1

The 555 timer

The 555 pinout and internal schematic the are shown in the accompanying figure.
  • Compare your completed schematic diagram of the mystery circuit with that of the figure. Note any differences.
  • Label on your circuit schematic the connections (nodes) of the mystery circuit that correspond to the pins of the 555 timer.
  • Note any differences between the mystery circuit and the 555 timer chip.
2

Astable operation

Your mystery circuit was configured as an astable oscillator, as is the 555 timer shown in the figure. It outputs pulses of period T = ln(2)*(RA + 2*RB)*C. These pulses can be used, for example, to sequence the operation of digital circuits.
  • Construct on the second breadboard an astable pulse generator using the 555 timer chip, as shown.
  • Connect Vcc and Reset to +5V, GND to 0V.
  • For direct comparison, use for RA, RB and C the same R4, R5 and C that were used by your mystery circuit.
  • Be sure to include the output pull-up resistor RL=270Ω.
  • Monitor the output with the logic probe or scope.
  • Verify that the 555 astable circuit functions the same as the mystery circuit, i.e. that the two circuits output similar waveforms at their equivalent test points.
3

Monostable operation

Another application of the 555 timer is as a monostable pulse generator. A high-to-low transition on the TRIG input causes the OUTPUT pin to go high for a time T=1.1*RA*C. TRIG must go high before T elapses. This pulse can be used to debounce noise arising from the mechanical opening and closing of switch contacts:
  • Monitor TRIG with CH1 and OUTPUT with CH2. Set gains at 5V, the trigger source to CH1, mode to single.
  • Connect a switch centre pin to TRIG, the ends to GND,+5V.
  • Toggle the switch from +5V to GND then quickly back to +5V
  • Note the TRIG bounce duration, if any, then select R,C so T exceeds this duration.
  • Sketch the TRIG and OUTPUT pulses. How long is a typical switch bounce oscillation? Does OUTPUT obey T? Explain.
  • Change R,C so that T>1s and quickly toggle TRIG. Does OUTPUT obey T now?
4

Some final tasks ...

As part of this lab book entry, include:
  • a one-page summary of your experience with the mystery circuit and comments on how this experience may have enhanced your understanding of electronics;
  • some suggestions regarding any changes and improvements that you think would benefit this lab course.

Lab test

The one-hour open-lab-book test will take place during your next lab session. Review the "Final test" lab manual entry and check that you can login to Brightspace to access your lab book.
A sample black-box containing the circuits to be examined will be available for you to practice on, if you so choose.