khayo@sonia.cs.ucla.edu (Erazm J. Behr) (11/23/87)
Is there any way to wire the 555 to get an "on" time of
less than 50% of the cycle? (I know, "just add an inverter" -
but I'd like to know if it can be done without adding extra
active parts). Sorry for this, I'm sure the answer is in the
library somewhere ...... 8-O
-----------------------------------------------------------
>>>>---------------> khayo@MATH.ucla.edu wolfgang@mgm.mit.edu (Wolfgang Rupprecht) (11/24/87)
In article <9376@shemp.UCLA.EDU> khayo@MATH.UCLA.EDU (Erazm J. Behr) writes: >Is there any way to wire the 555 to get an "on" time of >less than 50% of the cycle? (I know, "just add an inverter" - >but I'd like to know if it can be done without adding extra >active parts). Sorry for this, I'm sure the answer is in the >library somewhere ...... 8-O 1) Don't use the open collector pull down to discharge the capacitor. 2) Run the charging resistor to Q. (you now hve a 50% duty cycle osc.) 3) To make the charge faster than the discharge add a diode with series-resistor in parrallel to the charging resistor. -- Wolfgang Rupprecht UUCP: mit-eddie!mgm.mit.edu!wolfgang (or) mirror!mit-mgm!wolfgang ARPA: wolfgang@mgm.mit.edu (IP addr 18.82.0.114)
rep@genrad.UUCP (Pete Peterson) (11/25/87)
In article <9376@shemp.UCLA.EDU> khayo@MATH.UCLA.EDU (Erazm J. Behr) writes: >Is there any way to wire the 555 to get an "on" time of >less than 50% of the cycle? (I know, "just add an inverter" - >but I'd like to know if it can be done without adding extra >active parts). Sorry for this, I'm sure the answer is in the >library somewhere ...... 8-O >----------------------------------------------------------- Information from Signetics applications note: Normal astable operation has a resistor Ra from Vcc to pin 7; a resistor Rb from pin7 to pin6/pin2; a capacitor from pin6/pin2 to ground. To get shorter duty cycles you connect a diode from pin7 (anode) to pin 6 (cathode) and it is recommended that you put another diode in series with Rb (anode of diode to pin6, cathode to Rb.). The signetics apps note claims you can get duty cycles from 5% to 95% with this configuration and recommends a minimum value of 3k for Rb. This configuration gives charge path: Ra->diode->capacitor and discharge path: capacitor->diode->Rb->discharge transistor (in 555), so you can determine high and low times more or less independently. pete peterson {decvax,linus,wjh12,mit-eddie,masscomp}!genrad!rep
root@CS.UCLA.EDU (Charlie Root) (11/25/87)
-----------------------------------------------------------
>>>>---------------> khayo@MATH.ucla.edu king@dciem.UUCP (Stephen King) (11/27/87)
In article <9376@shemp.UCLA.EDU> khayo@MATH.UCLA.EDU (Erazm J. Behr) writes: >Is there any way to wire the 555 to get an "on" time of >less than 50% of the cycle? <...> Yup. I did this at school some time ago, let's see if I remember...Oh, yes! I used a pair of diodes to route discharge current for the timing capacitor through a different resistor than the one used for charging. Good enough? ...sjk -- * Defence & Civil Institute * ...!utzoo!dciem!king * of Environmental Medicine * Stephen J King - Simulation & Training Group - (416) 635-2149