Where to get decent 120uF cap for 555

why not hook the 555 to a 4017 then the cap needed would be at least 1/10 ie 12uF?? Run it in astable and use the last decode output to hold the reset low/high.

Hi again all,

I can't believe how confusing I'm finding this simple circuit

Okay, yes, I did read the data sheet and understand the basics of 1/3 vs triggering etc but that's about it, I didn't understand any more there that would help me.

The application, the simple way I can think to explain is is a bit like courtesy light auto on when you turn the engine off. The circuit is powered by 12v live and normally it has a 12v feed to the trigger side. At this point the output is low. When the ignition is off the 12v feed to the trigger is cut, this needs to cause the output to go high which activates a relay and switches the interior lights on for a time between 1 and 60 seconds roughly, set by the trimmer.

I thought on the attached circuit that removing the 12v trigger feed would cause the 1k resistor to feed 0v temporarily via the cap to the trigger side till the 10k resistor has chance to pull it back up to 12v once the cap is charged? Am I wrong? If so, what do the 1k attached to 0v and 10k attached to 12v across the 1nf cap do?

Thanks again in advance,
Greg.

Yes, the 1k pull down resistor will pull it down to 0V, when the 12V is removed from the capacitor.

I don't see why you're using a 9V battery, just feed the 7555 of a permanent 12V feed and disconnect the trigger.

When triggering this circuit from voltages higher than the power supply be careful. This circuit already feeds double the power supply voltage into pin 2 for a short period of time but most ICs have protection diodes (especially the CMOS versions) so it isn't an issue.

If you have any problems, then add a diode across the 10k pull-up to clamp it at 0.6V above the supply to the 555.

Thank's Super, that's a bit beyond me at the mo though! May come back and investigate it if I hit a brick wall this way though

Super_voip said:

why not hook the 555 to a 4017 then the cap needed would be at least 1/10 ie 12uF?? Run it in astable and use the last decode output to hold the reset low/high.

Click to expand...

Cheers Hero.

Do you think it's better/I need the diode in?

I'm NOT using a 9v battery, I don't know where that came from!

Please see attached, I have annotated which hopefully makes my supply and also componant choice clearer. I have dropped the cap and upped the R and hopefully everyone will agree this should be okay?

If this will do the job, the only thing I'm stuck on now is I can't find Tant caps for C1 & C3, the 1nf & 10nf. Should I use a different/inferior type of cap or can I use different values so I can use Tant?

I have put the type of componants on the diagram as well for comment if I picked anything stupid.

Nearly there now...
Greg.

Hero999 said:

Yes, the 1k pull down resistor will pull it down to 0V, when the 12V is removed from the capacitor.

I don't see why you're using a 9V battery, just feed the 7555 of a permanent 12V feed and disconnect the trigger.

When triggering this circuit from voltages higher than the power supply be careful. This circuit already feeds double the power supply voltage into pin 2 for a short period of time but most ICs have protection diodes (especially the CMOS versions) so it isn't an issue.

If you have any problems, then add a diode across the 10k pull-up to clamp it at 0.6V above the supply to the 555.

Click to expand...

That looks fine.

I've built similar circuits using the TS555 and NE555 that uses an AC coupling capacitor with no diode and haven't had any problems.

I don't know about the 755 though, I assumed has protection diodes, it might not. The datasheet says the maximum voltage you can take the trigger above the supply is 0.3V so strictly speaking you should use a small Schottky diode like the BAT85 which will drop <240mV when the current is <0.1mA (which it will be 9V/10k = 900:mu:A).

By the way, I used Crocadile Clips which is a kid's simulation package (I got the student version many years ago when I was at at school). It isn't much use except for simulating and drawing very simple schematics, I wouldn't recommend it as a main package. I just use it for quick and simple things and I know it's limitations so it's alright.

Thanks for the info.

I can pick another type of 555 if you think that would be better? I just picked this as it was there at RS and said it handled up to 18v and 200ma out, which seemed to be what I needed.

As the trigger voltage is connected to Vs, I don't understand how it can ever get higher than Vs? Again I'm not using a 9v battery, the trigger is a line from the 12v Vs supply through a switch so the trigger should have the same (or less, if switch contacts have a touch of resistance) voltage as the supply right? Unless I'm missing somethign ;-)

If I'm best advised to add in that diode maybe you would be kind enough to indicate it on the diagram, or if what I said above makes sense and I don't have to worry about it even better!

Greg.

Hero999 said:

That looks fine.

I've built similar circuits using the TS555 and NE555 that uses an AC coupling capacitor with no diode and haven't had any problems.

I don't know about the 755 though, I assumed has protection diodes, it might not. The datasheet says the maximum voltage you can take the trigger above the supply is 0.3V so strictly speaking you should use a small Schottky diode like the BAT85 which will drop <240mV when the current is <0.1mA (which it will be 9V/10k = 900:mu:A).

By the way, I used Crocadile Clips which is a kid's simulation package (I got the student version many years ago when I was at at school). It isn't much use except for simulating and drawing very simple schematics, I wouldn't recommend it as a main package. I just use it for quick and simple things and I know it's limitations so it's alright.

Click to expand...

hi Gregg,
Your original request was a little misleading, the way it read, is that the car battery connection was cut, where in fact IIUC, its just the ignition thats been switched OFF.

If you just wanted to detect and start a timer when the ignition was switched off, then we could have advised you better.

I also understood that you wanted a precise timing of around 40ish seconds, you are now saying a 'rough' setting of 1 to 60 seconds, which is it.?

I would be pleased to draw a circuit for you, but please tell us what exactly what you want to it do,..

Hey Eric,

I'm sorry for being confusing! I'm very clear in what I want (in my mind!), perhaps didn't explain it well though. I never wanted a fixed time or 40 seconds, I want 1-60 sec adjustable, however I think I was meaning that even a max adjustment of 45 sec would be sufficient. My criteria is give or take a few seconds as the nominal time it will be trimmed to is 20 - 40 seconds, so I was just adding a bit of margin on either end if that makes sense, so fixed the criteria to a rough 1-60 sec ;-)

Not sure if you read this but hope it help:
The application is like courtesy light auto on when you turn the engine off. The circuit is powered by 12v live and normally it has a 12v ign feed to the trigger side. At this point the output is low. When the ignition is off the 12v feed to the trigger is cut, this causes the output to go high which activates a relay and switches the interior lights on for a time between 1 and 60 seconds roughly, adjustably set by the trimmer.

The attached should show the feed/trigger lines as above and as I understand it produce an output in the range I require.

Maybe if I had explained myself better in the beginning there would have been an easier solution? Other than not being able to find tant caps for the small caps I think I'm okay with this though providing it does what I think it does. Breadboard has been ordered!

I hope I explained a bit better, if not tell me and I'll try again!

Greg.

ericgibbs said:

hi Gregg,
Your original request was a little misleading, the way it read, is that the car battery connection was cut, where in fact IIUC, its just the ignition thats been switched OFF.

If you just wanted to detect and start a timer when the ignition was switched off, then we could have advised you better.

I also understood that you wanted a precise timing of around 40ish seconds, you are now saying a 'rough' setting of 1 to 60 seconds, which is it.?

I would be pleased to draw a circuit for you, but please tell us what exactly what you want to it do,..

Click to expand...

The components you've listed are fine.

If you're going to use the 7555 (which is probably a good idea due to the long time delay), then I'd strongly recommend using the BAT85 diode which can be bought from RS and isn't expensive.
https://uk.rs-online.com/web/search...thod=searchProducts&searchTerm=BAT85&x=29&y=4

Another thing, is I'd also recommend getting some spare components - don't just buy one 7555, get two, just in case you make a mistake and destroy one.

Thanks Hero, I have put 5 of the Bat in the shopping basket. I have already made sure that I have 2 of everything just in case I short something out ;-)

2 q's, where do I connect the BAT diode? Also the 1nf and 10nf, I couldn't find them in Tantalum can I use different values? If not what sort of cap should I get for those two?

Thanks,
Greg.

Hero999 said:

The components you've listed are fine.

If you're going to use the 7555 (which is probably a good idea due to the long time delay), then I'd strongly recommend using the BAT85 diode which can be bought from RS and isn't expensive.
https://uk.rs-online.com/web/search...thod=searchProducts&searchTerm=BAT85&x=29&y=4

Another thing, is I'd also recommend getting some spare components - don't just buy one 7555, get two, just in case you make a mistake and destroy one.

Click to expand...

Just use ceramic capacitors for the 1nF, 10nF and 100n; I know it isn't on the schematic but you need a 100nF capacitor across the power supply.

I told you where to put the protection diode several posts ago, here's the schematic.

Sorry I missed that in earlier post, was tired. Okay I think I'm at purchase and breadboard stage now

Hero999 said:

Just use ceramic capacitors for the 1nF, 10nF and 100n; I know it isn't on the schematic but you need a 100nF capacitor across the power supply.

I told you where to put the protection diode several posts ago, here's the schematic.

Click to expand...

How long do you expect the battery to last?