Circuit 31 – Dry sensor (Transistor).

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Circuit 31 – Dry sensor (Transistor).


This circuit could be used to detect if soil is too dry.

Circuit diagram


SW1 = Toggle Switch

VR1 = 1M ohm

R1 = 470 ohm

R2 = 2200 ohm

R3 =1k ohm

Transistor = BC108

5mm Standard LED


This circuit uses a transistor (BC108) as an electronic switch.

Transistor action.

When the voltage on the base of the transistor is less than 0.6V the transistor is switched off.  No current can flow through the transistor from the collector to the emitter, so no current can flow through R1 and the LED.  Thus the LED is unlit.

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If more than 0.6V is applied to the base, the transistor switches on.  Now current can pass easily from the collector to the emitter, so current can flow from the top rail, through R1, the LED and the transistor, back to the battery.  The LED is now lit.

Resistance of the probes

Resistance of the probes will be referred to as RP.  In dry conditions resistance RP will be extremely high, whereas in moist conditions resistance RP be relatively low.

Consider the variable resistor (VR1) and RP in series.  They form what is known as a voltage divider circuit.


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