Help me to find the time constant of this circuit.

[sel010913]

I am a student trying to build the traffic light circuit using NE555 timer unit.
I want to find the time constant of the following circuit.
I just learned that Time constant is R*C other than that I have no Idea where to start.
Can I get some help?

 

[D.A.(Tony)Stewart]

look at the history, then the datasheet at the bottom

Wikiwand - 555 timer IC

The 555 timer IC is an integrated circuit used in a variety of timer, delay, pulse generation, and oscillator applications. It is one of the most popular timing ICs due to its flexibility and price. Derivatives provide two or four timing circuits in one package. The design was first marketed in...

www.wikiwand.com

 

[KlausST]

Hi,

I agree ... reading the datsheet is essential and should give all information you need.

And that´s what I am worried about: What is the "information you need"?

RC time constant is defined by about 37% error to a step response. (I.e. after 63% of "movement")
But are you looking also for this "37%" related time constant?

So to get the most useful answers .. you need to tell us what / how you want the circuit to operate. Not only functionally, but also regarding timing.

Klaus

 

[FvM]

There's more than one time constant involved. The left 555 is acting as independent "master" oscillator, controlling the right 555 by switching it's power supply on and off. Both 555 are using a 50 % duty cycle configuration.

The oscillation period without considering output voltage drops, would be achieved with CMOS 555 and no output load, can be calculated as 1.4*RC = 2*RC*(ln 2/3 - ln 1/3), about 14 seconds for the master oscillator. Real period can be expected slightly larger.

As a rough estimation, the sequence is 7 s red, 3.5 s green, 3.5 s orange. Considering capacitor of right oscillator is discharged during red state by 555 substrate diodes, we get about 5 s green, 2 s orange.

 

[danadakk]

The attached might help :

 

[D.A.(Tony)Stewart]

If you are learning electronics, I suggest you avoid websites that have schematics that look like the one described below.
There are several problems:

1. It doesn't work. When Output #1 is high, the red LED turns off and powers U2-8. When Output #2 is high, it powers the green LED. This situation turns on the internal discharge pin 7 to low, pulling down the cathode on the "orange LED" since its anode is at 12V. This creates a logic error with both LEDs being on simultaneously.
2. There were no specifications given for the LEDs. Older LEDs using GaP green were 2V, while newer ones are brighter, made with a different substrate, and typically 3.2V.
3. 5mm LEDs are most commonly used and are rated for 20mA or 65mA. If you have 2 to 3V on the unknown LEDs, then you have 9 to 10V across the resistors, which is five times the power. Using 1/4W resistors in this situation would cause them to overheat. Normally, when a resistor is used at 100% of its power rating, it will reach 125°C above ambient room temperature. Thus, it is wiser to use 5V instead of 12V or use 1W resistors.
4. Connections with three lines should always have dots, and it is preferable not to connect four lines at one node. Crossing lines without gaps can cause simulators and PCB design programs to think there is a broken logic line.
5. Part details such as part number, tolerance, power ratings, and whether they are CMOS or TTL are missing. This information is crucial.
6. There are many other clues why you should ignore this website and instead use the links provided by myself and others who are professionals in this field.
7. Beware of CMOS inputs with capacitor voltages on ICs that are switched on and off to function as a timer. This is unwise because CMOS inputs must not exceed a Schottky diode voltage above Vdd (+).
8. Traffic Engineering does not use orange lamps in any continent. They use amber, which is a warmer yellow with a wavelength of 592 to 595 nmD. The "D" in nmD stands for dominant wavelength, corrected to standard eye response centered around green.

I tend to regard any use of a 555 timer as amateurish (prone to many bad habits) and chose never to use or needed one in 45 years of Design, until trying to help others have fun.

However, the 555 timer has some neat features: the two comparators have internal resistors that create a 1/3 and 2/3 of V+ threshold. The 2/3 threshold is close to the exponential asymptote threshold of 63% of target voltage, Tau = RC. These ratios also mean changing the supply voltage doesn't affect most 555 circuits because the design is "ratiometric" using differential amplifiers. So the ratio matters, not the absolute values within specified limits. The CMOS version and dual 555 versions are preferred as the outputs go rail-to-rail but may not have the same current drive capacity as bipolar NPN/PNP outputs.

Good luck and have fun.

Tony
EE since '75

 

[FvM]

Don't think post #1 circuit is an instructive design, but it works at least in LTspice simulation with NE555 transistor level model. Behavioral NE555 model supplied with LTspice shows incorrect behaviour of discharge pin with switched supply voltage. There might be also a problem of damaging the right 555 when discharging the timing capacitor capacitor through substrate diodes.

There are wrong assumptions in above analysis. Discharge pin and output are not activated simultaneously.

 

[D.A.(Tony)Stewart]

Don't think post #1 circuit is an instructive design, but it works at least in LTspice simulation with NE555 transistor level model. Behavioral NE555 model supplied with LTspice shows incorrect behaviour of discharge pin with switched supply voltage. There might be also a problem of damaging the right 555 when discharging the timing capacitor capacitor through substrate diodes.

There are wrong assumptions in above analysis. Discharge pin and output are not activated simultaneously.

OK Murphy's Law, If anything can be Dyslexic, it will be. The drivers are both active LOW but the LEDs are NEVER ON at the same time). The OUTPUT STAGE is also an INVERTING complementary Buffer.


Even TI's block diagram is missing the point or a little bamboozled.

If they ever decided to update this datasheet ... The top NOR gate is a SET register after the inverted buffer output, while the lower comparator can RESET the output is SET is not active, or one can override this with an external Reset.

I recommend users follow this updated part and datasheet if you want the BJT version.

https://www.ti.com/lit/ds/symlink/lm555qml.pdf?ts=1716652519856&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM555QML%253FkeyMatch%253Dlm555

But beware to buy from this link for the "military" version leads to many options such as Mil-Std-883 versions or just 883 which includes X-RAY precap inspection. But it is still the same chip. Buying one will help increase military spending.

 

[danadakk]

When you progress at a later point in school here is an example of using'
state machine design to do a sophisticated intersection design. This shows
what modern tools look like, not rocket science, and a lot of fun. But
dont despair because it may seem foreign to your current state of learning,
just tuck it away for future reference.

Regards, Dana.