. Indication that the pump is on is shown by Amber LED ‘Topping Up’.
System Disable
If IC1 is held in reset via RST pin 4, the NE555 monostable connected output should be held off. To this end the ‘Enable / Disable’ switch will either pull RST high (‘E’) or allow the upper 1K resistor to pull it low when in the ‘D’ position, thus illuminating the ‘Disabled’ LED via the lower resistor.
System Fault Detection
The relay used to drive the pump is DPST – the two poles are mechanically linked. If the pole that drives the relay were to become welded shut in a failure condition of the relay, then the other pole should also close. This can be used to our advantage: The second pole is therefore used to monitor the pole driving the pump motor. With the use a 1K pull up resistor, the signal ‘Status’ is created. It is now a simple question of logic to determine system failure if used with the ‘Control’ signal. Truth table is therefore:
If the relay is closed (the pump pumping water) then ‘status’ is logic ‘0’, under normal circumstances, this can only happen if ‘Control’ is at logic ‘1’. Therefore if the ‘status’ signal is ‘0’ AND the control signal is ‘0’ (off) then things are amiss. This is the truth table for a NOR gate (IC2):
S C | Alarm on = ‘1’
0 0 | 1
0 1 | 0
1 0 | 0
1 1 | 0
Alarm Output
The alarm circuit is centred around NE555 timer IC3 arranged as a 2Hz astable circuit (with a 66% duty cycle). It is normally held off, or in reset by Alarm signal from IC2D pin 13. When a fault is detected, the astable is taken out of reset and should start oscillating, indicated by Red LED flashing. Output is also provided for by use of an alarm output at CON2.
Notes
NE555 timers is I think capable of driving a 200mA (0.2A) load. Therefore, the mains relay should take no more than this less the current needed to drive the Amber ‘Topping Up’ LED, i.e.180mA (0.18A). Ideally I’d choose a 100mA rated Main relay coil.
Same applies for the Alarm output – use an alarm that takes no more than 180mA, ideally 100mA.
-----------
Hope this bloody works now!
Andy
