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What are 555 integrated circuits, how do they work and why are they called 555?

What are 555 integrated circuits, how do they work and why are they called 555?

August 01 , 2023 0

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two (556) or four (558) timing circuits in one package. The design was first marketed in 1972 by Signetics and used bipolar

junction transistors. Since then, numerous companies have made the original timers and later similar low-power CMOS timers. In 2017, it was said that over a billion 555 timers are produced annually by some estimates, and that the design was "probably the most

popular integrated circuit ever made".

What Is 555 Timer

The 555 timer is one of the most iconic and popular integrated circuits ever created.

One of the integrated circuits. It was designed by Hans R. Camenzind for Signetics in 1970. Today - half a century later - it is still a very popular circuit building block. It is estimated that more than one billion 555 timer circuits are built each year.

The 555 was the first commercially available integrated circuit of its kind and was immediately and widely used as a circuit "clock" oscillator and timing delay generator. The original Signetics NE555 datasheet described the chip's function as follows.

"The NE/SE 555 monolithic timing circuit is a highly stable controller capable of generating precise time delays or oscillations. Additional terminals are provided for triggering or resetting if required. In the time delay mode of operation, the time is precisely controlled

by an external resistor and capacitor. For steady operation as an oscillator, the free running frequency and duty cycle are precisely controlled by two external resistors and a capacitor. The circuit can be triggered and reset on falling waveforms, and the output structure

can generate or pot up to 200 mA or drive TTL circuits."

Since its release, the chip has gained a loyal following as a building block for all sorts of custom circuits and as an introduction to electronics - there are several books on the 555 alone.

Over the years, there have been many derivatives and descendants of the original 555 - low-power and low-voltage types, dual and quad 555 chips, ultra-small surface-mount versions, and countless implementations of the 555 circuit itself.

Among these derivatives are our Three Fives and 555SE kits: they are both discrete implementations of the "equivalent circuit" in the NE555 datasheet, built using resistors and individual '3904 and '3906 transistors. These are "discrete circuits", so to speak, and contain

essentially the same components you might find on the chip of a 555 integrated circuit.

As with the integrated circuit version of the 555, you can build working timer and oscillator circuits with the discrete versions and use soldered connections (or, in the case of the larger ones, even crocodile chips) to monitor what's happening on the pins. However,

unlike the chip version, it's also easy to insert your own probes into the circuit to monitor what's going on at any point in what might just be a black box.

This ability to peer inside a circuit makes these kits a unique educational tool. In what follows, we'll go through the circuit diagram, discuss the theory behind the workings of the 555 timer IC, and suggest some opportunities for further exploration.

High-level overview

Let's start by looking at the block diagram of the 555, Figure 1. Here we have reduced the many individual components of the 555 to a smaller set of functional blocks.

From this perspective, the 555 is a relatively simple circuit whose core is made up of these three basic elements.

- A voltage divider.

- Two voltage comparators, and

- ﬂip-ﬂop logic gates.

(In addition to these, there is a power amplifier, or 'output driver', which provides the output current, and a separate driver for the discharge pins.)

You're probably familiar with these core components, but let's take a look at them anyway.

Voltage Divider.

The voltage divider consists of three 4.7k resistors connected between Vcc (pin 8)† and ground (pin 1). Since these three resistors are in series and are equal, the voltage in ⅓ of the resistor chain starting from ground (measured between the lower two resistors) is ⅓Vcc.

Similarly, the voltage between the upper two resistors is ⅔Vcc.

Comparator.

A comparator is a circuit element that compares two analogue input voltages at its (+) and (-) inputs. It produces an output voltage that is logic high (or "true") if the voltage at the (+) input is higher than the voltage at the (-) input, and low (or "false") otherwise.

The two comparators in the 555 circuit test the input signals from the trigger (pin 2) and the threshold (pin 6) against the reference voltages of the voltage divider (⅓Vcc and ⅔Vcc).

Thus, the output of the lower "trigger" comparator is high when the voltage on the trigger pin is below ⅓Vcc, and low when it is above ⅓Vcc. Similarly, the output of the upper "threshold" comparator is high only when the voltage on the threshold pin is above ⅔Vcc.

