Anyone who starts playing with electronics sooner or later encounters on his way the chip numbered 555. According to many, this timer is one of the chips that changed the world. It is hard to disagree with this especially considering the scale of production and the multitude of applications, about which even separate books have been written. In this material I will tell you a little about this unique chip.
Where did the 555 come from?
Hans Camenzind (https://www.chipsetc.com/interdesign.html)
It was the summer of 1970, chip designer Hans Camenzind was working as a consultant for Signetics. At the time, the financial situation of the Camenzind family was not very good, and on top of that there was a chance that Hans would be fired, in order to keep his position he had to come up with something really good. And so he did. He designed one of the most popular chips of all time, the so-called 555. Still popular today, the chip was a hit, finding its way into kitchen appliances, toys, spacecraft and a mass of other things.
The idea for the 555 came when I was working on a circuit called a phase loop. With some modifications, such a circuit could function as a simple clock. Although the concept of an electronic clock sounds simple, no such circuit was available on the market at the time. Initially, Signetics’ engineering department rejected Hans’ idea. The company was already selling separate components that customers could combine to create timers. That could have been the end of it, but Camenzind insisted. He went to Art Fury, Signetics’ marketing manager, who liked the idea of a single chip.
555 chip core (https://spectrum.ieee.org/chip-hall-of-fame-signetics-ne555)
Camenzind spent almost a year testing prototypes. All circuit elements were created on paper, only later transferred to the appropriate masking films. “Everything was done by hand, without a computer,” Hans recalls. His final design consisted of 23 transistors, 16 resistors and 2 diodes.
The 555 chip hit the market in 1971 and immediately caused quite a sensation. Shortly thereafter, in 1975. Signetics was absorbed by Philips Semiconductors, now NXP.
How does the 555 work?
Internal schematic of the 555 circuit (https://www.ti.com/lit/ds/symlink/ne555.pdf)
The 555 can undoubtedly be described as something brilliant in its simplicity. Inside we find three resistors, whose task is to produce a reference voltage of 1/3 and 2/3 of the supply voltage, respectively. Further on, we will find two operational amplifiers to which just the reference voltage is connected, and the remaining inputs are brought outside the circuit. The main component of the 555 is a single, slightly modified RS flip-flop. Its inputs are connected to the outputs of the operational amplifiers, in addition, the element is equipped with a master reset pin. The output of the metering device via a buffer is brought outside the circuit, but otherwise it is connected to a single bipolar transistor.
One of the most popular uses of the 555 is to use it as a timer, just connect two resistors and a capacitor to the circuit. The operation of such a design is quite simple. The capacitor will alternately charge and discharge. When the voltage on it reaches 1/3 or 2/3 of the supply voltage, the corresponding input of the flipper will be activated, thereby changing the state of the output. When the output state is set high, the capacitor will be discharged through an internal transistor.
It is also worth mentioning that there is such a thing as a “double” 555. This is a chip numbered 556, inside of which the designers placed two timers. However, this design never became, as popular as its simpler version.
The simplest pulse generator
One of the most commonly built designs based on the 555 chip is an astable pulse generator. It will produce a rectangular signal, as soon as the power supply is turned on, and its parameters will depend on the value of resistors R1, R2 and capacitor C2. In addition to the astable generator, we can meet with a monostable design. In this variant, the chip will generate a single pulse when an external trigger signal is applied to it.
But let’s take a closer look at the astable generator circuit, the schematic of which you can see above. It is a simple design, consisting of five components – the chip itself and a pair of resistors and capacitors. However, to fully understand the operation of such a circuit we will need a second schematic, taking into account the internal design of the 555 chip.
Schematic of astable generator including internal design of 555 chip (https://commons.wikimedia.org/wiki/File:NE555_astable.png)
After assembling the circuit and starting the power supply, the capacitor will be discharged. The effect of this will be the appearance of a high state at the output of the second comparator, thereby setting a high state at the output of the flip-flop, which will also appear at the output of the circuit. The comparator will react in this way, because the voltage at its non-inverting input, is much higher than that at the inverting input, connected to the capacitor.
At the same time, the discharge transistor located inside the circuit is occluded, that is, it does not conduct current. On the other hand, the reset input of the flip-flop (R) has a low state, because the inverting input of the upper comparator has a potential of 2/3 of the supply voltage.
Over time, however, the capacitor, slowly charged, by the current flowing through resistors R1 and R2 will reach a voltage threshold at which the lower comparator will respond. Then a low state will appear at its output, but this will not change the signal stored by the flip-flop.
Only after a long time, when the capacitor has accumulated even more charge, will the first comparator react. By changing the state at its output, it will reset the flip-flop, thus setting the output of the circuit low. At the same moment, the aforementioned discharging transistor comes into play. It is put into a saturated state, and the current flowing through it slowly discharges the capacitor. This will continue for as long as the voltage on it does not exceed the lower limit, activating the second comparator that sets the flip-flop. Then the whole cycle will execute again.
