• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

Circuit 31 – 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.

Circuit diagramimage00.png

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.

...read more.

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).

...read more.

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:

image01.png

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.

...read more.

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

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Electrical & Thermal Physics essays

  1. Making, Calibrating and Testing a Sensor

    maintain your variables to ensure the experiment as fairly and accurately as possible. Firstly, in order to minimize errors it is important to identify and highlight any potential area which could create error. In this particular experiment the main factor which contributed to error was the light source.

  2. physics sensor coursework

    R1 = (6.39 � 1500)/ (10 - 6.39) R1 = 2655.12 ? At 30 lux: VA = VB + p.d. VA = 1.64 + 3.87 VA = 5.51 V R1 = (V1 R2)/ (V-V1) R1 = (5.51 � 1500)/ (10 - 5.51)

  1. Using an LDR to detect the intensity of plane polarised light allowed through a ...

    three times before I take the values of the grew rows, making sure that I do not have a limited set of values at the end of it. Preliminary Readings Status Beginning Reading (V) Final Reading (V) In Ambient Light No Polaroids One Polaroid Uncrossed Polaroids Crossed Polaroids Actual Experiment Angle of Polaroid (o)

  2. Making a Sensor

    see diagram below. The diagram on the next page shows how the experiment works. When I first asked for my apparatus, I was going to ask for a light gate but it was bought to my attention that there was not able to use one.

  1. Using a sensor to measure an angle.

    This is known as a non-linear calibration. Aim For this project we had to devise a sensor, which would enable us to measure something. With my experiment I plan to use a rotary potentiometer to measure the angle at which an object is leaning.

  2. Testing a rotary potentiometer for use as a depth sensor

    Circuit Plan: Plan: I will set up the circuit as shown above. I will measure, using a multimeter set to measure in Ohms, the maximum resistance of the potentiometer, and set the resistance box to a resistance as close to that as the dials allow.

  1. Investigate the properties of a sensor.

    We used a 6volt battery and placed an Ammeter and Voltmeter (connected in parallel to the potentiometer) in the circuit so that we could calculate the differing resistance as the input changed. Below is a circuit diagram for our experiment.

  2. Build a successful sensor that will measure the proximity of a light source.

    It has another loose tube inside with an attached LDR or photodiode at the end. Therefore can be moved away from the light source. It is also equipped with a stuck on tape measure so it will be easy, quick and reliable to measure and alter the distance.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work