ﬂip-ﬂop.

In general, a flip-flop is a circuit element whose output state (logic low or high) depends on its input value, but also on its previous output state. In effect, it is a digital logic gate with a built-in bit storage unit.

The ﬂip-ﬂop gate in the 555 has two main inputs, called "S" (or Set) and "R" (or Reset). These inputs are driven by the outputs of two comparators. The "S" input goes high when the trigger pin is below ⅓Vcc. This causes the ﬂip-ﬂop output to go high. It remains high

even if the trigger pin later rises above ⅓ Vcc. the "R" input has the opposite effect. When the voltage at the threshold pin is above ⅔Vcc, "R" goes high, causing the ﬂip-flop output to go low and remain low even if the threshold voltage later falls below ⅔Vcc.

When both the "S" and "R" inputs are low, the flip-flop output remains in any state, whether it is high or low.

There is also a separate reset input (pin 4) which, when pulled low, resets the ﬂip-ﬂop to its low output state.

† The name "Vcc" means "positive supply input to the 555". In this case, the DC voltage range is 4 to 18V.

There is an open collector discharge pin. Normally it is floating (high impedance), but when the output of the flip-flop goes low, it also goes low. It is useful when discharging external timing capacitors.

A control voltage (CV) pin is connected to the reference input (usually ⅔ Vcc) of the threshold pin. This is a good analogue signal input for PWM (pulse width modulation) circuits.

Overall Schematic

A detailed schematic of the Three Fives and 555SE boards is shown in Figure 2.It is helpful to keep this overall diagram nearby as we navigate through the details of the different modules that make up the circuit.

The schematic is roughly divided into the same blocks as the block diagram, but note that these divisions are only approximate. For example, a resistive voltage divider (which you'll remember from the block diagram) consists of resistors R7, R8, and R9. In Figure 2,

these three components appear within the "Flip-Flop Comparator" block, even though (as you may recall from the block diagram) the voltage divider is not actually part of that comparator.

You may notice - from the parts included in this schematic or kit - that the electronics in the circuit consist only of resistors and transistors. We'll briefly review these two components before we dive into the heart of the circuit diagram.

Recap. Resistors

Resistors are defined by the fact that they are circuit elements that follow Ohm's Law, which is V = I x R, where V is the voltage across the resistor, I is the current through the resistor, and R is the value of the resistance.

Resistors are characterised by the fact that they are circuit elements that follow Ohm's law, i.e. V = I × R, where V is the voltage across the resistor, I is the current through the resistor, and R is the value of the resistor in ohms. For example, if a 4.7kΩ resistor has 1mA of

current passing through it, then the voltage difference across it is 4.7V.

Recap. Transistors

Although there are many types of transistors, the transistors in this circuit are bipolar transistors in which a small current controls a large current to act as an amplifier or switch. There are two

There are two types: the NPN and the PNP; we'll pick the NPN to look at first.

The three terminals of an NPN bipolar transistor are named "base", "collector" and "emitter". The base and emitter pins form a diode, indicated by the small arrows on its schematic symbol. A diode is a unidirectional circuit element, so under normal conditions, current

can only flow from the base to the emitter, not from the emitter to the base.

In addition to this. An NPN transistor also has a second internal diode which It (only) flows from the base to the collector. In most use cases. Circuits using such transistors are designed to always keep (or "bias") the voltage at the collector higher than the voltage at the

base in order to

that the base-collector diode will not conduct current. As long as this bias is maintained, we can usually ignore the presence of the second diode. presence of the second diode.

An important detail is that there is a small voltage loss across the diode, typically about 0.7 V. This means that (a) as current flows from the base to the emitter, the voltage at the emitter pin is typically about 0.7 V lower than the voltage at the base (b) current does not

begin to flow from the base to the emitter until the voltage at the base is about 0.7 V higher than the voltage at the emitter. This typical voltage difference of 0.7V is often referred to as the "diode drop".

