555 timer internal working

[eptheta]

Hey,
I looked at the Oscillator sticky at the top of the page, but i didn't find the answers i am looking for...
I did a quick search on google for the theory behind the working of a 555 but i think i'm looking too broad....
How exactly does the 555 work ? Is it just an RC oscillator with a diode to generate a squarewave ? Why are there so many terminals then ? Is there any resource which can explain how the 555's insides work(and not just what they do) ?
I know this is more of a physics question than an electronics one but could anyone direct me to a resource where the working of resistor-capacitor timing circuits (like what the 555 implements) are explained nicely?

Also, whats the difference between mono-stable, bi-stable and astable modes of oscillation ?
I seem to be a complete newbie at this, but in-fact i have worked a lot with 555s in the past, but i never questioned its working before.
Thanks a lot.

 

[blocka]

The block diagram in the datasheet shows how it works. it has comparator and flipflop and things.

monostable - you only get one pulse out and then need to reset for it work again
bi-stable - have two states but you need to trigger to change states
astable - continually change states (ie produce a square wave) with no external trigger.

 

[Mitchekj]

Basically it's two comparators feeding a flip-flop, which switches a totem-pole output.

The comparators will switch depending on what mode you have it configured as, but they sense 1/3 and 2/3 of Vcc for switching. For example, in astable mode (a continuous oscillation) the charging and discharging of the RC network (external, that the user sets) changes the voltage levels on pins 7,6, and 2. That toggles the flip flop each time the voltage level falls to 1/3 of Vcc or rises to 2/3 of Vcc. The flip-flop then either drives the totem-pole high (vcc) or low (gnd) and you get your square wave.

The totem-pole would be two NPN transistors in the chip, one connected collector to Vcc and emitter to the output pin (this is where the high output comes from) and the other transistor connected collector to output, and emitter to ground (that's where the low output comes from.) The bases are controlled by the flip-flop, high-side on Q, low-side on Qnot.

At least that's how I've always understood it. I'm sure there's an actual schematic somewhere on the net of all the individual transistors, resistors, diodes etc. needed to make the comparators/flip-flop/etc.

As to the modes:
Monostable acts like a one-shot, using the trigger input.
Astable is the continous oscillator. (high-low-high-low, ad infinitum)
Bi-stable acts like a flip flop usins the trigger pin.

I'm sure there's much further in depth information on the operation, but I leave that stuff to the PhDs of the world. lol.

Edit: Here's a schematic of the die.

 

[eptheta]

when you say "they sense 1/3 and 2/3 of Vcc for switching", how exactly do they do this ?

Also, I have a few other questions:
-Why does the voltage drop in those pins ? (Shouldn't the pd remain constant ?)
I ran a few simulator programs and put a voltage scope on the terminals, and all of them note no change in voltage...... Are we just talking about current change ?
-After the RC network is charged and the capacitor starts discharging, where does the current flow ?
-Why specifically 1/3Vcc and 2/3Vcc, why not Vcc and 0 ?

Thanks a lot for your help so far.
Eptheta

 

[jackorocko]

Some suggest the 555 timer gets its name from the 3 - 5k Ohm resistors that make up the voltage dividing network that gives it 1/3 and 2/3 switching voltages of VCC.

wiki differs in opinion, but I don't believe in coincidences very much. http://en.wikipedia.org/wiki/555_timer_IC

 

[(*steve*)]

Why specifically 1/3Vcc and 2/3Vcc, why not Vcc and 0 ?

The reason is that an RC circuit will (in theory) never get exactly Vcc across the capacitor, nor will it ever get down to zero.

The voltage will get asymptotically closer to each. However you can calculate exactly how long it will take to get to a lower voltage. 1/3 and 2/3 Vcc are useful because they're symmetrical (so the calculations are the same) and because they maintain a reasonable voltage across the resistors so that leakage current is less of a concern.

 

[Externet]

This may be of help:

http://www.unitechelectronics.com/NE-555.htm

 

[imranahmed]

how to use 555 as delay timer,i want that when supply voltage is apply the timer will start after desired time?

 

[eptheta]

Anyone willing to answer the other questions?:

-Why does the voltage drop in those pins ? (Shouldn't the pd remain constant ?)
I ran a few simulator programs and put a voltage scope on the terminals, and all of them note no change in voltage...... Are we just talking about current change ?
-After the RC network is charged and the capacitor starts discharging, where does the current flow ?

Oh yeah ! Its an exponential curve.... Good point steve...
Externet, Thanks for the link... I should look through it properly but i only had a chance to glance through it.
No need to answer those questions if you think they are in the link...

And imranahmed, Honestly i would think you could either:
-Wait for a capacitor to charge up and then discharge, pick an appropriate value so that it matches your delay and then use that as an input to your 555 (if your delay is really small)
-Use 2 555 chips, one with the delay frequency and one with the desired frequency....
The delay frequency one could output to a latch (or relay) which then would output to the other 555... Thats all i can think of... (unless there is a delay function in the 555 itself... but i have no idea)
Good luck

Thanks

 

[imranahmed]

Thanku very much dear eptheta,

But i also want to know that when 555 use as astable(free running oscilllator)
the first pulse is greater than other incoming pulses! what is its solution????

 

[(*steve*)]

Remember that the trigger points are approx 1/3 and 2/3 Vcc. across the capacitor.

What is the voltage across the capacitor when you start up? Is it 1/3 Vcc, or something lower?

How long will it take for the capacitor to reach 2/3 Vcc from this point?

 

[imranahmed]

Thank you dear steve,
i make a circuit of 555 for starting my electric generator automatically when electricity is shut off.The function of my circuit is that i connect first 555 (astable configuration) its output to another 555 (its also astable configuration).first 555 generate first pulse of 21 seconds then off,Between 0-21 secs the other 555 generate pulses of 2secs high 5secs low.But problem is that when i connect relay to other 555, it shut off after 19 secs not after 21 secs? And other problem is that the first pulse of both is slighty longer than other incoming pulses???

 

[(*steve*)]

You are designing a 555 circuit where the period is very long. You should expect significant variability in the timing with temperature, humidity, voltage, anything... THe fact that you're only seeing 10% variation between 2 devices is perhaps lucky

the first pulse is always longer because the capacitor has to charge up from 0V rather than 1/3 Vcc.

 

[imranahmed]

ok thanks dear steve,

Do you know any other perfect oscillator IC,so plz refer me?

 

[jackorocko]

why not use a binary counter and pull both values off one oscillation cycle. I did something similar where I held the reset of the counter high until I needed it to start counting.

 

[imranahmed]

Thanks jackorocko,

Can i set or reset binary counter of my desired time? plz explain few hints about it?

 

[(*steve*)]

The basic idea of using a binary counter is to clock it with a higher frequency oscillator and reset it when it reaches a particular count.

Because the clock frequency is far higher that the frequency you're creating (after division), the difference in the first period is inconsequential.

You might like to look at this thread where we are discussing a similar issue (although the exact requirements may be a little different)

 

[imranahmed]

Thanks Dear Steve,

what is the basic function of inductor?

 

[(*steve*)]

In basic terms an inductor is the exact opposite of a capacitor.

A capacitor can be seen as storing energy as a voltage. It also restricts the passage of DC but allows AC.

An inductor can be seen as storing energy as a current. It restricts the passage of AC but allows DC.

 

[imranahmed]

Thanks dear Steve