# 555 timer questions

#### Firestorm

Joined Jan 24, 2005
353
Hey,
I have been looking at different 555 circuit on the web and have come across a few questions.
1) When using the timing formula, this only calculates the time the circuit is in high. Is this correct or does it depend on how the timer is connected?

2) Does a power on reset make the circuit jump into low when turned on?

3) If I were going to make a timer circuit that would delay the device from triggering, what all would I use? I was thinking that a relay and the LM555 would work but am not sure on how to make the circuit wait the first 15 sec. then give a diff. output(if so, how long would it have to be?), then to maye power it off somehow. Not really a project of mine, but thought itd be nice to know how to do it.

All help is greatly appreciated...thx again

-fire

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by Firestorm@Mar 24 2005, 04:19 AM
Hey,
I have been looking at different 555 circuit on the web and have come across a few questions.
1) When using the timing formula, this only calculates the time the circuit is in high. Is this correct or does it depend on how the timer is connected?

2) Does a power on reset make the circuit jump into low when turned on?

3) If I were going to make a timer circuit that would delay the device from triggering, what all would I use? I was thinking that a relay and the LM555 would work but am not sure on how to make the circuit wait the first 15 sec. then give a diff. output(if so, how long would it have to be?), then to maye power it off somehow. Not really a project of mine, but thought itd be nice to know how to do it.

All help is greatly appreciated...thx again

-fire
[post=6338]Quoted post[/post]​
1) If it is wired in monosable mode then only one formula is involved. It gives the time for which the output is high.

If it is in astable mode then there are three relevant formulas. One gives the frequency of operation, there's another giving the time for which the output is high, and a third which gives the time for which it is low.

See Tony van Roon's excellent write-up.

2) Briefly taking the reset pin (4) low resets the output pin low.
The reset pin must be returned high for the chip to accept a trigger.

3) To inhibit the chip from triggering at switch-on, connect the reset pin to the positive supply rail via a resisor, and connect a capacitor from the reset pin to 0V.
This will keep the reset pin at a low state until the C charges up to almost the positive supply voltage and prevent the chip from being triggered until that time.

#### Firestorm

Joined Jan 24, 2005
353
k thx for the help,
i will try to incorporate what all you said into a schem just to make sure i am on the right path..thx again

-fire

#### Firestorm

Joined Jan 24, 2005
353
I have been reading through the pages u suggestedand those which appeared on google...I have a few more questions and I guess something just isn't registering with me (hopefully once i understand the 555 i can understand any IC better )
1-what exactly does the trigger (read that it starts it but any detail left to add anyone?)
2-When we use the terms high and low we are refering to the amount of what?
3-What exactly fo the control voltage (pin 5) and threshold (pin 6) do for the circuit?
I hope if I can understand high and low stuff it will click :unsure:...thx l8er

-fire

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by Firestorm@Mar 28 2005, 12:54 AM
I have a few more questions
1-what exactly does the trigger (read that it starts it but any detail left to add anyone?)
2-When we use the terms high and low we are refering to the amount of what?
3-What exactly fo the control voltage (pin 5) and threshold (pin 6) do for the circuit?
[post=6459]Quoted post[/post]​
1) The trigger, which is a negative-going edge, changes the state of the lower comparator within the 555, which goes "high" - (see later.) When it goes high it "sets" the control flip-flop.
The output of the control flip-flop switches two things: the output stage so that it connects the output pin to the chip's positive supply rail, and the transistor who's collector is connected to the discharge pin. This latter action discharges the external timing capacitor.

2) The terms high and low mean the presence of a voltage (high) or the absence of one (low).

If the 555 is wired in the monostable configuration its "normal" state, i.e. its state between pulses is with the output "low", i.e. the output stage switches the output pin to the chip's 0V line (the "ground" pin.)
When it produces a pulse the output goes "high", i.e. it connects the output to the pos. rail as stated in 1).

3) The control voltage pin gives us access to the the potential divider which provides the reference voltage applied tp the upper comparator. Bringing this point out to a pin allows us to change the reference voltage (within limits) so that we can, for instance, modulate the frequency of oscillation in the astable mode.

The threshold pin applies the voltage developed across the external timing capacitor to the upper comparator where it is compared with the control voltage. Once the externally applied voltage reaches the threshold value the comparator output goes high, resetting the control flip-flop. This changes the state of the output pin and switches off the discharge transistor.

I am surprised that all the above wasn't understood from Tony van Roon's description.