|IC1 ……. CD74HC132||R3 ………. 3.9M|
|B1……… Two AAA alkaline cells, with holder||R5, R6 ….. 680|
|C1, C3 … 1nF (0.001uF) or 2.2nF (0.0022uF)||R7 …….… 15|
|C2 ……… 100uF/16V electrolytic||R8 …….… 47K|
|C4 ……… 220nF (0.22uF)||D1 …….… 1N4148 or1N914|
|R1 ……… 470K (all resistors 1/4W, 5%)||D2 ………. MV8191 orHLMP-D101A|
|R2, R4 … 100K||Q1, Q2 ….. 2N4403 or 2N3906|
Using a Veroboardmother-board about the same size as the battery holder, adaughter-board was added to hold the remaining parts:
1) IC1D is a CMOS Schmitt trigger oscillator at about2KHz. It starts and continues to oscillate with a supplydown to 1.24V (the lowest output voltage of my LM317variable power supply) or less.
2) IC1A is an inverter.
3) IC1B is a Schmitt trigger NAND gate. Its output is lowonly when both inputs are at, or higher than the upperSchmitt trigger threshold voltage. With 47 ohms or lessbetween the probes, an input is always low, so the outputis always high. With a resistance of only R8 between theprobes, the voltage across C3 is high most of the time,so the gate’s output is low for ½ the oscillator’speriod. With a resistance that is halfway, then C3 ischarged high by that resistance when the oscillator’soutput is high, then is discharged when the oscillator’soutput is low. When C3 is being discharged, then pin 12of the gate is high, and pin 13 is also high until thedischarging voltage of C3 reaches the lower Schmittthreshold voltage. During this time, the gate’s output islow. So the low time of the gate’s output depends on thevalue of the resistance between the probes. This isPulse-Width-Modulation of the low output of the gate.
4) IC1C is another CMOS Schmitt trigger oscillator atabout 2Hz. D1 and R4 discharge C4 quickly so that itsoutput is low for only about 15ms with a 3V battery, andabout 25ms with a 2V battery.
5) The series connection of Q1 and Q2 performs like a NORgate, so that the LED lights only when both inputs to thetransistors are low.
6) R7 is a current-limiting resistor for the 1.8V LED.With a 3V battery, the LED current is about 35mA.
1) When the soil is very dry, the LED flashes brightly,since the soil’s resistance is very high.
2) When the soil has been watered a few days before, butis drying, the LED flashes dimly,
3) When the soil is damp because it has been recentlywatered, the LED is off.
Note that different soils have a different resistance.Also, sometimes, watered soil will continue to have ahigh resistance until the soil absorbs the water, a delayof about one hour.
Although the LED’s current is 8mA with a 3V battery, itis lighted for only a maximum of only about 1/64th of thetime, so its maximum average current is only 550uA. Theremainder of the circuit draws 200uA. The total is 750Afor new batteries, and about 250uA for run-downbatteries. Therefore the exponential current of 300uAwill continue with 1000mA/hr batteries for 2000 hours, orabout 4.6 months.
The LED’s current is logarithmic with the soil’sresistance, so that when the resistance is one-half, thenthe LED’s current is one-tenth. If you water the plantswhen they need watering, then the average LED currentwill be very low, and the batteries should last for aboutone year.
1) Try to obtain the very bright and wide-angle LEDs thatare listed. Samples are available from Fairchild.
2) Use tinned copper 1.5mm diameter buss-bar wire about8cm long for the probes.
3) Use silicone caulking to attach and seal the Veroboardto the battery holder, and to seal the battery holder’scontact holes.
4) Perhaps the project can be mounted in a plastic bottlefor pills, available from a pharmacy (chemist?), with theprobes sticking out of its lid.
Additional information:A list of plants and their watering requirements is here: http://electronics-lab.com/projects/science/002/Usermanual_moisture_meter_1820.pdf