Now that you have a TTL compatible square wave source operating between about 1Hz and 10Hz (last week's lab) the folks in marketing want an enhanced model with selectable frequency ranges up to 100kHz. (Marketers rarely know how hard it is to bang this stuff out). Using the existing 555 based signal source you designed in lab 6, calculate new values for capacitor C1 (do not change any other components) to provide the following output frequency ranges:

Recall that your 555 output frequency is determined by the formula:

Assume you are using the same 1 KΩ resistor for R1 and the same 10 KΩ potentiometer for  R_var  (called R4 last week) that you designed for last week.  Also assume that you are still using the 150 uF capacitor for the 1 Hz to 10 Hz range.

1. Look at the equation and determine how to scale it to implement 10 times the original frequency range by changing just the capacitor (this new circuit will be just like the old one but will implement a range of 10 to 100 Hz instead of 1 to 10 Hz). Then repeat the process for 100x, 1000x, etc. to choose capacitors for each specified frequency range. Show all calculations to determine the capacitor values for each of the five frequency ranges (including the original 1 Hz to 10Hz range from last week) in your pre-lab.

   Here's how: If we need ten times the frequency on the left side of the equation, we need to determine how to alter the right side of the equation to make that possible. Figure out how to scale the capacitor value and use that to scale the 150 uF capacitor to other values. You should find that to change frequency range by a power of ten, you need to change the capacitor value by a power of a tenth (Because the C₁ value is in the denominator).

2. Now that you have chosen the capacitor values, plug them back into the equation (for both extremes of R_var ) to find the actual frequency ranges it will be able to implement. These will become the theoretical values for this lab, where we'll actually build the circuit with each capacitor and determine the experimental values for each frequency range.

   Here's how: For each frequency range, to determine the minimum and maximum frequency within the range, change the value of Rvar *not* the value of C1. Then change C1 to switch to the next frequency range and recalculate the min and max f. Remember that we accepted some error in the final frequency output to accomodate the parts we chose to use.

3. Draw an updated schematic, adding a capacitor bank to your lab 6 schematic in place of the original C1. An example capacitor bank schematic is shown in Figure 1 below, with the upper frequency for each capacitor noted. In this application, a jumper wire is used to select which capacitor will be used by connecting node A to the desired capacitor. When you build the circuit you will not use a bank, you'll simply replace the capacitor with another one.

Figure 1

End of Prelab

 

Lab Exercise 7: Multi-Range 555 Frequency Source

In this lab exercise you will complete the second stage of your EE101 semester project. Be sure to write up this lab thoroughly because you will need it to write your formal report.

Today you will build and test the 555 square wave source you designed during this pre-lab and the previous lab. As stated in the pre-lab, your function generator should cover the frequency range from 1 Hz to 1 MHz by utilizing all the capacitors determined in the prelab calculations. A sample schematic for the capacitor bank was provided in Figure 1. You are not required to build a capacitor bank as the caps can be easily switched out instead. If you would like to build a bank, the connections between node A and the various capacitors can be realized with a piece of jumper wire between the pot and the capacitor of choice.

Part I - Build the circuit

It is important to work off the actual schematic diagram from last week's p-spice simulation when you build this circuit. We want to build the exact circuit you simulated, so be sure you're using the same diagram from the p-spice file that you simulated. If you opted to add the correction resistor to control the max value of the potentiometer (last week's extra credit portion), be sure to use it in this lab too. Your instructor can help you find the exact value of resistor that you calculated.

Get a 555 timer chip and the capacitors and resistors you chose for your design and build the circuit on a proto-board. Try to keep your wiring under control and avoid the 'spagehtti board' effect.  (There is no need to include all the frequency control capacitors in your circuit at one time.) Start with the 150 uF capacitor so we can test the 1 to 10 Hz range first, then switch to each of the other capacitors for the remaining ranges. Use as few wires as possible, make them as short as possible, and take advantage of the red and blue Vcc and Ground bus lines provided on the breadboard.

Part II - Function Test Using Oscilloscope

To test the function of your circuit, we will use an oscilloscope to measure the frequency ranges given when each capacitor value is used.

Before proceding, have a TA check to see that your circuit is safely wired BEFORE turning on the power supply.

1. For the frequency range of 1 to 10 Hz (using the 150uF capacitor), we will measure the actual frequency range the circuit is capable of implementing. This becomes our experimental data set. Use a table to Compare this data set to the two data sets from lab 6 (theoretical values from the prelab and simulation values from P-spice). Calculate percent errors, and write a brief conclusion about how they compare. What method does a better job of predicting the 555 behavior--the equation from the manufacturer's spec sheet or the software simulation?

2. Testing Procedure:
   • Attach the positive O-scope test lead clip to the out-put pin of the 555 (pin 3), and attach the negative (ground) test lead clip to your ground.
   • After you have a stable waveform on the screen (ask the TA for help if needed) measure the period of your circuits waveform and then calculate frequency, (take 1/T).
   • Record your systems functionality (does it work as expected) and a brief note on quality of function (note what the wave looks like, is it a good, clean squarewave? if not what does it look like?).
   • Plot a sample waveform for at least one of your frequencies.
   • If your system doesn't work fully as expected, record what it is doing and where (at what frequency) it stops behaving nicely.

3. Repeat the testing process for the other frequency ranges by switching out the capacitor for each range. Compare this data to your prelab data from this week (you do not have simulation data because we did not simulate the other frequency ranges in last week's lab).

4. Demonstrate functionality of at least one range to the TA and tell them how your design behaved, the TA will initial your lab book after you show them how it went.

5. EXTRA CREDIT: Last week we discussed how the frequency range could be implemented more exactly by using a parallel resistor to limit the maximum value of the potentiometer. If you didn't already do it last week, it's not too late (but you will have to alter your schematic from last week before moving forward with Lab 9 next week). Calculate the resistor needed, add it to your circuit, and test one of the frequency ranges to see if it helped. Calculate new percent errors compared to theoretical values and summarize your results and their meaning.

   
    

    
   

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