Creative Commons Licence
Quad 555 Irrigation Controller by JF Morton is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License                                                            11 September 2023


A Pi-free, Arduino-free Crop Watering Solution

Commercially available soil moisture sensors provide a voltage output which varies with soil moisture.  This needs a programmable micro, an Arduino or Raspberry PI, to read the voltage and control an irrigation system.  The Quad 555 sensor uses the same 555 integrated circuit but instead of a voltage it gives a frequency signal. The Quad 555 Control Unit uses three more 555s to manage the water supply.

In other words Quad Q555 is entirely analogue, using no advanced hardware and no programming. 

This note sets out the principles and operating logic of the system.  A full breadboard layout suitable for an advanced DIY electronics project and build guidelines are given at the end.   You will need an understanding of how resistor-capacitor (RC) combinations are used to set the timing of a 555 operating in monostable and astable modes.  (Electronics for Dummies, Chapter 11 and are good places to start.)

Quad 555 draws on several ideas from the excellent book IC 555 Projects.   I owe a big debt to Mike Jordan for the loan of his copy.  Without it Quad 555 would never have existed.

                                                    Quad 555 at Work – End of the Season


Quad 555 uses a capacitive soil moisture sensor. The alternative, resistive sensor suffers from corrosion and its readings are affected by the soil chemistry, fertiliser applications in particular.

Figure 1 shows an outline of a co-planar capacitor.  For Quad 555 wet soil is the dielectric such that the sensor’s capacitance varies with the moisture content.  Figure 2 shows the sensor unit circuit.  It consists of a 555 in astable mode with the sensor probe as the capacitor in the RC combination.  As the capacitance of the sensor varies with soil moisture so does the frequency of the output signal. 

Capacitance increases with soil moisture.  The frequency-moisture relationship is inverse: the drier the soil, the higher the frequency.

Figure 1   A Co-Planar Plate Capacitor

Figure 2    The Sensor Circuit

With acknowledgements to


         The Sensor Unit in Operation


Quad 555 is based on a technique called re-triggering.  A monostable 555 circuit can be retriggered by connecting the input signal to a transistor which discharges the 555’s capacitor early.  So long as the interval between trigger signals is shorter than the 555’s timed period, that period is extended and the 555’s output is locked high.

IC555 Projects gives a circuit for Overspeed Warning using a pair of 555s to signal when a machine starts to go too fast.  This logic table sums up how it works with re-triggering on both 555s.  What follows is a highly abbreviated description.  IC 555 Projects gives fuller details on pages 21 and 85-87, .


  Normal - Long Interval

  High - Short Interval
  Timed Period < Input Interval
  Not re-triggered - Pulsing

  Period> Input Interval
  Re-triggered - Locked High


  Timed Period > IC555-a Interval
  Re-triggered - Locked High

  Times Out
  Goes Low

Quad 555 uses this circuit with the soil moisture sensor unit providing the input signal.  Instead of sounding a warning, a low output from IC555-b switches on the irrigation timing unit.  The result is this extended logic table.


  Low - Long Interval

  High - Short Interval


  Timed Period < Input Interval 
  Not re-triggered - Pulsing

  Period> Input Interval
  Re-triggered - Locked High


  Timed Period > IC555-a Interval
  Re-triggered - Locked High

  Times Out
  Goes Low, Sets Irrigation Unit




Quad 555 is tuned by a potentiometer on the IC555-a RC circuit to increase or decrease the Timed Period relative to the input interval.  This changes the soil moisture level at which the system goes from ‘wet’ to ‘dry’.


Quad 555 could control an irrigation supply directly, turning the water supply on every time the sensor signals ‘dry’.  To prevent frequent on-offs, a timing  unit has been added so that a ‘dry’ signal turns the water on for a fixed period which can be set by the operator.  The timing unit uses the fourth 555 of the Quad, together with a NAND gate IC and a decade counter to time the length of each irrigation. Once again the circuit is based on one taken from IC555 Projects (p25, section 2.8 Very Long Time Delays) .   It consists of:

- A 74HC00 set of NAND gates providing a set-reset flip-flop or latch.
- A 555 running in astable mode as the clock.
- A 4017 decade counter multiplying the 555 outputs to extend the clock period.

It operates in three steps:

        1) Control Unit goes Low and sets the flip-flop.
        2) Flip-flop turns on the irrigation pump and starts the decade counter.
        3) Decade counter times out and resets the flip-flop to switch the pump off again.

Different irrigation periods can be selected by switching between different resistors in the 555 RC circuit to lengthen or shorten the ‘on’ period of the decade counter.  


