EE101 - Lab Exercise 7:  Multi-Range 555 Frequency Source

Prelab Exercises

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 1MHz. (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:

10Hz - 100Hz
100Hz - 1000Hz
1000Hz - 10kHz
10kHz - 100kHz

Recall that your 555 out-put frequency is described by the formula:

fosc = 1.44 / [(R1 + 2*Rvar)*C1]

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

Show all calculations to determine the capacitor values for all five frequency ranges (including the original 0.5Hz to 10Hz range from last week) in your pre-lab. Also, 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, with the upper frequency for each capacitor noted).

Figure 1

Lab Exercise 7: Multi-Range 555 Frequency Source

In this lab exercise you will complete the second stage of your EE101 project. As with last week's lab, this lab will be important to you later in the semester. Be sure to exercise due caution in storing this lab and the information therein.

In this lab 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 .5Hz to 1MHz. A sample schematic for the capacitor bank was provided in Figure 1. The connections between node A and the various capacitors can be realized with either a few pin headers and jumpers or just with a little piece of jumper wire between the pot and the capacitor of choice.

Part I - Build the circuit

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.)

Part II - Function Test Using Oscilloscope

Test the function of your circuit using an oscilloscope to measure the frequency ranges given when each capacitor value is selected.

Before proceding, have a TA check to see that your power supply is properly hooked up to your circuit.

  1. 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.
  2. 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).
  3. 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?). If your system doesn't work fully as expected, record what it is doing and where (at what frequency) it stops behaving nicely.
  4. Demonstrate functionality to the TA and tell them how your design behaved, the TA will initial your lab book after you show them how it went.



February 2001

Copyright 2001, New Mexico Tech