2x 555 configuration

[daviddoria]

ok, i have a 555 setup in monostable mode. i want my second 555 to be low until it gets the "turn off" of the first 555, then it should go high. essentially what i want is to hit a switch and have about 10 seconds before the final output goes high.

TillEulenspiegel was trying to help me, but i didn't really understand. i kinda get that pin 3 (output) of the first 555 should go to pin 2 (trigger) of the second 555, but that would make them both be on at almost the same time.

does anyone understand my question?

david

also, does it actually matter if you connect pin5(offset) through a filter cap to ground, or even connect it at all?

 

[John Sorensen]

Have you looked at the block diagram for a 555? That may help make things clearer for you. If you give some more details, I'd take a crack at this, such as, what kind of switch (pushbutton, Normally Open?), what power supply, what load is being driven (a purpose would help: what are you trying to do?), what should the circuit do when power is applied? What parts are available? What constraints?

j.

 

[daviddoria]

hmm that block didn't really help... lol

ok here goes:

the power will be a 9 v batter, which can be connect through a NPN transisor on the output of the second chip before going to the igniting wire

yes, NO momentary switch. i want to hit the switch, and 10 seconds later have a pulse for about .5 seconds. the application would be a rocket engine igniter. i would go up to the rocket, push the button (there should be no pulse flickers at all during any of this as that would be dangerous.) then 10 seconds (+ or - 5 seconds or whatever, i can do the resistor calculations for that) have an output go high for a second or so then turn off

so the way i though to do it was have a 555 in monostable mode, and make its pulse length 10 seconds. at the end of this 10 seconds, i though i could trigger another 555 with pulse length of 2 seconds. i kinda got that to work by connecting the output of the first one to pin 4 (reset) of the second one, the only problem is that before i push the button, output 2 is high... which would have already ignited the rocket.... which is no good. also, once i push the button, output 2 turns off, output 1 goes high for the expected lengh, then output 2 turns on and output 1 turns off, but then output 2 never turns off.

do you understand the problem?

any help would be appreciated

thanks
david

 

[John Sorensen]

Sorry the diagram didn't help... it helped me to quit thinking of the 555 as a black box and instead recognizing that it's just a few comparators and a flip flop, knowing this you can do some neater things with it.

But anyhoo- I think I understand the app, the output must be low, then, ten seconds after a button push, it should go high for about 1 second.

I'll let you know, or there may be others already at work on this

j.

 

[daviddoria]

that actually is a good plan... the not thinking of it as a black box, but i think i will wait 1 more year (as next year is my second year of EE school) to try that.

as for now i just like to build stuff and not really know whats going on

 

[tavib]

The problem appear because the 555 is a level triggered monostable.
For use only 2 monostables cascaded you should use 2 edge triggered monostables, like CD4098 which have 2 edge triggered monostables in it, every monostable can be triggered on rising edge(low to high transition) or on falling edge(high to low transition).

If you want to use 2 555s then additional circuits must be added for have the desired work mode, because before you push the button, first 555 is in inactive mode(not triggered) and his output is low and the second 555 have the input 2(low trigger input) triggered so the second 555 have the output high all the time before hiting the button. After the button hit, the 1st output go low and free the 2nd 555 trigger input so the 2nd 555 output will go low after past of it time(0.5s) and will remain low until the 1st 555 time expire when it output go low and remain low, so 2nd 555 is trigger again and go high and remain high because it is trigger permanently.

If you use the pin 6(high trigger input) of 2nd 555 when hit button the 1st 555 is trigger, his output go high and imediately will trigger the 2nd 555 and is not god.

Use the both monostables from 4098, and connect 1st output to falling edge input of 2nd(TR- pin). When hit button, the 1st mono output go high and after it time his ouptut go low and only this high to low transition will trigger the 2nd mono. The trigger input can remain any time in low or high state, only follow edge(in case of use TR- input, or rising in case of use TR+ input) will trigger the monostable.

Hope this explains help you.

 

[John Sorensen]

Are we allowed to use any parts we want, or are we anchored down to using 555s? I'm really most concerned about the safety aspect.

j.

 

[daviddoria]

i only have 555s, i guess i could get other chips if they are common and good to learn about, what do you guys think?

 

[daviddoria]

**broken link removed**

i found that...

not exactly sure what all the pins are... anyone know where an explanation of this chip is?

 

[John Sorensen]

https://www.electro-tech-online.com/custompdfs/2004/04/CD4098B.pdf

"Let Google be your friend." Let Digikey be your best friend.

j.

 

[tavib]

another datasheet for 4098
https://www.st.com/stonline/books/pdf/docs/2071.pdf

but another problem:
when connect supply voltage the monos can be triggered, so the reset input must be hold active during power up, for few milisec., using a RC circuit.

 

[gerty]

Take a look at the "rocket launcher" post.. It may be what you're looking for..