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1 DAY WORKSHOP: BUILD A LIGHT SENSITIVE DAY/NIGHT SWITCH WITH NPN TRANSISTOR

In the parts layout illustration below we can see a light sensitive day/night switch. Below the parts layout illustration we

can see a schematic diagram of the same circuit. We can replace the red-red-red-gold [3.3 kilo Ohm] resistor with a 10

kilo Ohm resistor [brown-black-orange-gold]. On the next page are two illustrations to build the circuit.

This is a schematic diagram of the same circuit, showing the parts represented by conventional symbols. Take a good

look at both drawings, and compare what the part really looks like with the symbol that represents the same part. The

way this circuit works is when light strikes the light dependent resistor [LDR] the internal resistance value of the LDR

part decreases, and then the path of least resistance at the base is towards GND or minus [-] or 0v which means the

transistor is biased LOW and won’t conduct and the light emitting diode will be off. The LDR is acting as a PULL DOWN

resistor. When the LDR is in shadow [or in the dark] its internal resistance remains HIGH and because the resistance

value of the LDR is therefore higher than the 33 kilo Ohm [or 10 kilo Ohm] resistor the PULL UP resistor is now

therefore the path of least resistance and the base [B] pin of the transistor will be pulled high and therefore switched

on which means current can flow and the LED will therefore come on automatically in the dark.

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Now you can build a temporary test version of the circuit on a prototyping breadboard, and check that it works

properly. Once you have built the circuit, get your instructor to take a look and then connect power to see if you have

wired it correctly. If it does not work check all your connections again.

Try changing values of the resistor between the base and the positive rail to see if you can make the circuit more and

less sensitive, trying different values for the 10 kilo Ohm resistor from B to + Positive. Note how much light is on the

LDR when the LED switches off, hold your hand over the LDR and place it in shadow to see if the LED comes on.

Once you have the circuit working properly on the breadboard above, you can transfer the parts to a small block of

wood with five drawing pins on it, as per the illustration below. The drawing pins are to hold everything in place. Use a

blob of PRESTIK to hold the block from moving around while you are working with the soldering iron. It is common

practice in electronics engineering to first build and test a circuit on a breadboard, which looks like a block of white

plastic with many holes in it like below, and test the circuit, before laying out a PCB.

If you are not familiar with how breadboards work, take a look at www.robotscience.co.za/oneday/breadboard.htm.

ILLUSTRATION OF A SOLDERED LAYOUT ON A SMALL BLOCK OF WOOD WITH 5 DRAWING PINS

Use the point-to-point method of construction soldering the parts onto drawing pins nailed into a small block of wood

as per the drawing below to gain practice using a soldering iron. Careful not to burn yourself!