Please check my 555 timer circuit

[ENovice]

Hello, i have made a couple of 555 timer circuits before that worked well, but this one is a little different because it must only be able to trigger no more than once every few hours. This circuit seems to work correctly according to the online simulator i tried, but i don’t know what value the capacitor on trigger pin 2 should be. I used 10uf, maybe that is way too big?

Thanks for your help

 

[crutschow]

it must only be able to trigger no more than once every few hours.

What does that mean?
Do you want to block any triggers the occur more often than every few hours?
I don't see any way that your posted circuit can do that.

555 circuits become problematic if you want time intervals more than a few minutes.

 

[ENovice]

The 13v supply is also supposed to be the trigger, and the output only stays on for just over 4 seconds. The 342k resistor is actually a 100k + 242k vr in series for adjustability down to about 2 seconds.
According to the simulator this works ok.

I put the 10uf capacitor on the trigger pinonly to keep the trigger high for roughly a few hours, so the output cannot be triggered more often than that. Maybe even 1uf would be plenty, I haven’t experimented on my breadboard yet, and I don’t know how to calculate how long that capacitor would keep pin 2 high for?

 

[Nigel Goodwin]

I put the 10uf capacitor on the trigger pinonly to keep the trigger high for roughly a few hours, so the output cannot be triggered more often than that. Maybe even 1uf would be plenty, I haven’t experimented on my breadboard yet, and I don’t know how to calculate how long that capacitor would keep pin 2 high for?

You don't how to calculate it, because it won't work - you also don't do things with 555's in 'hours' anyway, minutes is pushing it and requires HUGE capacitors.

Dump the 555, get an 8 pin PIC (so same size as a 555), throw most of the support components away, apply a little programming - and you can make it as long as you like, and MASSIVELY out perform a 555.

 

[ENovice]

So does that mean the time that capacitor will keep pin 2 high for can wildly fluctuate?

I was looking at your website and some of the links, lots of reading to do! I guess it’s about time i learnt how to use a PIC.

Are PIC chips reliable? As in multiple power cycles won’t erase or corrupt the programming?

 

[Nigel Goodwin]

So does that mean the time that capacitor will keep pin 2 high for can wildly fluctuate?

I don't see as it's going to keep it high at all?, and even if it did, it would be for a very short period.

However, the pin is held high by the 10K resistor, the capacitor will hold it low very briefly on power up, and then go high as the capacitor charges. It will then stay permanently high until power is removed.

I was looking at your website and some of the links, lots of reading to do! I guess it’s about time i learnt how to use a PIC.

Are PIC chips reliable? As in multiple power cycles won’t erase or corrupt the programming?

PIC's are EXTREMELY reliable, they are very widely used, and you've probably got a number of items in your house that actually use them

 

[ChrisP58]

With 10uF value shown, the lockout period will be about 60mS past when the 13 Volts drops out. If that is the time period that you need to extend, the you'll need to increase that value by a very large margin. The problem you'll run into though, is that electrolytic capacitors suffer from internal leakage currents that cause them to self discharge over time. That's always been a fundamental limitation in using 555 circuits in long period timers.

As mentioned, PICs (and many other microntrollers) are very reliable. They're used in many common objects. Microwave ovens, coffee makers, TV sets/DVD players. Dish washers, clothes washers and dryers. My bathroom scale that has a Bluetooth connection to my phone. Many childrens toys, etc. I put them in battery chargers and UPS systems,

They typically use flash memory to store their programming, similar to a USB flash drive. But, they usually require being physically connected to special programming hardware to erase or change the program, so it's not going to get corrupted by a few, or a few million, power cycles.

 

[audioguru]

Read the datasheet of a 555 timer to see that your circuit does not work the way your simulation wrongly shows. The 342k resistor and 100nF capacitor produce a timeout of only about 0.038 of a second, not 4 seconds. But since the 10k and 10uF capacitor on pin 2 over-rides the other RC parts, the timeout is actually about 0.11 of a second.
Your 100nF capacitor must be about 100uF (1000 times more) for a 4 seconds timeout. Your idea to prevent re-triggering will not work.

 

[ENovice]

I’m still reading and learning about PICs, but since it’s going to be some time until I play with PICs, I made a couple of changes to the circuit i posted and gave it a go.

The 100nf cap should be 10uf, not sure why i used 100nf. Different websites reccomended 100nf or more between pin 8 - pin 1 and another 100nf from pin 5 - ground so I added those. With 342k total resistance and 10uf, (and seemingly due to the larger 470uf i tested) the output time was approximately 4.5 seconds. The output time increased slightly but that’s fine. It’s consistent, and I’m happy the circuit is disabled for hours due to the charge in a small capacitor, while no external power is applied. It’s a much larger cap than I mistakenly thought I would need, but it’s smaller than a piece of popcorn, the whole circuit can easily fit inside a matchbox.

