• Soldering
• Reading Schematics
• Sketching
• 3D modeling
• 3D printing

What is an LDR

LDR stands for Light Dependent Resistor and is also known as a Cadmium Sulphide (CdS) cell, a photoresistor or a photocell. An LDR is made of a high resistance semiconductor. It is similar to a normal resistor with the exception that a normal resistor has a fixed value and the LDR’s resistance is dependent on ambient light, the light that exists in the LDR’s vicinity. So a photocell is a variable resistor, which produces a resistance proportional to the amount of light it senses. It is very hard to use an LDR to determine the exact amount of light in a given setting but these components are great for determining light in a broader sense, whether it is dark or light. There are two basic circuits that use LDRs:

• circuits activated by darkness
• circuits activated by light

Image from Adafruit

To measure the resistance of an LDR you will need a multimeter:



Image from www.doctronics.co.uk

1. Write down the resistance value you measure when the LDR is exposed to the room lights
2. Cover the LDR with your hand so that it is in the dak. The resistance of the LDR will increase. Write down the new resistance value
3. To create a voltage divider you would pick a fixed resistor in the middle of the the values you wrote down

Introduction to Transistors

A transistor is a semiconductor with 3 leads:

1. Emitter
2. Base
3. Collector

Transistors allow or restrict current flow with the application of voltage.

There are two types of transistors: bipolar and field-effect transistors (FETs). Unlike bipolar transistors, the leads of FETs are referred to as gate, source and drain.

Bipolar transistors can be divided into two categories: NPN and PNP. NPN transistors act like normally opened switches. PNP transistors act like NC switches.

A Transistor’s Function

Transistors can amplify the current or can be used as switches.

A Transistor’s Operating Values

These values vary and you must refer to the datasheets.

Transistor Basics

Bipolar Transistors are composed of three layers:

• An Input Layer, the emitter
• A Control Layer, the base
• An Output Layer, the collector

Layers can be N-type (negative) or P-Type (positive).

Layers are formed by doping silicon with traces of impurities. Silicon treated with Boron produces negative type material. It is negative because it has a surplus of electrons. Doping the silicon with Phosphorous produces positive material, or material with a surplus of electron holes.

Silicon transistors are considered semiconductors because they conduct only a small amount of current unless a change in voltage is applied at a juncture.

When current is applied to the input layer, or the emitter of an NPN transistor, electrons flow to the P-type base and fill the positive material, or electron holes.
When the holes are filled, the material no longer conducts current. But when
a small voltage is applied to the control layer, or base, that current will produce additional holes which in turn allows current to flow from the emitter through the base to the collector.

The difference between a PNP transistor and an NPN transistor is the polarity, or proper biasing of the junctions. Transistors restrict the amount of current passed according to a smaller, controlling current. The main current from collector to emitter (PNP) or from emitter to collector (NPN) depends on the type of transistor it is. The small current that controls the main current goes from base to emitter (PNP), or from emitter to base (NPN), depending on the kind of transistor it is.

The arrow always points against the direction of electron flow.

Bipolar transistors are called bipolar because the main flow of electrons through them takes place in two types of semiconductor material: P and N, as the main current goes from emitter to collector (or vice versa). Two types of charge carriers: electrons and holes, comprise the main current through the transistor.

All currents must be going in the proper directions for the device to work as a current regulator. The small, controlling current is usually referred to as the base current because it is the only current that goes through the base wire of the transistor.

The large, controlled current is referred to as the collector current because it is the only current that goes through the collector wire.

NPN transistor

• The base-emitter junction behaves like a diode.
• A base current I_B flows only when the voltage V_BE
  across the base-emitter junction is 0.7V or more.

• The small base current I_B controls the large collector current Ic.
• The collector-emitter resistance R_CE is controlled by the base current I_B:
  • I_B = 0   R_CE = infinity   transistor off
  • I_B small   R_CE reduced   transistor partly on
  • I_B increased   R_CE = 0   transistor full on (‘saturated’)

Transistors are not a good choice when switching large currents (> 5A). When switching large currents, you need to use relays.

Switches

If there is no current through the base of the transistor, the transistor shuts off the controlled current like an open switch and prevents current through the collector. A base current, turns the transistor on like a closed switch and allows a proportional amount of current through the collector. Collector current is primarily limited by the base current, regardless of the amount of voltage available to push it.

Part List (BOM)

• Breadboard—Jameco
• Wire—Jameco
• Battery Clip—Jameco
• 9 Volt Battery
• BC548 or 2N2219 or 2N2906 transistor—Jameco
• 470Ω resistor—Jameco
• 1KΩ Resistors—Jameco
• 1 LED—Jameco
• 1 LDR—Jameco
• 1 10K potentiometer—Jameco
• Dual Mini Board with 213 Holes Protoboard—RadioShack

The BC548 is a general purpose silicon NPN BJT transistor.
Which transistor is the BC548:

Instructions

Schematic

1. Breadboard the circuit first to test it.
   1. Place the trimpot in the breadboard so that each lead is in its own row:
      
      

      ---

   2. Connect the center lead of the trimpot to the power bus:
      

      ---

   3. Connect the anode of the LED to the center lead of the trimpot:
      

      ---

   4. Connect the cathode of the LED to the 470Ω resistor
      

      ---

   5. Place the transistor in the breadboard so that the cathode of the LED is connected to the collector of the transistor through the 470Ω resistor:
      

      ---

   6. Connect the base of the transistor to the right lead of the trimpot through the 1K Ω resistor:
      

      ---

   7. Connect the base and the emitter of the transistor through the LDR:
      

      ---

   8. Connect the emitter to GND
      

      ---

   9. Connect a 9volt power supply and test. You may need a screwdriver to adjust the trimpot:
      

      ---

   ---

2. Freeform solder or use the protoboard to make your design more permanent.
   1. Connect the cathode of the LED to the collector of the transistor through the 470 Ω resistor:
      

      ---

   2. Connect the base and emitter of the transistor through the LDR:
      

      ---

   3. Connect the 1K to the base of the transistor:
      

      ---

   4. Connect the other side of the 1K to the right lead of the trimpot
      

      ---

   5. Connect the anode of the LED to power:
      

      ---

   6. Connect the anode of the LED to the center lead of the trimpot:
      

      ---

   7. Connect the emitter to GND
      

      ---

   8. Attach a battery and test:
      

      ---

   9. If everything was fine, clip and strip the power line and attach a switch:
      

      ---

   ---

3. Use a digital caliper to measure your components. Write down the dimensions.
   

   ---

4. Think about how people use night lights.
   

   ---

5. Design an enclosure.
   

   ---