555 IC Timer Resistor Value Help

[drewbagd]

Hello there!

I'm just getting into electronics and I decided to jump in with the 555 ic timer contest. I've been reading the site and all about this stuff for a few months and finally just bought a bunch of shit to get started.

What I'm looking to calculate is the appropriate resistor values in a 555 ic timer circuit that would allow it to be off for 55sec and on for 10sec, repeating as long as there is power (astable mode, ya?). I -really- want it to be ON for 55sec but I've read that it cannot be on for longer than it's off. To compensate I was planning on trying to hook up the output from the 555 to a ground, triggering the unit on when the output was 0.

I found a couple formulas that look like they might be useful for this but I got a little lost--how to I pick the right capacitor value?

t1 = .0693(r1+r2)C
t2 = .0693(r2)C

If my math is right (highly improbable) then 779.22/C=r1. Does this mean I can pick almost any capacitor value and use that to determine the necessary resistor values? Another issue I'm facing here is that I'm not certain of the units of measure in these equations (ohms or mohms? uf or ...?).

Any help is appreciated!

[PhilKll]

http://www.csgnetwork.com/ne555timer2calc.html
I've been working on my entry for the last week or so, used this site a lot but it doesn't seem to want to go below 50% duty cycle. You can have a duty cycle less than 50%, but it requires a diode be added to the circuit in parallel with R2. With 55 off 10 on, I believe that would be a 15% duty cycle which is really really low, maybe it would be easier to do 55 on 10 off, and then just invert the signal.

http://www.doctronics.co.uk/555.htm#more_astables
This site gives more information on the lower duty cycle, I would just worry the values start getting extreme, with a huge capacitor or something.

[Ernie]

It is a good thing to worry. Depending on the accuracy you need in your timing that diode can be a disaster.

By design, the 555 has a very linear well defined and temperature stable method of operation. The diode introduces a non-linearity into the timing path that can easily cause problems. It will continue to work, just not as accurately as you may wish.

10 seconds is very long for a 555 and you need a very low leakage cap. In fact, the wrong choice of cap may even fail to oscillate.

[drewbagd]

Psh, leave it to me to come up with an idea that I can use to break into something I've been interested in doing and it's WAY more complicated than expected :/

Is there a better ic timer to use for something like this? I'd like to be able to enter it into the 555 contest so if it fits into that family that'd be great but if not it's still something I want to put together.

Thanks for the help!

D

[PhilKll]

Probably depends on exactly what you're trying to do, I have no personal experience with any other sort of timer chip myself, so no recommendations there. I would probably use a micro controller, as you should be able to set the timing for anything you want, but I would search around, as a micro could be over kill for the job.

[drewbagd]

essentially i just want to flash some leds on for 10 sec and off for 55 repeatedly. Space is an issue, so a micro would be out.

[PhilKll]

http://www.csgnetwork.com/ne555timer2calc.html
since its just an LED, accuracy probably isn't a big issue, I got the timer really close using
.0001 Farads (100uF)
650000 Resistor 1 (650k)
150000 Resistor 2 (150k)

which got
10.395 seconds low
55.440 seconds high

Which is the opposite of what you want, but since you are going to need a transistor anyhow to control the LEDs, you should be able to use a PNP transistor which is on when the base is low, and off when its high. Which would invert the timing for you.
http://www.kpsec.freeuk.com/trancirc.htm
Scroll down to "Choosing a suitable PNP transistor" It shows how to hook it up.

[drewbagd]

Thanks again PhilKll! There is some great information here and I love soaking it up! That info is pretty spot on and that calculator is awesome when I've got a few starting points. Quick question: why would I need a transistor to control the leds? Why couldnt I power them directly?

[PhilKll]

Actually looking at my datasheet for a 555 it looks like it can source or sink 200mA so you should actually be able to drive a few LEDs off that easily. I was thinking it was lower, like 20mA or so, which could do one, but if you wanted more than that, it would burn up your chip. The transistor is used, because it doesn't draw as much current, turning it on and of, but can handle a lot more depending on the transistor used, so you can hook up a lot more LEDs to the transistor, and use the timer to turn it on and off, with out fear of burning out your timer chip. Depending on how many you plan on using, and of what type, you'll need to figure out the load created by the LEDs, and find out if the timer chip can handle it. But in your case, the PNP transistor would create the inversion you need, while also handling the current. Also don't forget the necessary resistors on the LEDs that will keep them from burning up also.

[drewbagd]

Very cool, thanks again so much for letting me pick your brain! Can't wait till all my goodies get in so I can start.