DESCRIPTION

1

IC1

LM555, bipolar or CMOS

1

IC1-SOCKET

8 PIN SOCKET

2

* R1- R2

2

R3 - R4

2

C1 - C2

1

C3

1

C4

1

D1

1

TR1

1

TR2

1

PCB

1

BOX

4

Screws

1

-

2

* R1 - R2

 

Circuit description for " Automatic Water Level Sensor & Pump Driver Circuit"
by Les Banki

I designed this circuit for automatic re-fill of electrolyzers in ‘water fuel’ systems. However, it can be adapted to many other uses as well. Despite looking deceptively simple, it is quite ‘sophisticated’.
I wrote this detailed circuit description mainly for the benefit of those who want to understand how it works, make their own and perhaps wish to modify the circuit to suit different applications.
However, modifying it does require some knowledge of electronics.

In brief:

The task itself is simple. We have a vessel (tank/container) we wish to fill automatically, between pre-determined minimum and maximum levels, as set by the probes.
Three (3) probes are needed: GROUND (or ‘Common’), HIGH and LOW.
The GROUND probe is the reference.
[Note: should the vessel be made of metal, (NOT coated with plastic on the inside) it can be used as ‘reference’ and a GROUND probe is not needed.]

We found solid copper teflon coated wire (16 gauge), worked well for the 3 probe wires,

Initial condition: the pump is required to start automatically if the vessel is empty (or the level is below the required minimum) when the circuit is powered up for the first time.

When the HIGH level probe is reached, the pump must STOP.

IF the tank/container is already FULL when power is first applied, NOTHING should happen. When your circuit is finished and your probes are in place in your hydrogen cell. Fill your water tank with water, and fill your cell with the circuit plug in or turned ON.

*Note this circuit is always ON or OPEN. WHen you apply 12v to the 2 IN wires the" pump motor" or "gravity feed solenoid switch" should start filling your cell. As soon as the water level touches the high level probe, the pump stops. The water pump should NOT turn on again until the level of water reaches the low level probe tip.

* Note : This circuit comes with 2 sets of resistors R1 and R2, it was designed for water with a approx. 0% to 10% electrolyte mix. with the change of R1 and R2 to 510K ohms for 20% solution of electrolyte commonly NaOH or KOH, by weight measured to 100% water by weight.

In greater detail:

The circuit is based on a 555 IC (Bipolar or CMOS) for sensing the minimum and maximum water levels and turns a MOSFET on/off which directly controls a 12V DC pump motor.
OR, it can power a relay coil to switch high currents/voltages, DC or AC.
[A Bipolar 555’s output can source/sink up to 200mA current. It can drive a LARGE relay coil directly, without TR1 & TR2. (A CMOS 555 can only supply about 10 mA.)
The relay coil is connected between its output and the positive supply rail.]

As the detailed operation of the 555 is explained in numerous publications, there is no point repeating it here.

Suffice to say that here we are using the ‘Trigger’ and ‘Threshold’ pins (2 & 6) to detect the maximum and minimum levels, respectively. The two voltage comparator op-amps inside the 555 control the output, turning it on/off..

Looking at the circuit diagram you will notice that the ‘Trigger’ pin (2) is marked ‘HIGH probe’, despite being triggered (output goes HIGH) when the voltage drops below 1/3 of the supply voltage and, the ‘Threshold’ pin (6) is marked ‘LOW probe’ while it is ‘reset’ (output goes LOW) when the voltage rises above 2/3 of the supply voltage. If this appears to you as being upside-down, IT IS!

The circuit works as follows:

Three (3) probes are immersed in the vessel. (usually from the top)
One is the ‘GROUND’ probe, going to the level a little lower than the minimum desired level. This is the ‘common’ (or ‘reference’) probe. The LOW and HIGH probes are set at the desired levels.

Now suppose the vessel is EMPTY.

Resistors R2 and R1 (1M) tie the ‘Trigger’ and ‘Threshold’ pins (2 & 6) to the positive (+) rail (supply). In other words, both pins are HIGH. Remember (from above), to make the output of IC1 go HIGH, the trigger pin (2) needs to drop below 1/3 of the supply voltage. (4V with a 12V supply)
Since the trigger pin is still HIGH, the output remains LOW.

We need to fill the vessel when IC1’s output is LOW.

TR1 is OFF. The GATE of the MOSFET switch (TR2) is connected to the supply rail (+12V) with R4 (10k).

TR2 is thus turned on and the pump motor is running.

TR1 (BC547) is connected between the IC1s output (pin 3) and the TR2’s GATE.
Its purpose is phase reversal. It means that when IC1’s output is HIGH, TR1 conducts and pulls its collector/TR2’s GATE junction LOW, so TR2 is OFF. Since the pump (or relay coil) is connected between the positive rail (+12V) and TR2’s DRAIN, the pump/relay coil is NOT energized.