Finally, there is the issue of switching and amplification. When current flows from the base to the emitter, that current ("base current", symbol IB) is known as the transistor's "switch": it allows current to flow from the collector to the emitter. The maximum amount of

current that can flow from the collector to the emitter is given (very closely) by IC = h × IB, where IC is the "collector current" (the current that flows from the collector to the emitter), and h is a gain factor that depends on the particular transistor. If the value of h is 30,

then the transistor is acting like an amplifier, and a small change in IB will result in a 30-fold change in IC.

Another type of transistor - the PNP - works in almost exactly the same way. Its schematic symbols are similar to the major changes pointed to by the small arrows. In a PNP, the emitter and base form a diode.

In a PNP, the emitter and base form a diode, and current can only flow in one direction, but this time only from the emitter to the base (again, in the direction of the arrow). Also, when current flows through this diode, it allows current IC = h x IB to flow from the

emitter to the collector.

The specific components we use are NPN transistors of type 2N3904 or MMBT3904 and PNP transistors of type 2N3906 or MMBT3906.These are some of the most common and well known types of bipolar transistors.

threshold comparator

The first module on the schematic is the "threshold" comparator, which looks at the voltage on the threshold pin and compares it to the ⅔ reference voltage of Vcc from the voltage divider (again, R7, R8 and R9). Note that the control voltage (CV) pin is wired directly

to this reference voltage, so it can be modified externally.

The comparator circuit has two main sections. The input differential amplifier and the second stage differential amplifier (with current mirror).

The Darlington pair

The first thing to note is that a number of transistors (e.g. Q1 and Q2) are connected together in what is known as the Darlington configuration.

In the Darlington structure, the emitter of one transistor is connected to the base of the other. This effectively makes two new "super-transistors" as it is the amplified current of the first collector that acts as the base current of the second transistor. If the gain of a

single transistor is 30, then the effective gain of the Darlington pair would be 30 x 30 = 900.

This huge gain improvement is not without cost. Since there are two transistors, there are also two diode drops to overcome; the Darlington pair does not begin to conduct until its base is at least 1.4V above the emitter.\

Differential Amplifier

Two Darlington pairs, Q1/Q2 and Q3/Q4 form a differential amplifier. The use of the Darlington pair (with its high gain) reduces the current from the comparator input.

The differential amplifier itself acts a bit like a seesaw: when the voltage at the threshold input is higher than the voltage at the reference input (⅔ of Vcc), the current in the circuit comes mainly from Q1 and Q2 on the left side. (Remember, the circuit is symmetrical).

When the threshold voltage is lower than the reference voltage, the circuit changes state and most of the current flows through Q3 and Q4.

We can use a simplified differential amplifier to help illustrate this main idea.

In the left case, the inputs to the amplifier are 2V (left input) and 1V (right input). Because of the higher input voltage on the left, the left transistor turns on more strongly, with more current through the collector. The voltage at the emitter saturates to 1.4V (2V, minus

one diode drop), turning off the other transistor (because the emitter voltage is higher than the base voltage).

If the input is changed to the right-hand side of the case, the opposite is true when the inputs are 1V (left input) and 2V (right input), and current flows down the right-hand side of the circuit.

In both cases, you should note that the current is always flowing, but the branch of the circuit it flows through (left or right) depends on the value of the input voltage.

A valid question at this point is, "If this is a differential amplifier, how is the difference actually being amplified?" The answer is that the difference between the input voltages controls an amplified current that flows through one of the two resistors near the top of the

diagram. Ohm's Law tells us that the voltage drop across these "load" resistors depends on the current through them, so the amplified difference can be read off below the resistors (i.e. as the voltage difference between the collectors of the two transistors).

Now let's return to the main circuit. Whichever set of transistors conducts current through the differential amplifier, the current always reaches ground through R5.

The voltage drop from each input to the top of R5 is two diode drops due to the presence of Darlington pairs. For proper operation, at least one of the inputs must be at least 1.4V above ground (minimum "common mode" input voltage). Since the other comparator

input is connected to ⅔ of Vcc, this condition is met. This is why typical 555 timer datasheets specify a limited voltage range for the control voltage (CV) input.

