555 timer curcuit

[layedbackkustomz]

Hey guys i got a question. Im trying to make a simple curcuit with a 555 timer chip. the curcuit has to use 5v or 12v imput and have a output thats enough to trip a relay. now the curcuit needs to turn on for 5 seconds and turn off for 5 seconds and repeat. When the curuit turns on i need it to trip a relay and let a 3.3v current pass through. now i know that the timing is all about getting the resistance for R1 and R2 correct and also has to have the right capacitor. with a curcuit calc. program i can get it close but cant get it right on. and even if i could get it right i dont know how to make the curcuit keep repeating itself. if someone can help me with a sch. or a pcb layout for such a curcuit it would be a great help and also a part list for the curcuit. and if your wondering what its for its to a homemade decoy. Thanks LBK Edited April 26, 2013 by layedbackkustomz

[mike_au]

Sounds like you're after an astable oscillator (a circuit that switches backwards and forwards, as opposed to a monostable or bistable which needs external input to keep switching). There are schematics all over the internet, I seem to recall that it might even be one of the demo files included with Eagle CAD.

 

Depending on how accurate you need to get it, I would say grab a couple of trimpots and tweak them until the timing is right. You should be able to get close enough that you couldn't tell the difference with a stopwatch but you are unlikely to get it perfect (i.e. it would probably lose/gain a few seconds per hour).

 

If you need more accurate timing than that, you probably need something crystal based rather than a 555.

[layedbackkustomz]

well i looked through Eagle and i didnt see it. so im still stuck. if i had a schematic of this curcuit i could make it but im just stuck. Astable is what im looking for just dont know how to do it. so please if anyone can make this curcuit please help.

[Peret]

http://en.wikipedia.org/wiki/555_timer_IC#Example_applications

 

There's an example on that page. Connect pin 2 to pin 6 to self trigger. It's very difficult to get an exact 1:1 mark space ratio with a 555. These days I'd use a 50 cent microprocessor, not a 555.

[mike_au]

Peret is quite right (it's been a while since I played with a 555). Most of the common schematics won't do a 50% duty cycle, I believe there is a trick to separate the two halves of the cycle (don't remember the exact details but it was only one or two extra components, I'm sure google will turn something up), or you can use two timers, one with a low duty cycle that goes high every 10 seconds, the second as a one shot that stays on for 5 seconds.

 

A PIC10 micro would certainly be simpler, assuming you have a way to program it.

[Arthur]

For a flashing decoy, a flashing LED to emulate the car alarm "armed" flash will suffice. Other flashing LEDs exist. Retail in the UK these are from £1 - 4! It's not worth warming the soldering iron for that.

http://www.maplin.co.uk/productsearch?criteria=flashing+Led will give some clues though specific to the UK.

[layedbackkustomz]

alright think i got it figured out. R1=1K ohm resistor R2= 10K ohm resistor And C= 470 uf capacitor. also using 1n4001 and 1n4148 diode with 5v spst relay. should get me at . Frequency: 0.146 Hz Cycle Time: 6.840 s Duty Cycle: 52.38 % Time High: 3.583 s Time Low: 3.257 s

 

And thats good enough for govn. work. plus closer to the timings in the vid.

 

 

Arthur im trying to do this.

watch how it turns on and then off. Edited April 27, 2013 by layedbackkustomz

[senem]

555 Astable circuit.

 

 

http://i.imgur.com/2x7l0Em.png

 

Positive Time Interval (T1) = 0.693 * (R1+R2) * C

Negative Time Interval (T2) = 0.693 * R2 * C

Frequency = 1.44 / ( (R1+R2+R2) * C)

 

As T1 and T2 is 5 Sec in your application, you can get the remaining values i.e.R1, R2, and C by using above formula.

[GalFisk]

If you want a 50% duty cycle, pass a diode across R2 in the schematic, with anode to pin 7. Pin 7 is the capacitor discharge pin, and with the diode the charging current will go through only R1, not both R1 and R2. Then remove R2 from the positive time interval formula.

Edit: I see you've found a solution good enought for you, so i leave this tip to others.

Edited August 15, 2013 by GalFisk

[layedbackkustomz]

GalFisk,

I'm glad you wrote and i did find something that works for me but i think ill try this out on a breadboard because it might work better then what i got. i assume the diode is a 1n4004. also i just got a arduino uno kit today so im trying to learn it and i got arduino mega coming in the mail tomorrow. both are clones but the uno seems to work just fine. i got the mega to make a 48 channel christmas light controller. ill make a new post about this adventure later.

 

-LBK-