Is there a way to configure a 555 timer IC to output 50% duty cycle 1 second high and 1 second low?

Question

I'm using an NE555 timer (or actually several of them) and I was looking at the equations for bistable, 50% duty cycle operation (which is what I need) and figured that I needed to set C1 (ceramic cap)=10^-7F, R1 (1/4W resistor) to as low as possible (to satisfy the equation, it needs to be zero ohms) and R2 (1/4W resistor)= [((ln2)^-1)(10^7)]. However, this configuration isn't working! R2's tolerance is near 1% (its actually an array of +-1% metal film resistors), so I know the device isn't just being killed by inaccuracy. I really appreciate any help, thanks!

Extra info:

The configuration is as follows:

Pin8: Connected directly to Vcc +5VDC

Pin7: Connected to Vcc through R1, which is (theoretically) zero ohms.

Pin6: Connected to Pin 7 via R2, which equals 14-million-something (don't remember right now, dont have calculator on me) ohms. The exact value is, as listed above, [((ln2)^-1)(10^7)]

Pin5: None.

Pin4: Connected directly to Vcc

Pin3: Output, not connected to any other pins

Pin2: Connected directly to pin 6, connected to ground through one 10^-7F ceramic capacitor (C).

Pin1: Directly Connected to ground.

The equation with which I'm deriving the t(high) and t(low) values is:

t(high)=(ln2)(R1+R2)(C)

t(low)=(ln2)(R2)(C)

Thanks again, any help is appreciated!

Answer 1

Putting R1 to zero isn't going to work. R1 is the on time, R2 is the off time. In the standard circuit, they are tied together. BTW, I would always go for the low power CMOS versions with extended range rather than the older versions. One of these might work with the excessive R you have.

There are several circuits used to do this with a single 555. Best is to add a diode across R2 to isolate R1 and R2. I would start with 1M or 500k pots, then size the ceramic (not electrolytic because they leak) cap at about 2 to 4.7 uF. For 4.7uF, R=300k. Use an oscope to tweak the on pulse and off pulse times exactly. See circuit and sizing calc at bottom of:

http://wolfstone.halloweenhost.com/TechB...

You can google "555 50% duty cycle" and similar to get lots of hits.

Answer 2

You may have overlooked the maximum values for the resistances, which depends upon the voltage Vcc.  I think you'll see the maximum Ra+Rb resistance must be less than 3.4 Megohms with Vcc of 5 volts.  

I'm not super comfortable with the R1=0 approach.  If I want 50% duty cycle, I'd consider running the timer at twice the desired frequency, and then using a divider (divide by 2) to generate the quite precise 50% duty cycle.  This could be a CMOS or TTL "flip flop" divider.  

Answer 3

The flip flop idea is probably the best way to do it.  

When you say it didn't work, which bit?

You seem to have the right idea.  Did you remember to put a logic '0' on the 'direct set' and 'direct clear' pins of the flip flop? If you don't, it's not going to work.

Answer 4

i cant get my head around everything you wrote. dont worry i am glad you gave details. i will look over it if i have time. your problem seems to be duty cycle. after you calculate the total time you need, you can adjust the duty cycle by adjusting the values. one second can be made by combing a million values; only one will give you a 50/50 duty cycle. here is the formula for DC:

DC=T2/T1=Rb/(Ra+2Rb)

T1=0.693*(Ra+2Rb)*C

T2=0.693*Rb*C

Total time: Tt=T1+T2

that is why you cant get a 50% duty cycle. R1= approx 0 and R2 = what did you say, 14 million?. balance the current flow between the thresholds and the 555

EDIT. oh, you already had the formula. lol, i am making myself loof like a fool. use a larger size electrolitic cap, it doesn not have time to charge properly and it forces r1 aprox=0. try this. it is the design i always used. DC=50%, at one Hz.

between pins 7&8: Ra/R1=100 ohms

between pins 6&7: Rb/R2=72k

pin six to GND: C= 10 micro

pin 5 MUST be connected it is the control voltage: connect to GND through a 0.01 mic cap

pin2: connected directly to pin 6 (no cap)

pin 3 must be tied to groun through a resistor while testing.

5 min in MS Paint:

http://i61.photobucket.com/albums/h68/ra...

Answer 5

Use R1=1K, R2=10M and C=0.15 Microfarad.OR R1=1K, R2=9.2M and C=0.15 Microfarad. This will give almost same timing for low and high of 1 second. Error is less than 4%. For more accurate timing use R1=680 ohm, R2=8.2M and a ten turns wire wound potentiometer of 2M in series with R2 and adjust the time by varing the potentiometer.  But if you want very very accurate timing then you have to use CD4017 decade counter.

Abdullah Al Baki