Price - Stock No 110

Price - Stock No 111 with Push Button Switch




  1. Check  the  continuity  of  the  tracks  with  an ohm-meter or an electronic circuit tester
  2. Turn the PCBoard over so that you are looking at the plain side.  From  the  drawing, find the locations of the components.  You will need to turn the PCB  around  so  that  it  coincides  with  the  placement  sheet.  Remember that you are seeing the copper track through the PCB.
  3. Identify  the  values  of the two resistors by their colour bands and/or an ohm-meter - see Resistors.  Bend the legs of the resistors to fit their respective locations.  Push them through and firm the resistor down against the PCB and solder in place - see Soldering Technique.
  4. The diode is polarised and must be inserted in the correct orientation - see  Diodes.  Solder in place.
  5. Offer the transistor into its position with the flat on  the  body the  way  round  that the drawing indicates.  Bend the legs to fit into the holes and solder.  
  6. The LED is polarised - a semiconductor - and will not work if  placed  the  wrong  way round.   Push the legs just through the PCB so the LED stands high off the board.  You should check that the small flat on the flange  at  the base  of  the globe is the same way round as the drawing shows.   Solder in place. 
  7. Depending on wheather you are using a switch or not, refer to the correct PCB placement diagram and identify the locations for the PCB Pins.  Insert and solder in place.
  8. The battery snap connection points also vary between the two models, solder the battery snap to it's PCB pins.
  9. If a switch is included this can now be soldered to it's PCB pins (a small amount of hookup wire can be used if the switch is to be mounted away from the PCB - ensure enough hookup wire is left for the next step).
  10. Cut the hookup wire into two equal lengths and bare the ends of the hook-up wire.  Bare and tin the ends of the wires - see Tinning wire.  Solder one end of the wires to the PCB pins for the probes.  Soilder the other ends of the wires to the teardrop terminals ready to attach to your probes.
  11. Connect  a 9 volt battery and test. When you bring the two probe wires together the LED will glow brightly.  Wet your finger  and move the probe wires apart on  the wet area.   The LED will dim off, and its brightness will reflect the wetness (or dryness)  of your finger. 


If it doesn't work check  that you have all the locations correct and that you have the LED and the diode in the correct polarity, and that the flat on the transistor is as the drawing shows.



The LED is a Diode which is a semiconductor.  This means  that it  will  pass  current  in  one direction only.  It has a Positive Leg (+) (A for Anode) and a Negative Leg (-) (K for Kathode).  Check the letters  A & K on the drawing.  The LED will emit light when current is passed through it.  The 390R resistor limits the current through the LED and ensures it is not damaged by excessive current.

The power diode IN4002 as well is also a semiconductor.  As with the LED the power diode only allows current to flow in one direction.  This component ensures the circuit id not damaged by accidently connecting the battery the wrong way around. 

The  transistor BC548 has three legs C (collector) B (base) E (emitter).  The middle one is the Base leg.  This is an NPN transistor; the E leg connects to the Negative battery pole.  In this circuit it acts as a variable switch.  The more current leakage between the two probes  (the wetter  the  soil), the more it switches on, and the LED glows brighter - see  transistors.  The 100K resistor works in conjunction with the transistor by biasing it to the off state, this ensures the LED is only illuminated by the current leakage between the probes.

If it doesn't light up in your potplant, then get the watering can fast!

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