With an appropriate relay Quad 555 could turn on a mains irrigation pump or an electronic tap.  It is being tested connected to the small Irrigatia solar-powered unit shown in the photo.  Operating at 3.6v this pumps water from the rainwater tank to a set of 12 irrigation drippers.  The unit's internal timing circuitry is bypassed so that Quad 555 controls the pump directly.


Figure 3 on the next page presents the Quad 555 circuit in barebones form.  You can follow the system logic through the pink circuit taking the signal from Pin 3 on the Sensor Unit 555 to Pin 2 on the Control Unit IC555-a and then from Pin 3 to Pin 2 on the right-hand IC555-b.  The Control Unit output goes from Pin 3 to Pin 1 on the Flip Flop and through to the system output. 

Note how the pink circuit also links to the two BC560 retriggering transistors.
The timing logic can be followed through the orange circuit from the Flip Flop to start the Clock Counter at Pins 13 and 15.  When the count reaches Pin 12 on the counter the yellow/green circuit takes the signal back to Pin 5 on the Flip Flop and resets it.         

The Sensor Unit and the Clock 555s have an equal-mark circuit using only one resistor instead of two, making the on and off periods equal.  (See IC555 Projects, pp13/14)  This layout is used to save space on the Sensor Unit circuit.  This has to be attached to the sensor probe and needs to be small.

Figure 3    Quad 555 Barebones Circuit


Quad 555 uses a double sided co-planar moisture sensor.  Figure 4 shows the principles involved.

Figure 4  Co-planar Capacitor Design


Sketch c shows the optimum layout, with the positive plate on one side, in red, facing the positive plate on the other side, and negative facing negative.  This gives the wet soil, ie the ‘external medium’, the maximum influence on the sensor.

Acknowledgements to

The internet shows a number of possible designs for a capacitive soil moisture probe: ranging from one made out of an old coke tin to sophisticated designs made by professional PCB manufacturers.   While undoubtedly the best option, the latter is expensive, in the UK at least: £200 for three probes.

For the Quad 555 test system a DIY sensor has been made using two pieces of Vero board cut to have 6 tracks, 5.75” long.  Headspace is left for attaching the sensor’s 555 circuit to the probe.  Figure 5 shows the layout

Figure 5        Vero Board Layout for a Co-Planar Capacitor

    A Side

    B Side

On the A side, the purple tracks – tracks 1,3 and 5 - are bridged to form the +ve plate of the capacitor. The pink tracks – 2,4 and 6 - are bridged as the –ve plate.  The bridges are standard breadboard connectors soldered to the tracks. 

The B side layout is mirrored so that when it is glued to the A side back-to-back +ve tracks are opposite +ve, and –ve opposite –ve.  The red pads show where the back-to-back tracks are linked with a through-pin soldered to the track on each side.


Sealing the probe can be difficult.  It must be watertight without the coating being thick enough to reduce the electric field.  Internet sources do not offer a clear solution.  One describes soak testing the solder mask on a manufactured PCB for 12 hours and concluding ‘so far so good’.  Given that the probe will spend months in wet soil, 12 hours in pure water is not much of a test. 

Sealing the DIY capacitor is particularly tricky because the holes in the Vero track offer weak points for moisture to enter. 

                    A Section of Sensor Track

Fabrication involves the following steps:

1.  Gluing the two sides together – eg with Araldite – ensuring the tracks are exactly aligned back-to-back.
2.  Filling the Vero track holes.
3.  Soldering up the connections and adding the terminal wires.
4.  Sealing the unit.

For the unit under test masonry filler was used for Step 2.  This may have been a mistake.  The filler is porous and likely to absorb moisture if the sealant is penetrated.  Several different sealants have been tried: nail varnish, hand-applied solder mask, acrylic conformal coating and polyurethane varnish.  After each sealant was applied it was soak tested in water with repeated measurements of the unit’s resistance and capacitance. 

The capacitance results were good.  In air residual or internal capacitance was between 30 and 40 pF.  In pure water capacitance was measured between 0.3 and 0.8 nF, enough to give a clear frequency signal through a 555.   Capacitance was clearly responsive to changing levels of soil moisture. 

None of the sealants tested provided total water proofing.  Some passed the 12-hour test.  Over longer periods all lost resistance.  Two coats of polyurethane varnish did best.  After several days, resistance dropped to around 10 mΩ but it appeared to stabilise at that level. 10 mΩ is high enough.  It just needs to be much higher than that of the resistors in the 555’s RC combination. 