The maximum common-mode range (how high both inputs can go at the same time) is close to Vcc. However, if you pull the voltage on either input too high, you may forward bias the base-collector junction† of either Q1 or Q4, and the current on that side of the

comparator will quickly drop to zero. This manifests itself as "comparator inversion", where the output jumps to the wrong state.

ammoscope

Before we discuss Q5, Q6, Q7, and Q8 at the top of the Threshold Comparator, we must talk about a circuit block called the "Current Mirror," which appears repeatedly in this section and throughout the rest of the schematic.

A current mirror is called a current mirror because it "copies" the current through one circuit element to the current of another. Let's start with a relatively simple example of a current mirror. Look at how Q19A and Q19B are connected.

Note that transistor Q19B is connected "as a diode" with its base shorted to its collector. Even so, it still allows current to pass through its collector.

Since their emitter and base are connected together, both Q19A and Q19B have the same base-emitter voltage. Symmetry dictates that the emitter-to-base current should be the same for each transistor. Therefore, both transistors allow the same amount of current to

pass through their collectors. If the current IQ19B comes from the collector of Q19B, the same amount of current will flow through the collector of Q19A: IQ19A = IQ19B. In this sense, the current through the collector of Q19A "mirrors" the current through Q19B.

† Remember that the base-collector of an NPN transistor also forms a diode. In this case, this actually becomes important; if the base voltage is higher than the collector voltage, then that diode will begin to conduct.

Second stage differential amplifier

The output of the first stage differential amplifier in the threshold comparator goes to a second stage differential amplifier, consisting of transistors Q5, Q6, Q7 and Q8 with resistors R1, R2 and R3.

This differential amplifier looks different for a couple of reasons. Firstly, it is "inverted" compared to the one we saw earlier. Second, its input stage is a current mirror circuit. However, it works the same way: it amplifies the signal from the first differential amplifier and

increases the overall gain.

One current mirror is made up of Q5, Q6, R1 and R2. The other is made up of Q7, Q8, R2 and R3. Transistors Q6 and Q7 do double duty - they are part of the current-mirror circuit, but work in conjunction with R2 to also act as a differential amplifier.The collector of Q6

is the output of the amplifier and is fed to the ﬂip-ﬂop block.The collector of Q7 is grounded and is not used, but it can be thought of as the 'inverting' output.

In essence, this part of the circuit is actually three circuits superimposed on each other. Two current mirrors are mixed with a differential amplifier.

Problems and Experiment 1

1. Measure the resistance from the CV (control voltage) pin to the Vcc pin. You can use a real 555 timer for this experiment. If you can find one, you can use a CMOS 555 timer such as a TLC555 or LMC555 for this experiment.

2. measure the voltage across R5. How much current is flowing in the comparator? Does it change when you adjust the voltage at the threshold input? What happens if you force the CV pin and the threshold input below about 1V?

3. Measure the current flowing to the threshold input. Connect a variable power supply to the threshold input and connect an ammeter in series so you can measure the current at different input voltages.

4. Short-circuit the base and emitter of Q1. Repeat these steps for Q4. Can you describe what effect this change should have on the circuit? Now measure the input current on the threshold pin. How does this affect the behaviour of a 555 circuit such as an oscillator?

5. what happens to the voltage on R2 when the comparator changes state?

6. measure the offset voltage. Try putting a voltmeter on the inputs (threshold and control voltages) and record the voltage when the comparator changes state. This will directly affect the timing accuracy of the chip as it will cause the comparator to jump slightly later

each time.

7. 7. Given an unlabelled bipolar transistor and a multimeter, how do you determine whether it is a PNP or NPN type and which pin is which?

8. download the datasheet for the NPN transistor (2N3904 or MMBT3904) used in the kit. What transistor gain value do you actually expect? Can you measure it somewhere in the circuit? What does it say about the "diode drop" voltage? Can you measure a typical diode drop in a circuit?

trigger comparator

The Trigger Comparator works similarly to the first (lower) part of the Threshold Comparator, except that it is inverted and uses a slightly different arrangement of PNP transistors. The reason they are inverted is to ensure that the common mode input voltage range

extends all the way to zero. This is important because the reference voltage terminals come from R7, R8 and R9 at ⅓Vcc. The two inputs to the differential amplifier are that ⅓Vcc reference voltage and the input from the trigger pin.