Now, back to the condition when the IC1’s output is low, TR2’s GATE is HIGH (+12V) and conducting. The pump is operating and water is being filled. As the water level rises, a water ‘bridge’ is formed between the GROUND (common) probe and the ‘LOW probe’ (Threshold, pin 6) This ‘bridge’ constitutes a low resistance, relative to the high resistance of R2 (1M), bringing the voltage at this pin to a low level (at least below 1/3 supply but actual voltage depend on the conductivity of the water). However, this is IGNORED by IC1 since its output is already LOW (in the ‘reset’ mode)

When the water level reaches the ‘HIGH probe’, a water ‘bridge’ is formed between it and the GROUND probe. Just as with the LOW probe, this ‘bridge’ constitutes a low resistance, relative to the high value of R1 (1M), bringing the trigger voltage to below the required level (1/3 supply voltage) and IC1 triggers, its output going HIGH. Now Tr1 is turned on, the bias voltage/current of TR2 is removed and the pump STOPS. The filling cycle is completed.

The emptying cycle is as follows:

As the water level falls, the ‘shorting’ water ‘bridge’ between the GROUND probe and HIGH probe (‘Trigger’, pin 2) is removed and the voltage rises above the 1/3 supply level (+4V).
This is IGNORED by IC1. [This pin needs to drop below 1/3 supply (4V) to trigger IC1. However, as long as the voltage on this pin remains below the trigger level (1/3 of supply), IC1 stays ‘triggered’, its output stays HIGH.] TR1 is conducting and so there is no bias supply to TR2’s GATE and the pump is OFF.

Once the water level drops below the LOW probe (‘Threshold’, pin 6), the ‘shorting’ water ‘bridge’ disappears and the voltage rises to 2/3 supply voltage (8V), IC1 ‘resets’. (its output goes LOW)
TR1 is now OFF, having no bias current. Its collector voltage rises to the supply rail and TR2’s GATE is now biased so it is turned on and the pump is operating. This is a new filling cycle.
The circuit will repeat these actions indefinitely, as long as power is applied.

To simplify:

When IC1’s output is HIGH, pump is OFF – IC1’s output LOW, pump is ON

Capacitors C1 and C2 (1uF), together with resistors R1 and R2 form an approx. 1 second time delay.
This delay serves two purposes: 1) Prevents spurious electrical noise signals and 2) minor level turbulence to trigger the circuit. *Note that the values of these 4 component can be tailored to suit different applications. For example: for highly conductive (very low resistance) electrolyte, the resistance values of R1 and R2 should be reduced considerably. (510K ohm for 20% NaOH or KOH to 100% water ) If you bought a B3 cell and want to follow Bob's electrolyte concentrate of 20% of NaOH to 100% weight of water. For very high resistance (pure) water, the values may need to be higher, ( we found 1M works with no electrolyte water , 7ph )depending on the probe arrangement (distance between the probes) (The value of C1 and C2 should also be altered to maintain the desired time constant. The slight pause before the pump motor kicks in.)

When selecting Bipolar or CMOS for this circuit, keep in mind that the 3 voltage divider resistors inside the 555 have very different values: 5k for the Bipolar and 100k for CMOS. Since the ‘water bridge(s)’ between the relevant probes and R1 & R2 (1M) are forming voltage dividers, it should be obvious that the internal voltage dividers resistor values ought to be considered.
But most importantly, when determining the shape, size, positioning and the distance between the probes, the conductivity/resistance of the water MUST be considered.

By removing the “phase reversing stage [simple modification: omit TR1 (BC547) and R4 (10k), replace R3 (10k) with 22 ohms and connect it directly to the Gate of TR2 (MOSFET)] this versatile circuit can also be used in reverse. It can activate a pump when the vessel/container is full (in order to empty it or transfer its contents to another container) and stop it when the desired minimum level is reached.

In this case, note that when power is applied to the circuit for the first time and the vessel is not full, the pump starts up briefly. (due to the time constant of R2/C2, apron. 1 sec.)
This happens since C2 is discharged and the ‘Trigger’ (pin 2) is momentarily low, triggering IC1, its output going high. Once C2 is charged enough through R2, the voltage on pin 2 (Trigger) rises well above the 1/3 supply voltage and IC1 returns to the ‘reset’ condition with its output low.
(To eliminate this brief start-up, C2’s value can be reduced to a value which still provides filtering of spurious noise pulses.)

As stated above, this circuit (as presented) was designed to drive 12V DC pumps directly.
For most applications, the ready made units (using the MTP3055 MOSFET) can comfortably supply at least 6-8 Amps. (Current depends on the MOSFET used and the size of its heath sink. However, the current carrying capacity of the connectors used MUST also be considered.)
For really high currents, and/or high voltage, AC or DC, use an appropriately sized relay to drive ANY pump.

"Automatic Water Level Circuit" Schematic

Les Banki (Electronic Design Engineer)
Water Fuel & LBE Technologies

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