The polyurethane-sealed unit was put into the Quad 555 test system.  It has now been in wet compost continously for over three months.  Although there are some signs that the sensor’s capacitance has drifted, it is still within the range of the Quad 555’s tuning potentiometer.

The Test Sensor Unit

The Sensor 555 circuit must be mounted directly onto the co-planar probe so capacitance in the connections does not muffle the sensor signal. You will need to solder the circuit up on a small PCB, ideally no wider than the probe itself and no more than a couple of inches long.  A small, single IC  prototype board such as the Datak 21-114 suits well.  Once the circuit has been mounted the PCB can be cut down to size.

The photos show the very basic setup installed in the Q555 test system including the mounting platform glued to the probe and the rubber band holding the 555 PCB onto the probe. 

Sensor Circuit Board and Mounting Platform


Figure 6 shows a complete Quad 555 circuit diagram laid out to suit a breadboard set up.  A single IC556 is used in place of the two Control Unit 555s. As well as the main breadboard a smaller one suitable for a single IC circuit will be useful. A multimeter will be essential, ideally one with options to measure capacitance and frequency.

The diagram and the Parts List below show the resistor and capacitor values in the test Quad 555 currently in operation.  You will find it helpful to have a full range of resistors, from 10Ω to 2mΩ, and capacitors, from 100pF to 470µF, available.  The build will involve quite a bit of trial and error with different resistor-capacitor combinations.  Extra LEDs will also be useful as tell-tales at different places in the circuit.

IC555 Projects notes that standard 555s and 556s have an “antisocial habit of drawing a large current pulse from the supply when they change state.”  This noise can cause problems elsewhere in the logic.  This pulse does not happen with CMOS 555s.  To guard against circuit noise, a CMOS 555 is used on the sensor unit and 0.01µF decoupling capacitors are placed across the power supply on all the other ICs.  

As well as the CMOS 555, the flip-flop uses a CMOS 74HC00.  You should check the standard precautions for handling static-sensitive CMOS devices before you install these two ICs.

Parts List

  a) Integrated Circuits
   - CMOS 555 x 1
   - 74HC00 x 1
  - 555 x 1
  - 4017 x 1
  - 556 x 1
  b) Transistors
  - BC560 PNP x 2
  - BC108 NPN x 1
  c) Capacitors
  - 0.001µF x 1   
  - 0.01µF x 7
  - 0.0047µF x 1      - 100µF x 1
  d) Resistors
  - 1.5mΩ x 1   
  - 220kΩ x 2       
  -  330Ω x 2   

  - 1mΩ x 1   
  - 100kΩ x 1  
  - 220Ω x 1   

  - 470kΩ x 1
  - 10kΩ x 3
  - 47Ω x 1
  e) Trim Pots, LEDs, etc
  - LED x 2 (1 green, 1 red)
  - 5-way rotary switch
  - N4001 Diode x 1

  - Trim Pot 100kΩ
  - Holder for 4 D batteries

  - Momentary push button switch


Q555 is a complicated build.  You may find it best to do it in stages.  This makes it easier to check for mistakes and be sure the logic is working as expected.

The circuit diagram shows two tell-tale LEDs, one on the output of the right hand 555 (IC555-b) of the Control Unit and one on the output of the Flip-Flop.  When the soil is wet, the Control Unit LED is lit green.  When the Q555 switches to dry, the Control Unit LED goes off and the Flip Flop LED goes red to show the pump is operating.  At early stages in the build it can help to add more LEDs, on the output of the Sensor Unit 555 and on the left hand 555 (IC555-a) of the Control Unit.  This makes it possible to see the state of each 555 and the point at which it changes. This helps you to understand how the re-triggering is working.  Starting with RC combinations operating at low frequences will let you see what is going on.

The Main Breadboard in Operation – Red Light = Dry

I suggest you start the build with a dummy Sensor Unit and the Control Unit only.  If you build the dummy sensor on a small, separate breadboard, you will be able to connect it to the co-planar soil probe at the last stage of testing.  However, the probe will operate the sensor in the kHz range, too fast to be visible.  Initially, therefore, the dummy should be set up with a standard 100μF capacitor and resistors between 10kΩ and 100kΩ.  This way it will work  at visible frequencies: > 1 sec per cycle.  To match this, resistors and capacitors on the Control Unit 555s will also need to be higher than the final values shown in the circuit diagram.  The Tuner Potentiometer may also need to have a wider range during testing.