I added a diode above the 10k resistor, and the 470uf instead of 10uf went below. After two hours the capacitor has discharged nearly two volts, at this rate it would take approximately 6 hours until the circuit will allow the output to be triggered high again. I will experiment tomorrow with a smaller capacitor, i’m aiming for about 3-4 hours.

 

[gophert]

You don't need a PIC to use a microcontroller. A $5 Arduino microcontroller is already mounted on a PCB and plugs into a USB connected to your laptop. No extra hardware and you can have everything up and running in under one hour with all the clear and we'll produced video tutorials on YouTube.

Good luck

 

[Mickster]

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[audioguru]

Did you read the datasheet for the 555 and do the math?

342k x 470uF x 1.1= 177 seconds. But 342k x 10uF x 1.1= 3.8 seconds. 342k is not a standard value but 360k is standard and with a 10uF capacitor the timing will be 4 seconds. But an electrolytic capacitor needed for 10uF or more is not accurate.

The datasheet for an ordinary LM555 or NE555 shows that the timing resistor can be as high as 10M ohms. Then for 4 seconds, the resistor can be 3.6M and the timing capacitor value should be 1uF. Use a film type of capacitor for accuracy and for it to hold a charge for a long time.

 

[Pommie]

If you tell us what you're trying to do then we'll be able to give you a suitable solution.

Mike.

 

[AnalogKid]

Your requirements still are not clear.

During the "few hours" when the circuit cannot be retriggered, is the circuit powered up continuously or not?

During the few hours, can the circuit be powered up and down multiple times?

ak

 

[dr pepper]

A delay in the order of hours might be a bit too long for a 555, capacitor leakage might well mess up the time period, or stop thing thing from working altogether.
If you can still get them in your area a 4541 might be a better option, for a delay of hours this would be better as the oscillator can run faster and then the delay then multiplied by the built in counter.
Google the chip number you might be able to find a circuit someones already done & published.

 

[ENovice]

I’ll probably get some time to have a play with arduino during the next school holidays, thanks for the info guys.

My circuit is working fine btw, with the small changes i mentioned above, except for a 100uf instead of the 470uf. The exact value of the capacitor between pin 2 and ground depends on the type/quality, I had different disabled times with the same voltage and uf across different brands of capacitor, some had so much leakage they only lasted minutes!

The circuit is for a car and meant to be disabled for a few hours without power connected, so it will ignore any multiple retriggers, and only be active if the car is left alone all day or all night.
The circuit seems consistant and fairly accurate, but of course if i was to carefully check with a stop watch there would most likely be some variation in the disabled time, but that doesn’t matter. The output time is very accurate since it’s short and the 100k + 242k vr combo is fine.

 

[gophert]

I’ll probably get some time to have a play with arduino during the next school holidays, thanks for the info guys.

My circuit is working fine btw, with the small changes i mentioned above, except for a 100uf instead of the 470uf. The exact value of the capacitor between pin 2 and ground depends on the type/quality, I had different disabled times with the same voltage and uf across different brands of capacitor, some had so much leakage they only lasted minutes!

The circuit is for a car and meant to be disabled for a few hours without power connected, so it will ignore any multiple retriggers, and only be active if the car is left alone all day or all night.
The circuit seems consistant and fairly accurate, but of course if i was to carefully check with a stop watch there would most likely be some variation in the disabled time, but that doesn’t matter. The output time is very accurate since it’s short and the 100k + 242k vr combo is fine.

What part of a car is this for? Some type of security system?

 

[Mickster]

Bear in mind that modifications to automobiles can potentially invalidate warranties and insurance coverage.
As a general guideline, devices which connect via standard means such as a cigarette lighter socket/power outlet tend to be okay, but when taps are made to the existing wire harness, that's where potential problems could arise in the future.
Insurance companies do not like to pay out, and will wriggle at each and every opportunity when a claim is made.
Manufacturers can sometimes follow suit and deny warranty claims, if the vehicle is modified in any way.
Check your warranty brochure and insurance documentation, to make sure any alteration(s) that you make will not invalidate your insurance, or your warranty.

Just a heads-up.

 

[gophert]

Bear in mind that modifications to automobiles can potentially invalidate warranties and insurance coverage.
As a general guideline, devices which connect via standard means such as a cigarette lighter socket/power outlet tend to be okay, but when taps are made to the existing wire harness, that's where potential problems could arise in the future.
Insurance companies do not like to pay out, and will wriggle at each and every opportunity when a claim is made.
Manufacturers can sometimes follow suit and deny warranty claims, if the vehicle is modified in any way.
Check your warranty brochure and insurance documentation, to make sure any alteration(s) that you make will not invalidate your insurance, or your warranty.

Just a heads-up.

Or, if you are one of those €#&%ing guys that drives without insurance, how much worse can things get?

 

[Mickster]

If you are one of those guys who drive without insurance, you deserve to meet Bubba, who is 'in' with the warders....