Example Project: Disco Lights
Zip File ©Christopher Leigh
9v Power Block
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Here is another example of how to use system blocks. We will construct a simple set of disco lights. We start with the power block – using a 9v PP3 battery, but we could use 6v or 4.5v using AA or even AAA batteries. You could even use 3 volts. Most of the blocks are standard system blocks, just one was specially created. Your GCSE project should use standard blocks as far as possible. Any extra ones will need to be built and tested separately before putting the whole system together. The arrangement of blocks used here is just one possibility – there other ways of doing things. Question to ask here - what is the best voltage and power supply for these lights?
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Microphone Input
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This block uses an electret microphone which includes a built-in amplifier. This needs a power supply, provided in this block by the 10k resistor. The 1k resistor and 100uF capacitor are there to make sure the power supply is clean, otherwise you might get feedback resulting in a howling sound. The output of the microphone is ac coupled using another capacitor. 1uF should give reasonable base response; you could use a smaller capacitor, say 100nF. Note that the symbol used here in the Crocodile Technology diagram is not the correct one. Question to ask - is an electret the best microphone to use; what about using a loudspeaker?
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Inverting Amplifier
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This block uses an Operational Amplifier (OpAmp) as an inverting amplifier. The circuitry is a little complicated so that we only need one power supply instead of the usual two with an OpAmp. This amplifier will increase the size of the signal from the microphone depending on the setting of the feedback variable resistor. This should make it easier for the comparator block to be set to turn the lights on in response to the music. This block's question: would it be better to use a non-inverting amplifier?
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Rectifier
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This block takes the rapidly varying output of the amplifier and turns it into a slowly varying dc voltage. The diodes make sure only the positive half of the amplified wave gets through to charge up the capacitor. This has a large resistor across it so that it discharges relatively slowly. You will want to test out different capacitor resistor pairs to get the best response to the incoming music from your disco lights.
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Easy-Adjust Comparator
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This circuit works exactly the same as a standard comparator, except that the extra resistors allow the precise switching point to be set far more easily. However, with the resistors given the previous block needs to provide a voltage which is in the middle of the range, around 4.5v. You may need to adjust the resistor values to get things working properly. Question: would a Schmitt trigger comparator be better?
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Transistor Driver
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With a transistor driver if the voltage at its input rises above about 0.6 volts, then the transistor will start to turn on. Give it a reasonable input voltage of say 2 volts and it should be turned on fully. If you are using the 741 for a comparator, then this circuit may not work properly because the 741 output low is not low enough! Change the base resistor to 4k7 and put the 2k2 resistor from base to 0v. The transistor can supply about 100mA which is plenty for our circuit. Here you could use a FET driver - which would be better?
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4 LEDs
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As each LED requires about 2volts, if you are using a 9volt battery supply (a PP3), then you can drive 4 LEDs not just one. You need a current limiting resistor, here 82R. Theoretically this will give you 12mA if each LED takes 2volts. In practice some LEDs take less than 2volts so the errors in the calculation become significant. The internal resistance of a PP3 battery can also be significant. This means that the current could be much less than 12mA. So in practice you may need to use a smaller resistor to get a decent current. How small?
That's your disco project just about done! You may say LEDs aren't much good although a careful choice of modern ones will provide a very bright light in a darkened room. You may want more than one set, or put a frequency filter in the circuit so they respond to only bass, or only high frequencies. You may want to investigate driving a 12v halogen light using a FET or a relay. There is still plenty of scope for design and development! And then there's the casing!
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