555 Unusual Timing Required

[micro571]

Hi,

I need a 2s pulse, followed by 4 seconds of dowtime ... repeated.

The way the 555 timer works as I have used it is a long positive pulse, followed by a shorter low period. Is there any way to make the low period shorter than the high period? If not, is there any way to create these conditions using other ICs as well?

Any suggestions would be greatly appreciated.

 

[Optikon]

Hi,

I need a 2s pulse, followed by 4 seconds of dowtime ... repeated.

The way the 555 timer works as I have used it is a long positive pulse, followed by a shorter low period. Is there any way to make the low period shorter than the high period? If not, is there any way to create these conditions using other ICs as well?

Any suggestions would be greatly appreciated.

There are other IC's that perform similar monostable behavior.. heard them called one-shots.. 74xx123 where xx is some TTL family like LS.
Cmos versions are available I'm sure..

 

[audioguru]

It sounds like you need an oscillator (quote: repeated) with a 2sec to 4sec duty-cycle. For such a low frequency, CMOS is best since the timing cap can be small so it doesn't leak, and the resistor can have a high value. A couple of CMOS inverters or gates can be connected as a "classic two-inverter CMOS oscillator", with a series resistor and diode connected across the timing resistor to adjust the duty-cycle. The polarity of the diode connections determines whether the short pulse goes high or low.

 

[red4925]

This can be done with a 555 timer. Look in the datasheet for your 555. There should be equations under monostable operation that dictate the duty-cycle and period based on two resistors and a cap. If your datasheet sucks, then go to national.com and search for the 555 datasheet there - i know those have the equations or you could just google it.

But you may want to listen to audioguru about using CMOS. I don't know how well 555 operates at low freq.

 

[Roff]

See below.

 

[micro571]

I am familiar with the 555 timer.

Isn't the first pulse high, followed by a shorter low pulse? Is that circuit different than that?

 

[Roff]

I am familiar with the 555 timer.

Isn't the first pulse high, followed by a shorter low pulse? Is that circuit different than that?

Not this one. This will have 2 seconds high and 4 seconds low (approximately, depending on component tolerances). The capacitor discharges through R1 only, but charges through R1 in Parallel with R2. When you turn it on, the very first positive pulse will be about 60% longer than all successive ones, due to the fact that the capacitor starts out with zero volts across it, but subsequently never gets below VCC/3.