555 timing help

[bunghole]

i dunno how many times I've tried, using the programs I've found, even sitting at a desk with a calculator for half an hour, but i still need help!

I am trying to make a timer that oscillates at 1Hz, 1 second for high and low time. This is used a a headlight hazard timer on the following train: https://en.wikipedia.org/wiki/VLocity
where the left and right headlights alternate every second. i know because i had a job testing them at the factory. they use a telemecanique timer.

I have a Dick Smith catalogue and this is what it says:

Output HIGH is:

t1 = 0.693(R1+R2)C1

Output LOW is:

t2 = 0.693(R2)C1

Total Period:

T = t1+t2 = 0.693(R1=2R2)C1

The frequency of oscillation is:

f = 1/T = 1.44/(R1+2R2)C1

The Duty Cycle is:

D = R2/R1=2R2

can someone PLEASE help me???

 

[MalcolmV8]

I found this page very helpful when working with 555 timers.



Malcolm

 

[ericgibbs]

hi,

Download from the philips web site Application Note: AN170

Which version of the 555 are you using,,, TTL , CMOS?

 

[Krumlink]

555 timers are crap (believe me I use these things all the time so dont argue with me, I am TEH 555 MASTER!) when it comes to precise timing, or anything at all

I reccomend a 1Hz circuit using a 32khz crystal. Search in the search box for
Digital clock, and find teh post by me. In the schematic it has a clock portion for a very precise clock circuit. But if you want to use a 555 timer use a 10uf cap across pin 1 and 2 and 10k ohm and 100k ohm across pin 8-7 and 7-6. Keep mixin up the resistor locations until you find teh right spot.

 

[Tomble]

Hi, FWIW despite what Krumlink says there, 555s should be more than adequate for just flashing lights on and off... (though I'm a bit confused, are you talking about in model trains, yes??). But you wouldn't want to use them for anything that needs really accurate timing.

In case there was any problem with the appnote Eric recommended, I think where I got the circuit for getting 50% duty cycle from the 555 (which is what you're after!) last year must have been http://www.williamson-labs.com/555-tutorial.htm
because I remember using the diode like that. NOTE: despite what that page says, using plain old breadboard and stripboard shouldn't be a problem unless you're doing something fancy, like maybe up in the MHz range or thereabouts. Or if you're using ultra-small capacitors? At 1Hz, it should be completely irrelevant Anyway, the diode approach worked well enough for me, wherever I got it.

There was also this page, **broken link removed**
which has absolutely tons of circuits, and various calculators too for different contexts. It does have at least one 50% duty cycle circuit, though I don't think I used his ones myself. Also ISTR this is the feller who does a lot of model-railway oriented circuits. So you may find some of that (incl. his other pages) useful for future projects.

BTW if it's not obvious: if these lights are drawing a lot of current, you shouldn't make the 555 drive them directly. 200mA is the stated maximum which should be plenty for many models but it depends what you're using, and near that current you might get overheating problems in the chip perhaps.

 

[Tomble]

Oops!

Sorry, I missed this detail

I am trying to make a timer that oscillates at 1Hz, 1 second for high and low time.

If you want 1 second high followed by 1 second low, then that's 0.5Hz, not 1Hz. Perhaps this might have made the maths more confusing than the 50% duty cycle issue?

If OTOH you just want 1 second for the total cycle of high+low, then yes that's 1Hz, but then you're not explicitly asking for equal on/off time like I'd thought you were (but maybe you'd still be wanting this anyway). If any problems remain, maybe you should clarify this?

 

[bunghole]

yeah, 1 second high, then then 1 second low (astable), so would that be 2Hz as the total time period? and TTL 555 as well.


thanks ev'rybody

 

[Nigel Goodwin]

yeah, 1 second high, then then 1 second low (astable), so would that be 2Hz as the total time period?

No, 0.5Hz.

 

[audioguru]

A 555 makes very close to 50-50 square-waves if its single timing resistor is driven from its output.

 

[Roff]

A 555 makes very close to 50-50 square-waves if its single timing resistor is driven from its output.

