The Ultimate Beginner's Guide to the 555 Timer: Build the Atari Punk Console and Other Breadboard Electronics Projects by Jesse Rutherford

Author:Jesse Rutherford [Rutherford, Jesse]
Language: eng
Format: epub, azw3
Publisher: Bent-Tronics
Published: 2017-01-10T17:00:00+00:00

Equation:

Period: T = 1.1*R1*C1

Note: R is in M Ohms, C is in µF, Time is in seconds

Now for a little math. Project 2 mentions that R1 and C1 determine the on time of LED1 and a rough estimate of that time. Using the equation above, calculate what this time should be using the components listed in Project 2, simplifying step by step.

T = 1.1*R1*C1

T = 1.1*1*4.7

T = 1.1*4.7

T = 5.17

T = 5.17 seconds

OK, so that wasn't near as bad as the equations in astable mode, right? What does all of that mean? In theory, LED1 was on for 5.17 seconds. This time is in an ideal world with ideal components.

Now take a look at the actual time on an oscilloscope in Fig. 4-4 to see what, if any, difference there was from our calculation.

Fig. 4-4

5.1 seconds actual, vs. 5.17 seconds calculated - that's pretty close!

•So what does all of that other stuff on the oscilloscope display mean? Trace 3 (labeled TRIG) shows even if S2 is pressed multiple times during the timing cycle, nothing happens. Trace 1 (labeled OUT) shows the output (pin 3) is on (HIGH) for 5.1 seconds. Trace 2 (labeled C1) shows C1 charging up to 2/3 (6 volts) of Vcc (9 volts), which triggers the timing cycle to stop. The actual voltage of 2/3 Vcc was 5.93 V. Trace 4 (labeled RST) shows pressing S3 (reset) forces the timing cycle to stop, and the output turns off (LOW). •

Your time may differ slightly from the calculation and this actual result. That is due mainly to variation in the tolerances of the components, and a few other factors. Using components with lower tolerance values should lead to more accurate results. Now, time to start experimenting with some other values.

Project 2a: Switch off S1, change out R1 to 150k Ohm. Switch on S1, and press S2. What changes, if any, do you see? You should see LED1 turn on for a shorter time than before. It should be about 1 second. That is because the resistor value is now smaller, and the time it takes to charge the capacitor, as a result, is quicker. Use the equation from above to calculate what the results should now be.

Project 2b: Repeat this process. Change R1 back to 1M Ohm and change out C1 to 10 µF. Pressing S2 should turn LED1 on for a longer period than in Project 2. The larger the capacitance of C1, the longer it takes to charge, and therefore is slower than having the 4.7 µF capacitor in place. What do you think happens if you change out C1 to a smaller capacitance? Try changing C1 to .22 µF. Did LED1 stay on longer or shorter? What is its duration? Remember your equation.

Feel free to experiment with the values of R1, and C1. Tip: A variable resistor for R1, like a rotary potentiometer, will vary the length of the timing cycle. You can use this circuit for anything that needs to turn on for a

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