BANKSTOWN

LED DISPLAY PROJECT

TAFE

INSTRUCTIONS

CONTENTS

1.    DUMMIES GUIDE:page 4

2.    ADVANCED GUIDE: page

3.    ALL OTHERS : page 2

4.    Operation of the Circuit: page 2

5.    CONSTRUCTION: page 1 construction manual

STUDENT NAME:

DATE:

FIG 1. Circuit of one section

If you look at the circuit, you will notice a column of resistors designated as 10k. Resistors resist the flow of current in an electrical circuit. In this case the circuit is from the 12volt positive to the 12volt negative. Resistors do not affect the voltage. The resist the current that the voltage may cause. If you place a wire between the 12V + AND � Current will flow (if high enough the wire will melt). For any given resistor the current can be determined using the formula  CURRENT=VOLTAGE/RESISTANCE. Similarly the VOLTAGE=CURRENT * RESISTANCE.

In this circuit, there are five 12k resistors, totaling 60k and the voltage is 9volts. Therefore the current  through the circuit will be 12/50,000 (50k).

We can now work out the voltage across each resistor using the formula. However, there are five resistors of equal value, if we divide 9 by 5 we get 1.8volts. There is 1.8volts across each resistor. These resistors form a reference for our comparator.

 LM 234  PIN OUTS                  

LM324

The LM324 is used as a comparator in this circuit. When the +input is higher than the �input the output is high. When the unit is turned on with no input then the +inputs are connected to the reference voltages and the input is at zero volts. The +input is then higher than the �input. The output of the comparator is high. The output of the comparator is connected to a LED(light emitting diode). The other side of the LED is connected to positive. If the output of the LM324 is high there is no current flowing through the LED and it will remain off. This is the turn on state without an input.

If we supply an input and the value is higher than the reference voltage to any of the LM324�s then their outputs will go low turning the LED�s on.  Each LM324 has a different reference voltage, so as the input is increased more LED�s will come on.

    The 470 ohm resistor in series with the LED is there to limit the current through the LED. The LED requires less than two volts to turn on and the supply voltages is 12volts. This protects the LED.

The input circuit is connected to a switch. This switch selects the input connector or a resistor connected to a capacitor connected to the supply. When the resistor capacitor network is selected via the switch, the capacitor charges slowly, as it charges the LED�s will come on displaying the voltage across the capacitor. Returning the switch to the input allows another resistor to discharge the capacitor. The capacitor will have no voltage across it the next time the switch selects it.

5.1

Look at the sample and NOTE the extra holes that are necessary for the switch. Mark these holes and drill.

PCB LAYOUT AND COMPONENT PLACING  img-5231502-0001.pdf 

5.2

Enlarge the two mounting holes so that the 3mm screws fit easily.

Look at the board layout noting the position of similar values of resistors. Select one of these values, 12k or 470 Ohm and bend the leads so that the resistor fits the holes. Orient all of the resistors the same way.

Do the same with the other values till all the resistors are placed. Orient all of the resistors the same way.

Insert the LM324 making sure it is in the right way. Check with the teacher. If correct solder.

Mount the LED�s, Noting the long lead is the ANODE  The anode connects to the Positive rail. Check all resistors are soldered and the IC�S are soldered. Check for �short circuits� and �dry joints�.

NOTE: THERE IS A 270K RESISTOR MOUNTED ON THE BOTOM LAYER.

If all is well, mount the switches.

There are two switches on the PCB, one to change the mode from voltmeter to self test. 

5.3

The plastic mounting block needs to be drilled and cut.

When in self test mode a capacitor charges up to the supply voltage and the led's display the votage increase over time. 

Check the sample measure and cut the mounting block carefully.

The push button resets this voltage to zero. 

NOTE: When using a saw you must take care not to cause damage to the work or yourself. Be aware of what you are doing and keep you mind on the job. Drills, saws, files and knives are all dangerous. Remember to mount the block securely. Take note of where other students are so that you can be safe and they can be safe at all times.

WATCH THE VIDEO 

Some holes will need to be punched. Select the correct punch and make the required holes.

Mount the required hardware

5.4

Check the position of the battery holder the battery terminal and the input connector. Wire the appropriate connections.

Connect these wires to the main �Circuit Board�.

Test the unit.