Especially if you use a CMOS 555. The equations are
Period: T=1.386*R*C
Frequency: F=1/T=0.721/(R*C)
You can solve for RC from either equation.

 

[3v0]

Instead of a 555 you could use a processor. It would be fewer parts.

Microprojects has canned code for the following

• 
  [*] HTC150: Four channel LED flasher.+SRC
  [*] HTC151: Four channel LED flasher.+SRC
  [*] HTC550: Monostable multivibrator.+SRC
  [*] HTC551: Power up sequenser.+SRC
  [*] HTC555: Clock generator.+SRC
  [*] HTC601: CPU supervisor.+SRC
  [*] HTC602: CPU supervisor.+SRC
  [*] HTC603: CPU supervisor.+SRC
  [*] HTC750: Melody generator.+SRC
  [*] HTC751: Programmable melody generator.+SRC
  [*] HTC752: Melody generator.+SRC
  [*] HTC852: Simple PWM controller.+SRC
  [*] HTC2500: Simple digital clock controller.+SRC
  [*] HTC2501: Simple digital alarm/clock controller.+SRC
  [*] HTC2502: Simple analog clock controller.+SRC
  [*] HTC2503: Simple analog clock controlle

 

[Krumlink]

Yeah, I dont doubt the obvious superiorty of a Microcontroller, but that is overkill! All he needs is a simple little circuit! I would like to get more people into electronics, but diving them headfirst into PICs and Microcontrollers can overwhelm them (like me). If he knows programming and PIC, folo 3v0s links. If you want a simple 555 timing circuit, play around with the 555 timer until you get what you want. You may also want to try a 2 inverter oscilliator. Google them. Its really easy to make one. Just keep trying and looking at your options.

Man 3v0 Your never going to find a nice signature

 

[Hero999]

You can also get perfect 50-50 squarewaves by sticking a divider on the output, although it's probably better to use a CD4060 and do away with the 555.

 

[Krumlink]

You can also get perfect 50-50 squarewaves by sticking a divider on the output, although it's probably better to use a CD4060 and do away with the 555.

Like what I said

Here is the Clock circuit

 

[Hero999]

That's a bit complicated for a 1Hz clock isn't it?

 

[Krumlink]

Lol no...

Mine has pretty colors and a smiley face so eat it!

 

[3v0]

Yeah, I dont doubt the obvious superiorty of a Microcontroller, but that is overkill!

It uses fewer parts, costs about the same. Do not see that as overkill.

... but diving them headfirst into PICs and Microcontrollers can overwhelm them (like me).
Man 3v0 Your never going to find a nice signature

Using the info on the site I linked to should not be all that difficult. It has canned programs that allow inexpensive PICS to emulate other devices but without a lot of external components.

One of these days we have got to get you to try your hand at programming again. It is not all that difficult, you just need to take it one step at a time.

When I have somthing to say I put it in my signature. As I do now.

 

[ecerfoglio]

You can also get perfect 50-50 squarewaves by sticking a divider on the output, although it's probably better to use a CD4060 and do away with the 555.

It's true that the "traditional" 555 astable circuit can't reach exactly 50% duty cycle....

BUT

If the "charging" (**) resistor (+V to pin 7) is a small one (say 1khm:, don't go bellow 1khm: as it is a current limiting resistor and you can destroy the 555),
and the "discharge" (**) one (pin 7 to pins 2&6) is rather big (say 100khm: )

Then the capacitor will carge across 101khm: and it will discharge across 100khm:

And the duty cycle will be 101/201 = 50.24%. You will need a very good stopwach to see the difference

PS: (**) I don't use the official "names" for the resistors (R1, R2) as I don't have the schematic at hand, and I don't know which one is called each name.

 

[audioguru]

Why use an old 5V TTL divide by 10 IC?
Use a 4017 or half of a 4518 Cmos IC that work fine with a supply from 3V to 18V. With a 9V battery then a current-limiting resistor is not needed for the LED.

 

[Krumlink]

Lol it was available...