The dummy sensor output will be fixed.  The Control Unit logic can be tested against that output by varying the speed of the left hand IC555-a using the Tuner Potentiometer. With the right sets of capacitors and resistors it will be possible to see the following:

Dummy Sensor
  Fixed Output Frequency   LED Flashing

          IC555-a Left
  Fast: period<dummy interval
  LED Flashing

        IC555-b Right
  Period>IC555-a interval
  Locked High, LED on

  Slow: period> dummy interval
  Locked high: LED on

  Goes low
  LED off

Time spent on trying different resistor and capacitor combinations to get to this result will not be wasted.  It is the best way to be sure you understand what is going on.

Once you have the Dummy Sensor/Control Unit set up working, it is time to add the Flip Flop, Clock and Counter.  Here too, LEDs on the different outputs will help you track what is happening.

The Flip Flop uses all four of the NAND gates on a 74HC00 IC.  Two form the set-reset flip flop or latch.  The other two are used as inverters.  The 74HC00 pin table is as follows.  The figure beside it shows the standard logic table for an SR Latch. 

Flip Flop   
   NAND 1  Pins 1-3    Set input from Control Unit on 1.
   NAND 2  Pins 4-6    Reset input from Decade Counter on 5.
   Reset Q’  Pin 6         Unused
   Set Q       Pin 3         Output to Pump Switch and NAND3

   NAND 3  Pins 8-10     Inverts Q to send low start signal to Counter
   NAND 4  Pins 11-13   Inverts Counter output to reset Flip Flop at Pin 5

Note the pull up resistor holding Flip Flop reset (Pin 5 on the 74HC00) high until a low signal is received.  The push button connecting Pin 5 to ground allows the operator to reset the Flip Flop and stop the timing count early. 

The logic is as follows:

    Sensor             Control Unit                Flip Flop                           Decade Counter   
    Wet                  High                            Reset  S1, R1, Q 0            Inhibited
    Dry                  Low                             Set     S0, R1, Q 1             Running

If the Control Unit has gone to Wet by the time the Decade Counter cycle ends, the counter’s signal resets the Flip Flop.  If the Control Unit is still at Dry, the reset is Invalid.  Q stays high and the count starts again.  The circuit diagram shows the Clock Circuit with a 100μF capacitor and a switch between four different resistors from 100kΩ to 1.5mΩ.  This should give timed periods of approximately 2, 10, 20 and 30 minutes.

The final step is to set up and test the irrigation pump switching circuit.  This is the 3.6v circuit shown at the bottom of the diagram.  Note the flyback diode guarding against reverse current flows. (See


Once the logic is working at every step from Sensor to Flip-Flop, Decade Counter and Pump Switch, it is time to replace the capacitor in the dummy sensor with the co-planar soil moisture probe.

The circuit diagram shows the Sensor Unit and Control Unit resistor and capacitor values from the Quad 555 currently under test.  What values will work for you may be different, depending on the capacitance of the sensor probe, the type of soil in the pot, the ambient temperature and other factors. A fair amount of trial and error will be needed; trying different resistors on the sensor breadboard and different RC combinations in the Control Unit.

To do this testing push the probe into the a pot of moist soil.  Connect the probe terminals directly to the small breadboard with the dummy sensor circuit, in place of the capacitor.  Three longer connections – Vcc, Gnd and Signal – from the Sensor to the Control Unit/Irrigation Unit breadboard will allow you to make changes and tests on that board without disturbing the Sensor. 

There are two key tests:

1. You can switch the Control Unit output from High to Low by varying the Tuning Potentiometer.

2.  With the Control Unit output at low, indicating dry soil, you can see it switch to high when you add water to the pot. 

 The Main Breadboard in Operation – Green Light = Wet


The test Quad 555 is set up with the Sensor Probe in one of nine 10 litre pots of compost in a green house.  By UK standards it is a hot green house.  Each pot has an irrigation dripper fed from the pump unit.  Six of the pots have tomato plants, including the sensor pot.  The other three have sweet peppers.

For testing, the Control and Timing units have been left on a breadboard.  The wiring is as follows:

    From Sensor Unit             – 3 strand.  Vcc, Gnd and Signal from Sensor 555 Pin 3 to Control Unit IC555-a Pin 2 and left retrigger transistor.
    From 3.6v Solar Panel     – 2 strand.  Gnd to Gnd,  3.6v to 3.6 Vcc rail
    To Pump                           – 2 strand.  3.6 Vcc to +ve, transistor collector to -ve  

Quad 555 operates at 6v.  This can by supplied from the mains through a transformer but the test system is powered by four D torch batteries.  With the system on 24 hours a day, the batteries last for over a month.

 Figure 6                        BUILD DIAGRAM FOR QUAD 555