Hydroponics Pump Timer

Hydroponics has myriad practical benefits which lead to an interest in pursuing this form of gardening (see the Hydroponics Section for further information). The actual hardware required for hydroponics generally involves a pump to re-circulate nutrient solution. Further, depending upon the technique being used (e.g. ebb and flow, nutrient film technique (NFT) etc) the pumping of nutrient solution is generally in discrete amounts of time and regular intervals. Therefore, a controller/timer is required for the hydroponics pump.

The DIY hydroponics system I started experimenting with only required a maximum pump head of ~1.5m and flow rate of a few litres per minute. Such a low pressure/flow is amenable to inexpensive pumps manufactured for use in garden water features (fountains) which are readily available in hardware stores and similar outlets. A 240V AC version pump is convenient being able to be powered directly from wall sockets, and a plug-in wall timer to provide the necessary control. Disadvantage of this approach is having 240V AC extension cords running throught the garden (depending upon exact location of hydroponics setup) and plug-in wall timers are very inaccurate, requiring frequent re-setting. Further, a 240V AC pump provides more difficulties if it is desired to make a DIY controller (any involvement of AC electrical power from house wall sockets should be avoided by DIY).

Alternatively, 12V DC version pumps have the advantage of being low voltage (safer to make controllers) and can be powered by a vehicle battery or similar (and recharged with a small solar cell if available). Further, locally 12V DC garden fountain pumps are very cheap and I had a small surplus solar cell which a friend used to use during camping.

This lead to the need to have a timer/controller that would turn-on the pump at regular intervals to distribute and circulate the hydroponic nutrient solution. Optimally, such a controller would be able to take into account the time of day/night, if there is currently rainfall or not, ambient temperature and other relevant factors to decide if pumping of hydroponic nutrient solution is necessary.

However, since initially the hydroponic system was only experimental (to enable focus on what type of nutrients required, which crops/plants were amenable, type of hardware needed, to gain practical everyday experience, etc) only a 'basic' pump timer/controller (i.e. only function is to provide various set time delays to be selected which then turns-on the pump for a set period) was desired, which could be quickly and inexpensively built, and possibly re-purposed later when superceded by a more sophisticated controller.

A microcontroller approach was initially considered, for example a cheap PIC 16Fxxx, which would only require the minimum support circuitry (oscillator and power supply) to control a relay. However, a circuit based upon a 555 timer provides a simpler solution. Except that a 555 timer is not particularly good for 'long' time delays (greater than 10 minutes as a rough guide) as leakage current from the necessary large value capacitors with the low charging current from high value resistors leads to inaccuracy and inconsistency. This can be overcome by using a ripple counter (CD4060 in this case) to provide the 'long delay' times (the eventual design allows for time steps 15min, 30min, 1, 4, 8 and 16 hours), which then uses a 555 timer to provide the 'pump on' time/relay control.

The combination of the CD4060 ripple counter with a 555 timer controlling a suitable relay, enables a very cheap, easily constructed pump timer/controller.

The circuit consists of three basic 'blocks' (see the Schematics Section). Firstly, the delay timer function enabled by U1 (CD4060 binary ripple counter) and associated discrete components. Secondly, the pump-on timer provided by U2 (NE555 timer) which controls the relay. Finally, the power supply provided by U3 (LM7809 linear voltage regulator).

Delay Timer Block (CD4060)

The CD4060 is a 14-stage binary ripple counter, which advances one count on the negative transition of each clock pulse. The clock pulse to the CD4060 can be from a simple RC network connected to the on-board oscillator stage (see the datasheet in the Bill of Materials Section). There is only a sub-set of the outputs from the binary ripple counter that are available on external pins of the IC package (1).

The first available output of the binary counter is at pin 7 (output 3) which has a frequency of the clock frequency (or oscillator frequency from the RC network) divided by sixteen (16). The frequency at each successive output is then half the preceeding output (and note output 10 is not available externally). This means that the range of possible output pulse frequencies is determined obviously by the input clock (oscillator) frequency and the limited number of outputs actually available at external pins.

In this case, the on-board oscillator is used with the external timing capacitor (C5) and resistor (R7 + RV2). The frequency of the oscillator is calculated using the formula:

where: (note the units)

• f = frequency (kHz) of CD4060 oscillator
• R_t = timing resistor (connected to pin 10) in k ohm
• C_t = timing capacitor (connected to pin 9) in uF

For the values given in the schematic, C_t is capacitor C5 which is nominally 33uF with a target R_t of 46.25 kohm (for the total of R7 and RV2) which gives a frequency of 0.2849Hz. The 'timing resistor' R_t is composed of the potentiometer RV2 (with R7 to provide a minimum resistance if the potentiometer is 'turned full off') to enable calibrating the frequency of the CD4060 oscillator to allow for variation in the actual capacitance value of C5.

