555 Astable help Please

Thread Starter

liteace

Joined Mar 7, 2012
171
Hi All, Im bagging my head on the wall here and need help. What I would like is to set up a 555 in astable mode, I have looked at 555 Astable Circuit Calculator but just cant get my head round it.
These dont have to be exact but what I would like is:

Frequency 900Mhz
Duty cycle 90%
Cycle time 1.25 second
Time high 1 s
Time low 1.25 ms

Can this be done, If so would someone please help me adding the correct value of Cap & the 2 resistors

Thanks in advance
 

w2aew

Joined Jan 3, 2012
219
Hi All, Im bagging my head on the wall here and need help. What I would like is to set up a 555 in astable mode, I have looked at 555 Astable Circuit Calculator but just cant get my head round it.
These dont have to be exact but what I would like is:

Frequency 900Mhz
Duty cycle 90%
Cycle time 1.25 second
Time high 1 s
Time low 1.25 ms

Can this be done, If so would someone please help me adding the correct value of Cap & the 2 resistors

Thanks in advance
You'll have to be clearer about what you want. The numbers you posted don't make any sense. High time plus low time should equal cycle time, and yours doesn't. High time divided by cycle time would give you duty cycle, but your numbers don't jive. And none of these numbers relates at all to 900MHz frequency. By the way, the 555 can't get anywhere close to 900MHz.
 

tubeguy

Joined Nov 3, 2012
1,157
Hi All, Im bagging my head on the wall here and need help. What I would like is to set up a 555 in astable mode, I have looked at 555 Astable Circuit Calculator but just cant get my head round it.
These dont have to be exact but what I would like is:

Frequency 900Mhz
Duty cycle 90%
Cycle time 1.25 second
Time high 1 s
Time low 1.25 ms

Can this be done, If so would someone please help me adding the correct value of Cap & the 2 resistors

Thanks in advance
You can't get 900MHZ from a 555 timer.

Do you really need 900MHZ and what is it for?
 

Thread Starter

liteace

Joined Mar 7, 2012
171
What I need to do is heat a nichrome wire to keep it at a constant temperature and stop it burning out, Im just starting out at this so maybe Ive got my figures wrong
 

tracecom

Joined Apr 16, 2010
3,944
This will get you close.

NE555N Astable Multivibrator

R1 = 2700 Ohms
R2 = 330 Ohms
C1 = 470 microFarads
Frequency = 0.91191 Hz
Duty cycle (% on) = 90.18
 
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Thread Starter

liteace

Joined Mar 7, 2012
171
This will get you close.

NE555N Astable Multivibrator

R1 = 2700 Ohms
R2 = 330 Ohms
C1 = 470 microFarads
Frequency = 0.91191 Hz
Duty cycle (% on) = 90.18

Thanks that worked, I dont think I can get it any better than that. I have here a HCF4541 as advised by member Antonv, what I need to do with this is have a 1 shot 5 min timer, any advise on how I go about that

Thanks

 

DerStrom8

Joined Feb 20, 2011
2,390
This will get you close.

NE555N Astable Multivibrator

R1 = 2700 Ohms
R2 = 330 Ohms
C1 = 470 microFarads
Frequency = 0.91191 Hz
Duty cycle (% on) = 90.18
He was asking for 900MHz, not mHz, wasn't he?

EDIT: Sorry, you're right. mHz--1/cycle time = 0.909 Hz.
 

tracecom

Joined Apr 16, 2010
3,944
Thanks that worked, I dont think I can get it any better than that. I have here a HCF4541 as advised by member Antonv, what I need to do with this is have a 1 shot 5 min timer, any advise on how I go about that

Thanks
You are welcome.

I have never used an HCF4541, but the datsheet has some application information. You can download it from the ST web site.

Unless I needed very precise time, I would just use another 555 for a 5 minute one-shot.
 

Thread Starter

liteace

Joined Mar 7, 2012
171
Thanks, So Im now looking for the 555 in Monostable mode, have I done this right:

Capacitor C, 47f
Resistor R , 6ohm

= 310 secs /60 = 5.1 mis ?

Thanks again
 
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Thread Starter

liteace

Joined Mar 7, 2012
171
Thanks, so the re-set stops it untill the trigger is pressed again or does it restart it.
Pin 3, output goes where on the astable circuit ?

Thanks again
 

tracecom

Joined Apr 16, 2010
3,944
Thanks, So Im now looking for the 555 in Monostable mode, have I done this right:

Capacitor C, 47f
Resistor R , 6ohm

= 310 secs /60 = 5.1 mis ?

Thanks again
That's right mathematically, but I don't know where you are going to get a 47F capacitor. :D

A more realistic selection would be 270k for R1 and 1000μF for C1, which calculates to 297 seconds. Of course, component tolerances will change the actual time. You can adjust the time by using a 220k resistor and a 100k pot in series.
 

DerStrom8

Joined Feb 20, 2011
2,390
Thanks, so the re-set stops it untill the trigger is pressed again or does it restart it.
Pin 3, output goes where on the astable circuit ?

Thanks again
If you have it set for a long output pulse, the reset would put it back to the starting state. But if you let the pulse time out on its own, pressing the trigger button again will re-activate the output.

Pin 3 is the output. It's where your signal is emitted from.
 

Thread Starter

liteace

Joined Mar 7, 2012
171
Thanks, I dont need it to reset. Sorry Im a bit lost on pin 3, is this going before, like 1st in line before the astable circuit, is pin 3 power ??

Thanks
 

DerStrom8

Joined Feb 20, 2011
2,390
Thanks, I dont need it to reset. Sorry Im a bit lost on pin 3, is this going before, like 1st in line before the astable circuit, is pin 3 power ??

Thanks
If you don't need the reset just connect it to +. This will hold it high, so it won't reset while in the floating state.

In your astable circuit you had an output. That's where your pulse came from. That's where you would measure the output frequency and duty cycle. It doesn't change for the monostable. Your output (pin 3) is where you measure the output pulse. When pin 3 is high, it is putting out a positive output (the positive part of the pulse). When it's low, the pulse is low. It's where your output signal appears. I don't think I can make that any clearer.
 

Thread Starter

liteace

Joined Mar 7, 2012
171
Yes sorry long day, as Im new to all this it takes its time to sink in, so when i scope Pin 3 Im going to have 12v for the set time & that is my 12v switch feed for the astable circuit :confused:

Thanks
 

DerStrom8

Joined Feb 20, 2011
2,390
Yes sorry long day, as Im new to all this it takes its time to sink in, so when i scope Pin 3 Im going to have 12v for the set time & that is my 12v switch feed for the astable circuit :confused:

Thanks
I'm not sure what the output voltage would be, I imagine if you have a 12v input voltage (power to the chip), the output would be closer to 8 or 9 v (was thinking 2/3 the source voltage, but I could be thinking something else). Not quite sure what you mean about "12v switch feed", but yes--connect your o-scope to the output (pin 3) of the 555 timer and you'll see the output pulse "ON" for a set amount of time, then go "OFF".
 

Thread Starter

liteace

Joined Mar 7, 2012
171
Here is a circuit with the 14541 in it set up for about 5 minutes.
Thank you very much for the circuit, I have ordered the other parts I need. Question R15 between Q1 & U3 is that 15 ohm or 15 K &
also I dont understand R3 & R4 "0805" do I use the resitors that I already have in the 555 curcuit & just 1 more thing if i replaced SW1 with a SPST would that damage anything ?

Thanks Again
 
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