555 one-shot continuous trigger?

[lade]

Hi.
I'm planning to build a relay timer with a 555 in monostable mode. I'm aiming at a pulse of 3-15 seconds, even if the trigger switch is kept on for 5-10 minutes.

I have not been able to find descriptions of a way to keep the timer from going continuously, such as the circuit in the attached diagram does.
Maybe my google-searching abilities are as limited as my electronics skills?

Does anyone know a thread/site that i can look at?

Another thing is the power consumption of the circuit on stand-by. I'm going to run this off a battery, and i'd like for it to last as long as possible. Can it be made to trigger the timer when power is switched on, pulse for 3-15 seconds, and then power off the circuit when the switch turns off after 5-10 minutes?

Hope that made any sense!

Thanks, Lade

 

[ericgibbs]

hi,
Look at this mod of your circuit.
You can make the mono trigger at power up, but to power down after a few secs will require extra components

 

[lade]

Thank you very much Eric, that was an amazingly fast answer!
At first i didn't get it to work, but i think the components you added on the attachment got mixed up. I put in a 47uF cap, and a 10K R, and it works like a charm!

In terms of getting it to power down, how many more components will it need? Is there a specific term i can search the forums for in order to learn more?

Thanks,
Lade

 

[ericgibbs]

Thank you very much Eric, that was an amazingly fast answer!
At first i didn't get it to work, but i think the components you added on the attachment got mixed up. I put in a 47uF cap, and a 10K R, and it works like a charm!

In terms of getting it to power down, how many more components will it need? Is there a specific term i can search the forums for in order to learn more?

Thanks,
Lade

hi Lade,
I am puzzled why you need a 47microFarad trig capacitor.?
It works fine with a10nF.
EDIT:
Or is it because the switch you have drawn is not in that position but is power ON switch.???

The easy solution for power drop out is to use a relay, which is energised when the power is applied and holds in until the final time period. The contacts on this relay supply the power to the 555 and itself.
OK.?

 

[lade]

OK it works with the 10 nF cap now!! I don't know what went wrong before.....

I'll use your advice with the relay if battery consumption becomes an issue.

Again, thank you very much for helping out!

 

[ericgibbs]

OK it works with the 10 nF cap now!! I don't know what went wrong before.....

I'll use your advice with the relay if battery consumption becomes an issue.

Again, thank you very much for helping out!

OK,
While you are considering future 'power offs' consider using P MOSFET in place of the suggested relay contacts

 

[lade]

OK, i will do that. Seems like a better solution than a second relay. I wasn't sure if a mosfet could be used to power both the 555 and the relay.

The rest of my components should be coming in tomorrow, so i'm looking forward to see the finished project!

Thanks for the assistance! Both in this thread and in others. It's really useful for me to read through answers given to other newbies in this field.

Lade

 

[lade]

Still issues with the monostable 555....

hi Lade,
I am puzzled why you need a 47microFarad trig capacitor.?
It works fine with a10nF.
EDIT:
Or is it because the switch you have drawn is not in that position but is power ON switch.???

The easy solution for power drop out is to use a relay, which is energised when the power is applied and holds in until the final time period. The contacts on this relay supply the power to the 555 and itself.
OK.?

Well..... I have made the circuit attached here , and it works fine. It's made on veroboard, and it's not pretty, but then again, it's my first! I'll have to do another one with better layout etc.
But in the part you suggested (cancelling continous triggering), i cannot get it to work if the capacitor is less than 20 µF. I don't get it....

Is the 47µF electrolytic cap. across the power source necessary, or should it be omitted?

Thanks,
Lasse

 

[ericgibbs]

hi lade,
I have downloaded your circuit, will get back to you.

OK.
Run tests with 20uF and 10nF no difference in re-trigger, single pulse only while switch is closed.

EDIT:
Rechecking your circuit, you have not drawn in the base resistor pin3 of the 555 to the transistor.???
Also you should have a 100nF cap,across the 47uF, its possible that 'electrical noise' from the relay circuit is retriggering the mono.

 

[lade]

hi lade,
I have downloaded your circuit, will get back to you.

OK.
Run tests with 20uF and 10nF no difference in re-trigger, single pulse only while switch is closed.

EDIT:
Rechecking your circuit, you have not drawn in the base resistor pin3 of the 555 to the transistor.???
Also you should have a 100nF cap,across the 47uF, its possible that 'electrical noise' from the relay circuit is retriggering the mono.

Thanks,

I have not made the actual circuit with a 10nF cap, only the 20 uF shown in the schematics. When i run a simulation it still does not work.... The only thing left i can imagine is something in Circuit Wizard going wrong. I'll make the actual circuit as soon as i get time.

thanks, for update on pin3 resistor and the 47 uF cap

Lade

 

[lade]

It works!

OK i tried to build the circuit and it works just fine!! Seems to be a bug in Circuit Wizard
I figured I would save time by simulating before building.... Yeah, right.
But I did get your other points and will use them in the new build.... And I guess I have learned alot more than i would if it had just worked right away. Always look at the bright side, eh?

Thanks,
Lade