Hi, I am new to electronics and would like to learn more, but I need help and advice now.
I have built a simple LED flashing circuit using a Arduino board and it is working great.
I took the circuit diagram from the net.
I am using a IC 7555, R1 680kΩ, R2 10kΩ, C1 10μF, Superbright Red LED with a 1kΩ resistor and a voltage of 4.5V. It is the very basic circuit diagram.
I would like to change the blinking rate which at the present is 5 seconds and blinks a Msecond to, say, 2 blinks within 1 second and OFF for 3 or 4 seconds.
I went to a Timer Circuit Calculator where I could change R1, R2 and C1 but I am still a little confused.
Is Time High when the light is ON and Time Low when the light is OFF? If I want 2 blinks within 1 second and OFF for 3 or 4 seconds which value should I have for R1, R2 and C1
Also, when I bought my IC, I asked for a IC 7555, but on the IC itself it is written 555. Are they the same? How can I differentiate them?

Thank you for helping.

Alain.

 

You control the ON time and OFF time with R1, R2, and C.

You cannot get two blinks followed by a long space. If that is your requirement you will need an additional IC to do that.

ON time and LED ON are not necessarily the same.
Post a schematic and we will be able to advise you.

What is the full part number written on the chip, including all letters and numerals?

Once we can identify the manufacturer and the part number we will be able to look up the data sheet for the formulas for frequency and duty cycle.

 

What circuit diagram? There's more than one blinking LED circuit diagram on the internet.

What are you using the Arduino for? How does it interact with the other components?

A schematic would be really, really, really helpful.

 

Welcome to AAC!

I am using a IC 7555, R1 680kΩ, R2 10kΩ, C1 10μF, Superbright Red LED with a 1kΩ resistor and a voltage of 4.5V. It is the very basic circuit diagram.

There is no standard way for denoting the timing and discharge resistors for a 555 timer. If you don't post schematics when talking about circuits, expect to get asked to post one.

Regarding when the LED will be on depends on how you connect it to the timer output. Again, there is no standard way for doing this because 555 timers will sink and source current If you had posted a schematic, we could have answered your question regarding when the LED would be on.

 

Well, I wasn't expecting so many replies. Thank you all.
I wasn't aware that 2 blinks per second required the use of 2 IC, so I will stay with one blink and one IC. My aim is to understand how to select R1, R2 and C1 to get a cycle of : 1 blink per second, then Off for 3 or 4 seconds.

The writing on the IC in between the 8 pins is : 05T19 or 05TI9
On the top of the IC is: 46A80PM and then TLC 555CP

I am using the Arduino to place all the components together as per the diagram. I was lucky I made it work.

Here is the diagram

Regards

Alain

 

TLC555 is Texas Instruments equivalent to Intersil ICM7555 which is the CMOS version of NE555 and LM555.

TLC555, ICM7555, LMC555 are all the same low power CMOS version.

Your circuit is correct for a short LED ON.
The way your LED is connected, the LED will be ON when the output on pin-3 is LOW.

Let me correct your requirement of "1 blink per second, then Off for 3 or 4 seconds". There is no such thing but we know what you mean.

What you mean to say is "ON for 1 second and OFF for 3 or 4 seconds".
Hence we want output LOW for 1 second and HIGH for 3 or 4 seconds.

Here are the formulas:

t-HIGH = 0.7 x C x (R1 + R2)

t-LOW = 0.7 x C x R2

With C = 10μF
R1 =290kΩ
R2 = 144kΩ

Don't bother trying to get the exact values.
Try
R1 = 330kΩ
R2 = 120kΩ

and see if that means your fancy.

R1 is the resistor between pin-8 and pin-7.
R2 is the resistor between pin-7 and pin-6.

 

You can also use a 556 timer IC which is simply 555 timers in the same package.

It would seem you are using a TI (Texas Instruments) CMOS 555 timer IC in a plastic dual inline (DIP) package.

http://www.ti.com/lit/ds/symlink/tlc555.pdf

TLC is Texas Instruments Linear CMOS, which is the fabrication process family used. 555 is the basic timer design. C is CMOS. and P is the package code.

The other marks give tracking information such as the date and production line it was produced on.

 

Thank you for the explanation. I understand now how to get the value of R1 and R2.
Just a thought. Can I replace R1 and/or R2 resistors with potentiometers and "play" with the duration of HIGH and LOW without damage the circuit and do I need to add more things along the way?
Regards,
Alain

 

Can I replace R1 and/or R2 resistors with potentiometers and "play" with the duration of HIGH and LOW without damage the circuit and do I need to add more things along the way?

Yes you can change the timing resistors to pots, or add pots in series with fixed resistors. It is important that R1 never go down to zero ohms, as this will overcurrent the 555 discharge pin during the low part of the output cycle. The datasheet might have recommendations for minimum timing resistor values.

ak

 

Thank you for the explanation. I understand now how to get the value of R1 and R2.
Just a thought. Can I replace R1 and/or R2 resistors with potentiometers and "play" with the duration of HIGH and LOW without damage the circuit and do I need to add more things along the way?
Regards,
Alain

Sure you can.