On the other hand, what about the capacitor C1 visible in the first diagram, but not in the second? Its function is to filter the voltage at the inverting input of the first comparator, and it is a matter of dispute whether it should be used. Some say that since the circuit is supplied with DC voltage, the potential at this input will never change anyway. Well this is true, but it is important to remember that the most that happens in the circuit is when the flip-flop is switched. The whole process is based on comparators, which also make their outputs dependent on reference voltages, which should never change. Therefore, despite everything, I stand on the side of those who argue that an additional capacitor of small capacitance (10nF – 100nF) should be placed in the circuit.
Testing several variants of the astable pulse generator, I used different 555 chips, in theory working identically, but as a test showed, this was not quite the case. In a circuit prepared to generate a signal with a frequency of 1Hz and a fill of 66.6%, I placed a contemporary chip manufactured by Texas Instruments at the beginning. The signal it produced was stable and did not differ much from the assumptions – a frequency of 0.97Hz and a fill of 66.9%.
It was more interesting when the 555 chip from the 1970s, manufactured at Warsaw’s CEMI plant, was placed on the board. The chip had problems generating a stable rectangular signal. The frequency fluctuated between 0.92Hz – 1.24Hz, while the fill oscillated between 64% – 72%. This is a kind of curiosity, but on the other hand the CEMI chip is already years old so it is not surprising that its parameters have changed.
Let's take a look inside the 555
Structure 555 under the microscope (http://www.righto.com/2016/02/555-timer-teardown-inside-worlds-most.html)
What the 555 chip looks like up close, you can see in the photo above. Here we have a silicon core, on which all the necessary components such as transistors and resistors are placed. The connections between them were made of metal, these are the white-gold areas. However, the conductive layer was not placed on the silicon directly, they are separated by a layer of insulator – silicon dioxide. Of course, in the appropriate places, holes have been made in the insulator, so that the metal layer connects to the silicon. On the edges of the structure we can also see electrodes connecting the core to the external leads.
Internal Diagram 555 (https://www.ti.com/lit/ds/symlink/ne555.pdf)
In this case, the integrated circuit is a silicon wafer with applied, metal connections, but where are the transistors and resistors mentioned? They are placed in the silicon. Through the process of doping certain areas, a P- or N-type semiconductor can be obtained, and from there it’s not far to a transistor.
How a bipolar transistor is constructed hardly needs to be introduced to anyone. In theory, we have a structure built from an appropriately doped semiconductor, arranged in an NPN or PNP type sandwich. In reality, however, transistors are built somewhat differently.
In practice, NPN bipolar transistors are formed from several layers of P-type, N-type and N+ semiconductor. The main difference between the N and N+ areas is the amount of doping, N+ being a richer, more doped semiconductor. The whole structure is placed on a P-type substrate and has little in common with the book NPN, although looking at the vertical slice under the emitter, we can see such a structure. An N+ type semiconductor is placed under the emitter, and it connects directly to the P area, which is the area connected to the base. The last element is the collector, it connects to the rest of the structure indirectly, that is, two N+ areas are separated by an N-type semiconductor.
One would expect a PNP transistor to be constructed identically to an NPN, with the difference being only the reverse designation of the semiconductor type. However, the reality is different. PNP transistors have a small, circular P-type emitter region surrounded by a collector ring, also P-type. The two areas are separated by an N-type semiconductor, connected indirectly to a slightly more doped base, N+. In the picture you can see a rather interesting transistor, in which between the base and the collector passes a path placed above the whole structure, on a layer of silicon oxide.
A transistor is not necessarily small. The three largest, in terms of area, elements in the structure of the 555 circuit are also transistors. These are so-called output transistors, whose power is much higher than the previously described structures. They are built with interlocking base and emitter areas surrounded by a collector.
As I mentioned, you can also find resistors in the silicon structure. However, these are quite problematic, especially by inaccuracy, which can reach up to 50%, depending on the chip. Therefore, designers usually do not consider their absolute value, but the ratio between, for example, two resistors, which, as a rule, will be the same. In the picture you can see a small resistor made of a P-type semiconductor, over which a single path has been placed.
Resistors can also be made in irregular shapes. In the picture you can see the outline of such a design in the shape of an inverted letter ‘L’.
Sources:
- https://www.chipsetc.com/interdesign.html
- https://spectrum.ieee.org/chip-hall-of-fame-signetics-ne555
- https://www.ti.com/lit/ds/symlink/ne555.pdf
- http://www.righto.com/2016/02/555-timer-teardown-inside-worlds-most.html
- https://hackaday.com/2018/10/10/the-555-and-how-it-got-that-way/
- https://www.circuitbasics.com/wp-content/uploads/2015/01/555-Timer-Datasheet.pdf
- https://0creativeengineering0.blogspot.com/2018/12/discrete-555.html
- https://forbot.pl/blog/kurs-elektroniki-ii-wstep-do-ukladu-ne555-id8202