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
- Rt = timing resistor (connected to pin 10) in k ohm
- Ct = timing capacitor (connected to pin 9) in uF
For the values given in the schematic, Ct is capacitor C5 which is nominally 33uF with a target Rt of 46.25 kohm (for the total of R7 and RV2) which gives a frequency of 0.2849Hz. The 'timing resistor' Rt 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):
| ||Output Period ||Rotary Switch|
|7||O3||56||0.9||0.02|| || |
|5||O4||112||1.9||0.03|| || |
|4||O5||225||3.7||0.06|| || |
|6||O6||450||7.5||0.12|| || |
|n/a||O10||7189||119.8||NA Ext.|| || |
|3||O13||57514||958.6||16|| || |
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.
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.
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.
||Microsoft Excel spreadsheet calculate CD4060 output frequencies: