shortened title due to posting bug.
Hi felow engineers Im building my first usefull circuit and having heaps of fun Its an adaptation from a 3.5 minute egg timer using a 555 ic.

Its purpose is to control a relay for a led 12v downlight on a timer so It will turn off by itself.
the time goes from about 1 minute to about an hour and has a led to shine on the switch and there is a switch on the reset line to turn it off manually tho not shown on the schematic.
I have made it up on a breadboard and it works nicely.
before I start soldering I thought I would post it here and mabye get some feedback/sugestions to improve it, also as its my first real practical circuit im just a tad worried it might burn something.

there is one improvement I would like to add but dont know how and that is I would like to be able to restart the timer during the cycle so I could extend the time it was on without waiting for it to finish and then restart it if u get what I mean.

below is the circuit although all the values are not quite accurate due to me not knowing how to use the design software

I plan to build it with perfboard tho i would like to bake a pcb so i could make a few of them easily but im not realy confident enough of the design.

 

It is called a retriggerable monostable multivibrator.
Use the RESET input.

 

Not sure where you are going to find a 1.2kF capacitor, even 47F is going to be a pretty huge component.

 

Good eye. I didn't even notice that. He probably meant mF and μF.
Only about 10^6 difference.

 

LOL its actually 1200μF
Tanks for the replies this forum is great

"retriggerable monostable multivibrator" <this i dont understand at all is it a different chip I only know how to use the reset pin to stop the cycle.

 

A monostable is just another way to use a 555 chip.

http://www.allaboutcircuits.com/vol_6/chpt_8/4.html

 

You may want to add a resistor to the relay since the current is limited only by the resistance of the coil.
BTW, what is the maximum current out put of the 555 ?

 

I think I have a monostable multivibrator, but I still dont understand how to make it retrigerable, do I need to add some circuitry onto the reset pin ?

 

You will find one solution here:

http://m8051.blogspot.ca/2013/02/retriggerable-555-timing-circuit.html

 

"high output current can source or sink 200ma"
I think its either 100 or 200ma depending on voltage.
acording to my shoddy multimeter the total current when relay is energised is about 40ma and 10 when its not.
I just connected the relay to 12v on its own and it was about 50ma

 

cool thats perfect thanks very much Mr Chips.

 

well I tried the simple solution to tie reset pin to the trigger to simultaniously pull both to ground to restart the cycle but it wont do anything. Is it possible that I have partly destroyed the chip

 

Can you post a schematic as well as a photo of your setup?

 

Hi mrchips

i am new here and i have an urgent circuit i need help can you please message me i dont know how to message you and i ried posting a new tread but its not posting??

You will find one solution here:

http://m8051.blogspot.ca/2013/02/retriggerable-555-timing-circuit.html

 

Sure I had given up on the reset pin so I decided to just have a short duration button that bypasses the pot and a long duration button that doesn't. Its a compromise thats what those blue wires are about in the photo.

I found this article that seems relevent here http://www.electroschematics.com/7195/quirky-555-timer-reset-function/
but its way over my head

Then while playing around I noticed that if I put a big resistor in series with the reset pin going to ground the cycle would never seem to end

I really have no idea but I can guess It might have something to do with the resistor that goes to + from the trigger switch

when I connect the reset pin as in the schematic it doesnt change anything just to be clear.
I dont think my spagetti photo will be mch help tho good for a laugh i supose.
Thanks for your interest mr chips.

 

I dont think you can msg other users on this forum

 

circuit has been rebuilt and simplified and is now working

 

Glad we could help.

 

Heres pretty much the same circuit but with a few additions....you can change the timing components to suit............

 

Attached is the reworked design schematic, everything is exact except for the doides being 4004's instead of 4007's and I dont have the 100μf decoupler cap on the end, I'm still working on the final resistor values for the timing but its working perfectly how I want it to.

I tried adding a resistor to the relay but it wouldn't power up with even a 220ohm resistor so I figure I don't need one.

Its been a lot of trial and error geting it to work how I want playing around with different values for resistors and caps, honestly I have no clue about what the different values mean, just from playing around I found what works.

Anyway the only hickup now is that when I put in the main power decoupler cap, it stuffs up the timing cycle and wont return to stable mode until I pull it out.
Does it matter where it goes, I have tried moving it around but no luck.
I 'm going to do some reaserch on the matter because I don't even know what it does.

Thanks for the enourmous help to everyone.
I'm still a huge noobie but have learn't so much from building this circuit,
one can read alot about electronics but the real learning doesn't start until you get hands on and make some smoke !

P.S that circuit looks a bit more complicated sheldon having a few extra transistors in there.

PEACE