Having established a nominal frequency of 0.2849Hz this means the first available output from the CD4060 (pin 7) will have a frequency of 0.2849/16 = 0.0178Hz or a period of 56 seconds. Subsequent outputs on the CD4060 have half the frequency (i.e. twice the period).

The following table summarises the results (the Downloads Section below has an Excel spreadsheet with the necessary calculations):

In order to ensure that the CD4060 starts counting from 'zero' when powered on, the CD4060 reset pin (pin 12) is connected to R5 and C4 which provide a logical '1' on power-up, resetting the CD4060, which then decays to low, enabling the CD4060.

The schematic shows that pin 7 of the CD4060 is connected to a current limiting resistor (R8) and LED (D1). This is to provide a visual indicator when calibrating the CD4060 (via RV2). The LED D1 should flash with a period of 56 seconds to provide the necessary output frequency at the other CD4060 pins as per the table above. For applications where battery power is limited (as opposed to the current situation where a solar cell is available for trickle charging), the jumper JP1 could be removed after the CD4060 has been calibrated.

Pump On Timer (NE555)

The desired frequency input pulse from the the CD4060 is selected via rotary switch SW1, which triggers the NE555 connected in monostable mode. The NE555 via output pin 3 turns on the relay (and hence connected pump) for the period of time specified via the NE555 timing controlled by capacitor C1 and resistors R1 plus potentiometer RV1.

The calculation of the NE555 monostable timing is well documented on the web (1) and in the case of the values specified on the schematic give a range of 'on times' between 4 and 6.6 minutes.

In order to start the NE555 monostable timer, a low pulse is required at pin 2 of the NE555. However, the CD4060 provides a constant 'high' or 'low' at the output pins (at the frequency determined by the oscillator etc as discussed above). Transistor Q1 (controlled via the base connection to the CD4060) and the RC network formed from R3, R4 and C2 convert the output from the CD4060 into a 'pulse' suitable to fire the NE555 monostable. Switch SW2 is provided so that the pump can be 'manually' started at any time (the pump operates for a single period of time as determined by the NE555 monostable).

The LED D2 is provided to give a visual indicator when the NE555 is triggered and hence the relay is energised and the pump should be operating. This is for calibration of the NE555 monostable and trouble shooting purposes only. The jumper JP2 can be removed to save battery power if such a visual indication is not required.

Power Supply

An automotive 12V battery is used to power the circuit and pump. The battery is trickle-charged using a small solar cell/regulator. See the Hydroponics Section for photographs. The pump is 12V DC and draws 3.8A when operating (as measured with DMM).

A Tyco relay was already available rated 10A so this gives plenty of 'head room' for any current surge at start up. Could possibly use a 5A rated relay, but the Tyco was already available. The relay nominally requires 12V (75ohm coil resistance) but operates satisfactorily using 9V. Therefore, the circuit is operated at 9V, produced via a linear voltage regulator (LM7809). The regulated 9V ensures stable, reproducible circuit operation, avoiding potential problems with fluctuating battery voltages. The connection of the LM7809 is as per the datasheet.

Calibration

Potentiometer RV2 allows 'trimming' of the oscillator input of the CD4060. Nominally, the frequency on pin 7 of the CD4060 should be 0.2849Hz in order to give the range of output times specified in the table above. If an oscilloscope is not available, the frequency of LED D1 could be measured with a stop watch to be nominally 56 seconds.

Potentiomter RV1 adjusts the 'on time' of the pump/relay when the NE555 monostable has been triggered. With the nominal value of 470uF for capacitor C1, RV1 will allow a range of approximately 4 to 6.5 minutes. This can be timed using LED D2 (or listening to the 'click' of the relay engaging/disengaging) and a stop watch or similar.

Downloads

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  Pump Timer Schematic

The project was constructed using veroboard (no PCB was used in this case).

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  Pump Controller - Veroboard

Pump Controller - Veroboard Layout

The pump/relay 'on time' can be lengthened by increasing the value of R1 and or RV1 (and or increasing the value of C1 - although larger value resistors are cheaper than electrolytic capacitors). Reproducible timings of ~10 minutes appeared to be achieveable when the circuit was prototyped on breadboard. However, the selected time of ~5 minutes suited the current application and matched component values that were readily on-hand.

After the timer had been running for approximately one (1) week, the absolute cycle times and approximate start/stop times were not distinguishably different from initial power-up, as determined by a stop watch.

The circuit consumes ~5mA when relay/NE555 are not operating and ~8mA if LED D1 is being used as a visual indicator. When the relay is energised (NE555 monostable has triggered and including LED D2 as visual indicator) the circuit consumes ~30mA. Measurements made with DMM on the constructed circuit on veroboard operating the actual pump.

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