Post by thread:[EE]: 555 with a duty cycle

I have finished a IR proximity sensor with a 556 (<=> 2x555), an IR led and an IS1U60. In order to save mA I need to reduce the duty cycle of my 555's. Somebody knows how to make a 555 oscillate with a duty cycle smaller thab 0.5 ? Note : duty cycle = ratio of the time duration when the ouput pulse is high to the total period. Thanxx ! -------------------------------------------------------------- Joan Ilari spam_OUTjoan.ilariTakeThisOuTterra.es Barcelona Voice: +34 93 431 96 39 Spain "I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhauser gate. All those moments will be lost in time, like tears in rain. Time to die..." -Blade Runner- --------------------------------------------------------------- -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 11:34 PM 3/14/01 +0100, you wrote: >I have finished a IR proximity sensor with a 556 (<=> 2x555), >an IR led and an IS1U60. In order to save mA I need to reduce >the duty cycle of my 555's. > >Somebody knows how to make a 555 oscillate with a duty cycle >smaller thab 0.5 ? > >Note : duty cycle = ratio of the time duration when the ouput >pulse is high to the total period. One way is to leave 7 open, connect 2 & 6 together & to cap, as usual, and connect two resistors in series with diodes, back to back between 2&6 and 3. This works better with CMOS 555 and output lightly loaded. 6 & 2 --x----|<|----- R1 -----x--- 3 | | x----|>|------R2 -----x Ton = k * R1 Toff = k * R2 Duty cycle = Ton/(Ton+Toff) = k * (R1/(R1+R2)) So, just make R1 < R2 to get < 50% duty cycle Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Joan Ilari wrote: > > I have finished a IR proximity sensor with a 556 (<=> 2x555), > an IR led and an IS1U60. In order to save mA I need to reduce > the duty cycle of my 555's. > > Somebody knows how to make a 555 oscillate with a duty cycle > smaller thab 0.5 ? > > Note : duty cycle = ratio of the time duration when the ouput > pulse is high to the total period. > > Thanxx ! Just change the timing resistor values. R2 Duty = --------- R1 + 2R2 I can't recall exactly, but I remember I used a 555 timing calculator on the web some time ago. -- Best regards Tony mICro's http://www.picnpoke.com salesKILLspampicnpoke.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Nuts and Volts had an article in the December issue on using the 555. I recently put together a circuit to flash LED's at a low freg to alert drivers when I'm on my bike. Try putting a 1k resistor as R1 and then put a pot as R2 and then you can vary R2 to change the freg as desired. {Original Message removed}

Wingate, F. Paine wrote: > > Nuts and Volts had an article in the December issue on using the 555. I > recently put together a circuit to flash LED's at a low freg to alert > drivers when I'm on my bike. Try putting a 1k resistor as R1 and then put a > pot as R2 and then you can vary R2 to change the freg as desired. Someone already posted the correct "official" solution, using diodes. A normal 555 astable only goes from 50% to 100% duty, as it charges C1 through R1 and R2 and discharges only via R2. It is hard to get a shorter charge time. You will need one diode across R2 to do it, so it charges via R1 and diode, and discharges via R2 (and diode is reversed). I've been playing with 555s since 10 years old. You can also use a single inverter from a logic chip to replace a 555 in 95% of circuits. For a 14pin chip you can get 6 oscillators vs 2 in a 556. -Roman -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

I don't have them on me, but if you refer to the data sheets for the 555 timer(at some place like http://www.onsemi.com) I'm sure you'll find the sort of "standard application" for the 555. It is a circuit that consists of R1,R2 and C, manipulating the values of R1 and R2 changes the duty cycle. I think the formula is duty cycle = (R1+R2)/(R1+2*R2) so increasing the ration R1:R2 will decrease the duty cycle. {Original Message removed}

> > > > Somebody knows how to make a 555 oscillate with a duty cycle > > smaller thab 0.5 ? > > > > Just change the timing resistor values. > > R2 > Duty = --------- > R1 + 2R2 As far as I know, the expression for the duty cycle of a 555 in astable mode is : R1 + R2 Duty = --------- R1 + 2R2 Thus, if R1 >> R2, duty -> 1 and if R2 >> R1, duty -> 0.5 Is this correct ? Joan -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

You can also simply "invert" the sense of your output - connect the load between Vcc and pin3 instead of pin3 and Gnd... Most 555s will sink as much current at they source (but CMOS 555s don't tend to source as much as they can sink.) BillW -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> > I've been playing with 555s since 10 years old. You > can also use a single inverter from a logic chip > to replace a 555 in 95% of circuits. For a 14pin chip > you can get 6 oscillators vs 2 in a 556. > -Roman You are completely right (this list is GREAT !) For instance, in www.users.dircon.co.uk/~doctron/chuckles.htm there is exactly the circuit I want to build (see the gated astable circuit) and I have two spare nand gates for other uses. Joan -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

this is easy. go to national ssemiconductor's site and you can see the datasheet of 555. in the datasheet, typical applications are found there. the duty cycle depends on the external components (can't remember the formula). jane --- Nick Torenvliet <.....torenvlietKILLspam.....SYMPATICO.CA> wrote: {Quote hidden}> I don't have them on me, but if you refer to the > data sheets > for the 555 timer(at some place like http://www.onsemi.com) > I'm sure you'll > find the sort of "standard application" for the 555. > It is > a circuit that consists of R1,R2 and C, manipulating > the values > of R1 and R2 changes the duty cycle. I think the > formula is > duty cycle = (R1+R2)/(R1+2*R2) so increasing the > ration > R1:R2 will decrease the duty cycle. > > > > {Original Message removed}