About 555 mode of operation

The threshold input detects when the voltage is greater than 2/3 of the supply voltage by setting the output to a low. The trigger input detects when the voltage is below 1/3 of the supply voltage and resets the output to a high. The discharge pin turns on a switch to the negative rail. when the output is low. If both the trigger in threshold pins watch the same voltage and the discharge pin is involved in changing that voltage, the circuit is probably an astable, since there are two levels involved in making decisions, and a means to alter the direction of change in that signal. If only the threshold or trigger are involved in monitoring a timing voltage, the circuit is probably a monostable. There are lots of possibilities.

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John Popelish

In article , kostas wrote: [...]

Follow it through for your self. A 555 always does this:

If TRG goes 1/3Vcc, the Q goes high.

If the THR goes above 2/3Vcc Q goes low.

Discharge is on when Q is low.

Notice what the RC circuits do and it should be clear.

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kensmith@rahul.net   forging knowledge

"kostas" wrote

but

Finally

The first is in true astable mode so that it triggers itself repeatedly. It generates a constant stream of trigger pulses that feed into the second 555. It is in one-shot or monostable mode (meaning that it isn't triggering itself), but the time timing interval is shorter than the period between trigger pulses feeding into it, so that it seems to output a PWM signal whose duty cycle is varied by the pot.

Opinion: Clever enough, but I'd use a PIC (or micro of your choice) because it's simpler, cheaper and generally will work better offering a multitude of future options like temp sensors, automatic control, LCD display etc.

Hello

What thing determines the mode of operation of LM555? For example if a

555 is triggered externally it is in monostable mode regardless if there is a resistor between pin 7 and 6?

Also if a 555 has connected pin 2 and 6 regardless of resistors again it's in astable mode, correct???

Thanks for your time

p.s. sorry for the crosspost in sci.electronics.basics but i need an answer fast before i give my project

repeatedly.

isn't

offering a

LCD

Yes, if I understand you. Monostable just means that 1 trigger pulse ->

1 timing cycle. Astable is when the trigger is somehow (via pin 6 in your circuit) reactivated by the completion of the timing cycle.

don't

Keep at it, they are more than worth the effort to learn them. Learn to program in assembler first, then use higher level languages if you wish.

Thanks for the reply

If it's not problem for you please have a look at the following link.

The guy says that the first 555 is astable and the second monostable but something isn't right with the connection of the second one. Finally is the second astable again. If this is true then the circuit is not frequency stable when you adjust the duty cycle

So finally the 555 if it's not triggered by itself (connection between pin 2 and 6) is in monostable mode.

yes microcontrollers are better choice and give more freedom but i don't know how to program one yet. I make my first steps with AVR

I'm interested in why you recommend that? With no relevant Assembler or PIC training, I've been taking the Basic route, and finding that tough enough. (In fact I've temporarily shelved it - again!)

Although exaggerating to make my point, isn't your approach like suggesting someone should learn how to dismantle and rebuild an auto engine and its electronics before taking driving lessons?

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Terry Pinnell
Hobbyist, West Sussex, UK

to

wish.

IMO, the problem with starting on a high level language is that it insulates you from how the hardware actually works. For example, PULSEIN doesn't give me clue me on what a CCP module is or how it operates. Assembler gives me the advantage of fully appreciating the time scale of the real world (uS in my own micro development world ;-)

I think it's more like learning to crawl, then learning to walk and finally run. ;-)

I don't have anything against high level languages, I just think that assembler should be learned first. I strongly believe that only then can you really take full advantage of a high level language on your chosen micro.