Two pairs of transistors (Q12/Q13 and Q11/Q10) are configured as Darlington pairs. They are directly grounded, and the bias current (i.e., keeping the current flowing through the transistors in the correct direction) comes from Q9, which acts as a constant current

source. The output current from Q11 is fed to the ﬂip-ﬂop block as the output of the comparator.

The current mirror, again 

Q9 gets its current through a "current mirror" with Q19A and Q19B, the two transistors in the ﬂip-ﬂop block that we discussed earlier when we talked about current mirrors.

How does Q9 fit into all this?

Current mirrors can be constructed with multiple "output" transistors, and Q9 is an additional output transistor that is connected in parallel with Q19A to track the change in current through Q19B.

However, unlike Q19A, the current through Q9 is not directly replicated, but is divided by a fixed ratio determined by R4. Because R4 is in series with the emitter, this circuit is known as the Widlar current source (invented by Bob Widlar, the legendary IC designer who

invented the basic building block circuits such as the IC op-amp, voltage regulator, and bandgap voltage reference).

A single Q19 on an IC is a rather strange beast called a dual-collector PNP transistor - a transistor with two collectors. IC designers use them because all you have to do is take a normal lateral PNP transistor, split the collector in half, and get two transistors for the price

of one (with roughly equal current in both collectors). However, two mirrored discrete PNP transistors are a good alternative for a kit, which is why we have Q19A and Q19B. Their presence also provides an excellent example of simple current mirroring, which helps to

draw out the role of Q9.

Finally, why does this comparator have only one differential amplifier instead of two?The Widlar current source is an "active" load (as opposed to the "passive" resistive load in a threshold comparator), which increases the gain of the amperage, making the second

stage unnecessary.

The Flip Flop

There are many analogue subtleties in this module. It is called a bistable circuit because it has two stable states. To simplify the analysis, we will observe the module in two possible states: the output pins are either open or closed.

The bias current for the module comes from the current mirror pair Q19A/Q19B. r10 sets the current through Q19B, and therefore through mirrors Q19A and Q9. (Recall that Q9 is the transistor that provides the constant current source for the trigger comparator.) 

The actual output of the ﬂip-ﬂop gate is the voltage at the collector of Q17, as shown in Figure 7. The three components to the right of this point (R12, Q20, and R15 in Figure 2) are part of the output drive and are not shown here.

Situation. Output pin on (Q17 on, Q16 off) 

The voltage at the emitter of Q18 is about one diode drop above ground.The current from Q18 flows entirely through the base of Q17 to ground, keeping the transistor on. R11 then acts as a pull-down resistor to keep the base of Q16 low and this transistor is turned

off. Since Q17 is turned on, the ﬂip-ﬂop output is low. The output pin itself is high because there is also an inverter in the output stage between the ﬂip-ﬂop output and the output pin.

In order to switch the ﬂip-ﬂop to the off state, the output of the threshold comparator must be turned on and current directed to the base of Q16. If there is enough current to overcome R11, then Q16 will turn on, pulling the base of Q17 low, which turns Q17 off and

changes the ﬂip-ﬂop output to a high state (the state of the output pin is low).

Another way to switch states is through the use of a reset circuit. If the reset input pin is low, it turns on Q25, pulls the collector of Q18 low, and shuts down Q17 by stealing base current from Q17.The base-emitter voltage drop of Q25 is "cancelled out" by the base-

emitter voltage drop of Q18, a diode-connected transistor.

Case in point. Output pin off (Q17 off, Q16 on)

In this case, Q18's current flows exclusively to ground through Q16. q17 has no base current, so it is off. r11 is pulled high through Q19A, supplying current to the base of Q16 to turn it on. Since Q17 is off, the output of the ﬂip-ﬂop is high.

In order to transition from this state to the on state, the output of the flip-flop must turn on. This turns on Q15 and pulls the base of Q16 low, turning it off and changing the state.

Problems and Experiments II

1. how do you probe a circuit and change the state of a flip-flop without using a threshold, flip-flop, or reset input?

