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# Circuit 31 &amp;#150; Dry sensor (Transistor).

Extracts from this document...

Introduction

Circuit 31 – Dry sensor (Transistor).

Function

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

Components

## SW1 = Toggle Switch

VR1 = 1M ohm

R1 = 470 ohm

R2 = 2200 ohm

R3 =1k ohm

Transistor = BC108

5mm Standard LED

## Operation

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.

If more than 0.

Middle

## 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.

### Operation of Voltage Divider Circuit

Consider the value of the variable resistor set so its resistance is very high.

Consider the probes in moist conditions, their resistance is very low.

With the large resistance (VR1) and resistance RD connected in series, a large voltage drop will occur across VR1, so voltage at connection of VR1 and the probes will be very small (less than 0.6V).

Conclusion

With 7V dropped across it, and the maximum permissible current of 20 mA flowing through the LED, the size of resistor can be calculated using Ohms Law:

Note

In electronic circuits it is not good practice to have components operating at their upper limits so a resistor value of 390 or 470 ohms should be used for R1

Remember the larger the value of R1 the less current is drawn from the battery.

So the battery will last longer at the expense of the LED being a little dimmer.

## Value of R2

By adjusting VR1 so its resistance is small, the voltage on the base could be very high. This high voltage could damage the transistor. It is for this reason a base resistor (R2) should always be inserted.

R3 is also placed in circuit as a safety precaution.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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