555-timer circuits generate pulse waveforms that are relatively stable in pulse duration and frequency. When the application requires fine adjustment, a variable resistor is wired in series with a fixed resistor so that the variable resistor provides a "fine" adjustment. For example, you can make the variable resistor 20% of the total resistance required while the fixed resistance is chosen to be 90%. This provides a resistance covering 90% to 110% of the nominal resistance.

You can make R2 a fixed value to provide the 1-second pulse duration. Then make R1 a series combination of a fixed and variable resistance, as explained above, to allow you to select the desired repetition rate.

Edit: On the other hand, if you want to adjust the time delay between pulses over 4 orders of magnitude, put a 1kΩ fixed resistor in series with a 1MΩ pot and use this for R1.

 

That's great. Thank you. I will try both options.

I have noticed on some website pin 5 is connected to the return or ground with a small capacitor which, they say, is not polarised.

What is the advantage(s) to have Pin 5 connected to ground? What actually does it do?

Regards

Alain

 

Pin 5 is NOT connected to ground. It is BYPASSED to ground with a capacitor. This is used to stabilize and reduce the noise in the resistive voltage divider that sets the two transition threshold. You can also drive this pin with a voltage in order to have some control over those thresholds.

 

The last one for the day. Which potentiometer is better for my application. A Linear or a Logarithmic?
Thank you
Regards
Alain

 

Probably a linear, but it really depends on the quantitative relationship you want between some aspect of the timing and the amount of rotation of the pot control.

 

The last one for the day. Which potentiometer is better for my application. A Linear or a Logarithmic?
Thank you
Regards
Alain

It really depends on the application. Either would work.

The idea of a log pot is that if you want finer control at one end of the range versus the other end, then one would use a log pot. Then it depends on if the taper is log or reverse log and which end of the pot you use as a variable resistor.

(Edit: To complicate things even further, you need to choose if you want finer control at lower frequency vs higher frequency, or lower delay vs higher delay. This is complicated by the fact that delay is the inverse of frequency.)

Log pots are used in volume control applications because the perception of loudness follows a log scale.

I would use a linear pot in your case.

 

Hi,
I was successful and progress well, however I have another question.

When selecting values for C1, R1 and R2 what is better, a higher C1 therefore lower R1 and R2 or a lower C1 value with higher R1 and R2 values.

Thank you.

Regards,

Alain

 

That depends on what constitutes one choice being "better" than another. One thing you'll discover is that engineering is about compromises, there are seldom solutions that are categorically better than others. Option A is better than Option B in some ways and worse in others, so which option is "better" depends one what is important.

Making the resistances small and the capacitance large means that things like the influence of parasitic resistances and capacitances will be less noticeable and, in particular, leakage current in the capacitor will be less of an issue. These considerations may be important to you or might not. Going the other way, large resistances and small capacitance will result is less power consumption, which might be critically important for a battery-powered circuit you you might not care at all for a mains-powered circuit.

The bottom line is that before you can answer the question of which is "better", you need to spend some time considering what, for your purposes, "better" even means.

 

It makes sense. I tried to apply the same thinking with "Voltage Supply" and can't come with any disadvantages similar to parasitics, leakage, consumption for R1, R2, C1 etc. So if I have the choice of different Voltage which one should I used.
Sorry for the hiccup, I push the wrong button and sent it before it was completed.

Alain

 

Hi,
I was successful and progress well, however I have another question.

When selecting values for C1, R1 and R2 what is better, a higher C1 therefore lower R1 and R2 or a lower C1 value with higher R1 and R2 values.

If you want repeatability over the long term, you do not want to use electrolytic capacitors for timing functions. Hence C1 should be 10μF or lower made with ceramic disk. For lower values, capacitors made with polyethylene, polypropylene, mylar or mica are preferred. This means that for the same time constant you need to increase the resistor values proportionately. CMOS 555-timers allow you to increase the resistances up to 10MΩ. That is not true with TTL 555-timers.

 

It makes sense. I tried to apply the same thinking with "Voltage Supply" and can't come with any disadvantages similar to parasitics, leakage, consumption for R1, R2, C1 etc. So if I have the choice of different Voltage which one should I used.

The supply voltage will depend on the components, application and power requirements.

For example, in the days when digital logic chips of the 7400 series were the dominant chips available, supply voltage was simply 5V, no choice in the matter.

Then came along extremely low power 4000 series CMOS logic with a supply voltage range of 3-18V. This meant that you could run your circuit from a single 9V PP3 battery.

Hence to answer your question, the requirements of the application comes first. You decide if you need battery or mains operation. For portable battery powered equipment you do a power analysis and determine your battery requirements. Then you choose the components to determine what your supply voltage should be.

In other words, the application will dictate your supply voltage. The choice of components will influence the supply voltage depending on the chip limitations and the power required for the application.