2. in the advanced overview section, we described the behaviour of the flip-flop in terms of the "R" and "S" input and output states. When you change R and S, test the circuit to see if it behaves as we described.

3. Compare the current in Q19B with the current in Q19A. Also compare it with the current in Q9. You will have to remove the leads of one transistor to connect an ammeter in series.

4. does the current flowing in R10 change when VCC changes? (You can measure the voltage across R10 and then use Ohm's law to calculate the current).

5. Build a simple oscillator circuit. Now change R10. what effect does it have on the operation of the circuit? (You can connect a resistor in parallel with R10 to reduce its value without cutting wires or desoldering).

6. the circuit has an available reset pin, but does not have a corresponding setup input pin. How would you add one?

output stage

Q20 receives the raw output of the ﬂip-ﬂop gate and creates a buffered (non-inverting) version and an inverting version of the signal. This also helps to analyse the circuit in both states.

Situation. The output pins are open (Q20 is closed, Q21/Q22 are open, and Q24 is closed).

The output of the ﬂip-ﬂop is low. As a result, Q20 has no base current and is turned off.Q21 and Q22 form a Darlington pair that is configured as a voltage follower. They attempt to follow the voltage on the base of Q21, which is pulled to Vcc via R12. the output

voltage in this state is Vcc minus the voltage drop across the two diodes. Q24 and Q14 are held off by R15.

Situation. Output pins are off (Q20 is on, Q21/Q22 are off, and Q24 is on).

The ﬂip-ﬂop output drive current into the base of Q20 turns it on. The current from Q20 turns on Q24 and Q14.R16 and R14 share the current from Q20 so that both transistors can be driven from one output.Q14 pulls the discharge pin low, while Q24 pulls the output

pin low.

As Q20 is switched on, Q21 and Q22 are switched off. the voltage at the base of Q21 is about one diode drop above ground; no current flows because two diode drops are needed to start switching on the Darlington transistor pair.

A brief note on Q23

Q23 is another diode-connected transistor. When it is driven low, it provides more current capability to the output stage. If the voltage at the output rises enough to forward bias Q23 (that is, if the emitter is at least one diode drop above the base), then the resulting

current flows into the base of Q24, making it work harder.

Reset Input

Pulling the reset pin low turns on Q25. This steals current from the ﬂip ﬂop, puts it into the on state (output pin low), and uses that current to turn on Q14 and Q24, which drive the output pin and discharge pin low.

Resistor R17 is not in the original 555 timer IC. It is included to help protect the reverse bias junction of Q25. In the kit, Q25 is a 3906 transistor that has a base-emitter junction with a maximum reverse bias voltage of only 6 V, whereas the original 555 can handle 18 V.

(The first version of the "Triple Five" kit did not include this resistor, so when the reset pins need to be connected to more than 6 V, an external 100 K resistor needs to be added.)

Problems and Experiment 3

1. try shorting R14. Does this change the function of the discharge or output pins? Try again, but this time short R16 instead.

2. compare the base voltage of Q21 with the output voltage. Now change the ﬂip-ﬂop to another state. If you have an oscilloscope, make an oscillator circuit and probe both nodes.

3. The original 555 IC did not include resistor R17, but it worked without an external resistor on the reset pin at voltages above 6V. How is this possible? Can you find a different transistor (instead of the 3906) that makes it so that R17 is not needed?

4. 4. What is the maximum speed of the 555? How could you increase it?

5. What components could you add to make the 555 slow? Add some (buffered) LEDs and maybe you can build a 555 timer that humans can watch.

6. can you solve the breakdown current problem with the output buffer?

7. what limits the maximum operating voltage of the circuit? And what is the minimum voltage? How could you change these limits?

8. the original 555 didn't have the reference input of the trigger comparator pinned out to a pin different from the CV pin of the threshold comparator. With the discrete version, you can connect anything you want to the hidden input of the trigger comparator! What

kind of circuits can you dream up? What kind of circuits can you dream up to take advantage of this feature?
9. How could you change a threshold comparator to use an active load (like a trigger comparator) instead of a second stage differential amplifier?