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Brought to you by:
Scott Medynski, Michael Vandegriend,
Gabriel Ricardo, and the number "e".
Looking to create a voice controlled widget? How about a sound-sensitive alarm? Going to do any small-signal metering? Well, you need some ADC or analog-digital conversion.
Click Here to get the Schematic
For those of you that are able to quickly dream up flawless interfaces from obscure parts, well, never mind this. For those of us who are looking to find a circuit that you can readily understand with the aid of our prerequisite courses, look ahead. This page looks to create an ADC input with the aid of readily available parts from the EE stores closet in CEB355 (or wherever it's going to be in the NEW building).
The discrete components and the methods that are used to design this circuit are taken from three places:
(1) the other ADC link in the student app note
(click here to check it out)(2) the information from classes you've already taken (or are taking)
(3) stuff I have added to clarify points made in (1)
THE STEP BY STEP METHOD TO CREATE THE ADC
1) Go to Radio Shack and grab a "Mini Condenser Microphome Element", it's part #270-085 and it should run you about $5.00.
2) Go to Morris Locker in (CEB355) and get yourself the following items
ADC0809
(analog-digital converter)LM348 chip (Quad 741 Op-amp) OR two LM741's
25k ohm potentiometer
(the small breadboard ones look nice, but the big ones will do fine)10uF electrloytic capacitors
(get 5, just in case)4.7 Volt Zener Diode
a bunch of resistors (as in the schematic above)
some wire
a breadboard
all the spec sheets for the various components
Now you'll want to assemble the circuit above as shown in the ciruit diagram above. Below is some discussion about the various parts that make up the ADC.
The microphone serves the purpose of transducing the voice into an electrical signal. The condenser microphone element is a variable capacitance. As such, you won't be able to perform design calculations with ohm's law. The spec sheet for the Radio Shack part is actually all that you need. It says to power it with around 4.5 Volts and it requires a 2.2k ohm resistor in series with it. The capacitor that is connected to the microphone only serves as a DC blocker and AC short circuit. Nothing fancy.
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The next step is to connect the operational amplifiers up. There really isn't much to tell if you've taken EE350. The gain of each stage is given by the value of the feedback resistor divided by the input resistor. The gain of the first stage is 1000. The gain of the second amplifier is variable. We set the potentiometer to a value of around 11.5k ohms, giving the second stage a gain of around 5.2. This let the voice signal of a spoken word reach around 5 volts peak to peak. Whistling is a good way to indicate operation, but unless your project involves whistling, it's not a good indicator of the amplitudes of a spoken word. Now to power the op-amps, we used a dual rail 12 volt source. This is recommended, but not required. The supply of the op-amps need not be larger than about 3 volts dual rail since we're looking to sample a signal that varies in about 5 volts peak to peak. NOTE: the supply
MUST be DUAL RAIL. If you attempt to use ground and 5 volts, you won't get the negative part of your swing.
The ADC0809 can only interpret voltage signals that vary from 0 to 5 volts. This means you'll have to add a DC offset to the signal. Now, you want to isolate the op-amp from any DC you're using to offset, so stick a capacitor in there. Now, using a voltage divider with resistors of equal values (I used 10k ohm, but anything will do really), we provide a node that is at approximately 2.5 volts. Run your signal through this node and it is now hovering at 2.5 volts. Your signal will now not normally go beyond 5 volts and ground. But just to be sure...
Add a zener diode between ground and 5 volts. This will limit the maximum swing of the signal to near 0 and 5 volts. Now the input can be safely assumed to be in the acceptable ranges of the ADC0809's requirements. The ADC0809 itself is powered by 5 volts and requires reference voltage inputs which you should tie to their respective values. The chip itself has a multiplexed input for the ability to swich between multiple inputs. If you're only using the one input, then tie the address select pins to ground and only use input 0.
The code we're using to fuel the fire in this hotrod's belly is a modification on the other code posted on the Student App Notes. You can use their
input reader and clock divider by clicking on them. However, slightly better functionality can be obtained by the modified versions. The modified ADC interface and clock divider are available here.