Discrete 555 Timer

by Rishabh_Sathe in Circuits > Electronics

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Discrete 555 Timer

As discussed in my earlier project:- https://www.instructables.com/Discrete-Operational-Amplifier/

Electronics dominantly deal with integrated circuits.

Circuit design is of two types, analog and digital circuit design.

Under analog circuit design, the most popular one is the 555 timer.

Here is a discrete transistor-level replica of the IC-555.

For waveform shaping circuits, controlled pulse generators, clocks, debouncers, etc.

we require precise timing circuits like the 555 timer.

555 timer internally contains operational amplifiers which are already covered.

Now, moving one step ahead.

Supplies

Materials required:-

1) Transistor 2N3904 (or any other equivalent NPN BJT) X 13.

2) Transistor 2N3906 (or any other equivalent PNP BJT) X 13.

3) Diode 1N4007 X 1.

4) Resistors 1K X 1, 10K X 1, 100K X 1, 15K X 1, 6.8K X 1, 3.9K X 1, 820R X 1, 4.7K X 7, 220R X 1, 100R X 1 (or suitable parallel/series combination).

5) Prototyping board /Breadboard.

6) Soldering iron, wires, soldering wire, wire cutter, and header pins.

Understanding the Block Diagram

The internal block diagram and schematic of the 555 timer are highlighted with the same color across all the drawings to clarify how the chip is implemented:-

Voltage divider

Between the positive supply voltage VCC and the ground GND is a voltage divider consisting of three identical resistors (5 kΩ for bipolar timers) to create reference voltages for the analog comparators. CONTROL is connected between the upper two resistors, allowing an external voltage to control the reference voltages:

When CONTROL is not driven, this divider creates an upper reference voltage of 2⁄3 VCC and a lower reference voltage of 1⁄3 VCC.

When CONTROL is driven, the upper reference voltage will instead be VCONTROL and the lower reference voltage will be 1⁄2 VCONTROL.

Threshold comparator

The comparator's negative input is connected to the voltage divider's upper reference voltage, and the comparator's positive input is connected to THRESHOLD.

Trigger comparator

The comparator's positive input is connected to the voltage divider's lower reference, and the comparator's negative input is connected to TRIGGER.

Latch

A set-reset latch stores the state of the timer and is controlled by the two comparators. RESET overrides the other two inputs, thus the latch (and therefore the entire timer) can be reset at any time.

Output

The output of the latch is followed by an output stage with push-pull output drivers that can supply up to 200 mA for bipolar timers.

Discharge

Also, the output of the latch controls a transistor acting as an electronic switch that connects DISCHARGE to the ground (convenient for discharging a timing capacitor) or leaves it disconnected.

Source:- https://en.wikipedia.org/wiki/555_timer_IC

Internal Circuitry of 555 Timer

Darlington Pairs

One of the first things to notice is that some of the transistors (e.g., Q1 and Q2) are hooked up together in what is known as the Darlington configuration. The emitter of one transistor is connected to the base of another. This effectively makes one new “super transistor” out of the two, since it is the amplified current out of the first collector that serves as the base current for the second transistor. If the gain of a single transistor were 30, then the effective gain of the Darlington pair would be 30 × 30 = 900. Since there are two transistors, there are also two diode drops to be overcome; the Darlington pair will not begin to turn on until its base is at least 1.4 V above the emitter.

