Make: Electronics by Charles Platt (read me a book .TXT) 📕

emits a positive pulse when the trigger (pin 2) drops below one-third of the supply voltage.

The 555 delivers the same duration of positive pulse every time (so long as you don’t supply a prolonged low voltage on pin 2).

A larger value for R4 or for C4 will lengthen the pulse.

When the output (pin 3) is high, the voltage is almost equal to the supply voltage. When the output goes low, it’s almost zero.

The 555 converts the imperfect world around it into a precise and dependable output. It doesn’t switch on and off absolutely instantly, but is fast enough to appear instant.

Now here’s another thing to try. Trigger the timer so that the LED lights up. While it is illuminated, press S2, the second button, which grounds pin 4, the reset. The LED should go out immediately.

When the reset voltage is pulled low, the output goes low, regardless of what voltage you apply to the trigger.

There’s one other thing I want you to notice before we start using the timer for more interesting purposes. I included R5 and R6 so that when you first switch on the timer, it is not emitting a pulse—but is ready to do so. These resistors apply a positive voltage to the trigger and the reset pin, to make sure that the 555 timer is ready to run when you first apply power to it.

As long as the trigger voltage is high, the timer will not emit a pulse. (It emits a pulse when the trigger voltage drops.)

As long as the reset voltage is high, the timer is able to emit a pulse. (It shuts down when the reset voltage drops.)

R5 and R6 are known as “pull-up resistors” because they pull the voltage up. You can easily overwhelm them by adding a direct connection to the negative side of the power supply. A typical pull-up resistor for the 555 timer is 10K. With a 9-volt power supply, it only passes 0.9mA (by Ohm’s Law).

Finally, you may be wondering about the purpose of C5, attached to pin 5. This pin is known the “control” pin, which means that if you apply a voltage to it, you can control the sensitivity of the timer. I’ll get to this in more detail a little later. Because we are not using this function right now, it’s good practice to put a capacitor on pin 5 to protect it from voltage fluctuations and prevent it from interfering with normal functioning.

Make sure you become familiar with the basic functioning of the 555 timer before you continue.

Figure 4-17. The top graph shows voltage on the trigger (pin 2) when the pushbutton is pressed, for different intervals, at different settings of the potentiometer. The lower graph shows the output (pin 3), which rises until it is almost equal to the power supply, when the voltage on pin 2 drops below 1/3 the full supply voltage.

Fundamentals

The following table shows 555 pulse duration in monostable mode:

Duration is in seconds, rounded to two figures.

The horizontal scale shows common resistor values between pin 7 and positive supply voltage.

The vertical scale shows common capacitor values between pin 6 and negative supply voltage.

To calculate a different pulse duration, multiply resistance × capacitance × 0.0011 where resistance is in kilohms, capacitance is in microfarads, and duration is in seconds.

47 µF

0.05

0.11

0.24

0.52

1.10

2.40

5.20

11.00

24.00

52.00

22 µF

0.02

0.05

0.11

0.24

0.53

1.10

2.40

05.30

11.00

24.00

10 µF

0.01

0.02

0.05

0.11

0.24

0.52

1.10

02.40

05.20

11.00

04.7 µF

0.01

0.02

0.05

0.11

0.24

0.52

01.10

02.40

05.20

02.2 µF

0.01

0.02

0.05

0.11

0.24

00.53

01.10

02.40

01.0 µF

0.01

0.02

0.05

0.11

00.24

00.52

01.10

00.47 µF

0.01

0.02

0.05

00.11

00.24

00.52

00.22 µF

0.01

0.02

00.05

00.11

00.24

00.1 µF

0.01

00.02

00.05

00.11

00.047 µF

00.01

00.02

00.05

00.022 µF

00.01

00.02

00.01 µF

00.01

1K

2K2

4K7

10K

22K

47K

100K

220K

470K

1M

Theory

Inside the 555 timer: monostable mode

The plastic body of the 555 timer contains a wafer of silicon on which are etched dozens of transistor junctions in a pattern that is far too complex to be explained here. However, I can summarize their function by dividing them into groups, as shown in Figure 4-18. An external resistor and two external capacitors are also shown, labeled the same way as in Figure 4-15.

The negative and positive symbols inside the chip are power sources which actually come from pins 1 and 8, respectively. I omitted the internal connections to those pins for the sake of clarity.

The two yellow triangles are “comparators.” Each comparator compares two inputs (at the base of the triangle) and delivers an output (from the apex of the triangle) depending on whether the inputs are similar or different. We’ll be using comparators for other purposes later in this book.

Figure 4-18. Inside the 555 timer. White lines indicate connections inside the chip. A and B are comparators. FF is a flip-flop which can rest in one state or the other, like a double-throw switch. A drop in voltage on pin 2 is detected by comparator A, which triggers the flip-flop into its “down” position and sends a positive pulse out of pin 3. When C4 charges to 2/3 of supply voltage, this is detected by comparator B, which resets the flip-flop to its “up” position. This discharges C4 through pin 7.

Theory

Inside the 555 timer: monostable mode (continued)

The green rectangle, identified as “FF,” is a “flip-flop.” I have depicted it as a DPDT switch, because that’s how it functions here, although of course it is really solid-state.

Initially when you power up the chip, the flip-flop is in its “up” position which delivers low voltage through the output, pin 3. If the flip-flop receives a signal from comparator A, it flips to its “down” state, and flops there. When it receives a signal from comparator B, it flips back to its “up” state, and flops there. The “UP” and “DOWN” labels on the comparators will remind you what each one does when it is activated.

Flip-flops are a fundamental concept in digital electronics. Computers couldn’t function without them.

Notice the external wire that connects pin 7 with capacitor C4. As long as the flip-flop is “up,” it sinks the positive voltage coming through R4 and prevents the capacitor from charging positively.

If the voltage on pin 2 drops to 1/3 of the supply, comparator A notices this, and flips the flip-flop. This sends a positive pulse out of pin 3, and also disconnects