Differential Amplifier

Two Darlington pairs, Q1/Q2 and Q3/Q4 form a differential amplifier. Using the Darlington pairs reduces the current drawn from the comparator inputs. When the voltage on the threshold input is higher than the voltage on the reference input (⅔ of Vcc), the current flow in the circuit comes mainly from the left side through Q1 and Q2. When the threshold voltage falls below the reference voltage, the circuit changes state, and the majority of the current flows through Q3 and Q4. We can help illustrate the main idea with a simplified differential amplifier: In the case on the left, the inputs to the amplifier are 2 V (left input) and 1 V (right input). Since the left input is higher, the left transistor turns on more strongly, pulling more current through its collector. The voltage at the emitter saturates to 1.4 V, turning off the other transistor. If the inputs change to the case on the right, when the inputs are 1 V (left input) and 2 V (right input), the opposite happens, and the current flows down the right hand of the circuit. In these two cases what you should notice is that current is always flowing but the branch of the circuit (left or right) that it flows through depends upon the values of the input voltages. Regardless of which set of transistors conduct the current through the differential amplifier, the current always travels through R5 to the ground. Because of the Darlington pairs, the voltage drop from each input to the top of R5 is two diode drops. For proper operation, at least one of the inputs must be at least 1.4 V above ground. Since the other comparator input connects to ⅔ of Vcc, this condition is satisfied.

Current mirrors

Before we get to Q5, Q6, Q7, and Q8 at the top of the threshold comparator block, we must digress for a moment to describe a circuit block called a “current mirror” that appears repeatedly both in this section and elsewhere in our overall schematic. A current mirror is called that because it “copies” a current through one circuit element to a current through another element. Let’s first examine a relatively simple example of a current mirror. Look at how Q19A and Q19B are wired up: Notice that transistor Q19B is connected “as a diode,” with its base short-circuited to its collector. Even so, it allows current to pass through its collector. Since their emitters and bases are wired together, both Q19A and Q19B have the same base-emitter voltage. Symmetry then dictates that the same amount of current should flow from the emitter to the base of each transistor. Accordingly, both transistors permit the same amount of current to pass through their collectors. If current IQ19B is sourced 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” that of Q19B.

Second Stage Differential Amplifier

The outputs of the first differential amplifier in the threshold comparator feed into a second differential amplifier formed by transistors Q5, Q6, Q7, and Q8, with resistors R1, R2, and R3. This differential amplifier looks different for a couple of reasons. First, it is “upside down” when compared to the one that we looked at earlier. Second, its input stages are current mirror circuits. However, it works using the same principle: it amplifies the signal coming from the first differential amplifier and increases the overall gain. One current mirror is formed by Q5, Q6, R1, and R2. Another is formed by Q7, Q8, R2, and R3. Transistors Q6 and Q7 do double duty – they are part of the current mirror circuit but, working with R2, also act as a differential amplifier. The collector of Q6 is the output of the amplifier and gets routed to the flip-flop block. Q7's collector goes to ground and is not used but it could be considered the "inverted" output. In essence, this part of the circuit is three circuits superimposed on each other: Two current mirrors mashed up with a differential amplifier.

The Flip Flop

There is a lot of analog subtlety in this block. It is known as a bistable circuit because it has two stable states. To simplify the analysis, we will look at the block in its two possible states: where the output pin is either on or off. The bias current for this block comes from the current mirror pair Q19A/Q19B. R10 sets the current through Q19B and consequently, through mirrors Q19A and Q9. (Recall that Q9 is the transistor that provides the constant current source for the trigger comparator).

Output Stage

Q20 takes the raw output of the flip-flop gate and creates a buffered (non-inverted) version and an inverted version of the signal. It will also help to analyze the circuit in two states.

Reset Input

Pulling the reset pin low turns on Q25. This steals current from the flip flop, putting it into the on state (for which the output pin is low), and uses that current to switch on Q14 and Q24, which drive the output pin and the discharge pin low. Resistor R17 is not in the original 555 timer integrated circuit. It has been added to help protect the reverse-biased junction of Q25.Q25 has a maximum reverse bias voltage on the base-emitter junction of only 6V, whereas the original 555 could handle 18 V. Therefore, when the Reset pin is tied high and the Vcc is high enough, it is possible to exceed this breakdown voltage, and R17 prevents damage by limiting the current.

Source:- https://shop.evilmadscientist.com/productsmenu/652