555 missing pulse cap

TC

Hello all,

I currently have a working missing pulse detector using a NE555 timer. I am currently using a 10uF aluminum cap. I am working on creating a board for everything. I am doing the board with as much SMD parts as I can. I would like to use a 10uF ceramic cap, but I don't know if it will perform the same.

So my question is.... Is there any difference between ceramic and aluminum caps for my application?

Thanks
TC

Mark_T

Is the capacitor part of the RC time-constant network or decoupling?

For decoupling its not critical.

High value MLCC capacitors have unstable capacitance (sensitive to voltage, temperature).
They are used for decoupling mainly. Some ceramic dielectrics are much more stable, but much
lower dielectric constant and not used in 10uF cap.

Al electrolytics are more stable but drift with time and aren't very accurate to start with. They have
higher series resistance and inductance.

For accurate stable capacitance plastic film or mica rule, but 10uF is too big.

If you use a 7555 you can increase the R value and reduce the C value since the leakage
current on the 7555 inputs is essentially zero.

TC

Mark_T wrote: »

Is the capacitor part of the RC time-constant network or decoupling?

For decoupling its not critical.

High value MLCC capacitors have unstable capacitance (sensitive to voltage, temperature).
They are used for decoupling mainly. Some ceramic dielectrics are much more stable, but much
lower dielectric constant and not used in 10uF cap.

Al electrolytics are more stable but drift with time and aren't very accurate to start with. They have
higher series resistance and inductance.

For accurate stable capacitance plastic film or mica rule, but 10uF is too big.

If you use a 7555 you can increase the R value and reduce the C value since the leakage
current on the 7555 inputs is essentially zero.

It is for the RC circuit. Accuracy is not important. I have it set up for 100ms. As long it gets a pulse around 100ms or less, I'm happy.

Mark_T

Use a 7555, 1M5 and 0.1uF plastic film? The 7555 has input leakage of way below 1nA, about 4 orders
of magnitude less than the 555. It also needs much less decoupling (555's are very harsh on the
supply rail and 100uF decoupling is recommended).

TC

Mark_T wrote: »

Use a 7555, 1M5 and 0.1uF plastic film? The 7555 has input leakage of way below 1nA, about 4 orders
of magnitude less than the 555. It also needs much less decoupling (555's are very harsh on the
supply rail and 100uF decoupling is recommended).

Well that works for me. It will make my life a little better. I can use a 0803 cap, instead of 1210.

Thanks.

kwinn

Or you could try a tantalum capacitor.

TC

kwinn wrote: »

Or you could try a tantalum capacitor.

Not a bad idea, but looking on DigiKey, the cheapest tantalum that is smaller than 1206, is $1.46. I know it is trivial, but I do not need, nor want expensive parts just for a very basic missing pulse detector. I want to be able to get more expensive/better parts for things that matter more (power supply, decoupling, audio amplifier, etc.) That is why I wanted to know if ceramic cap would work.

Peter Jakacki

This is the whole reason it just makes sense to use a cheap micro such as this and even at one off it's only 77 cents in an 8-pin DIP. Writing the missing pulse code for this is trivial and it will be much more precise just using it's internal oscillator than the 555 ever could be even with tantalums etc.
As for burning the chip itself you can always do it on the cheap and just connect 3 I/O from a Prop as you don't need any high voltages, and a simple bit of code to load a hex file into the chip. You might think it's too much trouble but once you setup for these simple micros you will thank yourself many times over.

Heater.

Yep, I'd use a 6 or 8 pin AVR but the point is the same.

A 555 might make sense if you have to run off 12 volts, say, and don't want the expense of another regulator and such.

TC

Peter Jakacki wrote: »

This is the whole reason it just makes sense to use a cheap micro such as this and even at one off it's only 77 cents in an 8-pin DIP. Writing the missing pulse code for this is trivial and it will be much more precise just using it's internal oscillator than the 555 ever could be even with tantalums etc.
As for burning the chip itself you can always do it on the cheap and just connect 3 I/O from a Prop as you don't need any high voltages, and a simple bit of code to load a hex file into the chip. You might think it's too much trouble but once you setup for these simple micros you will thank yourself many times over.

Heater. wrote: »

Yep, I'd use a 6 or 8 pin AVR but the point is the same.

A 555 might make sense if you have to run off 12 volts, say, and don't want the expense of another regulator and such.

I know nothing about programing one of those. Plus, the missing pulse is just a fail safe incase the prop freezes. I talked about it in this thread. I am starting to work on the board, and I am trying to make the board small (cheaper price).

Peter Jakacki

TC wrote: »

I know nothing about programing one of those. Plus, the missing pulse is just a fail safe incase the prop freezes. I talked about it in this thread. I am starting to work on the board, and I am trying to make the board small (cheaper price).

Well if you do get into working with this type of microcontroller you will never regret it and the thing is that at this level there is nothing overly complicated.

As for the watchdog circuit I'm surprised that you think the Prop might freeze but I would worry about buggy software "freezing" though. For these situations I use a small microcontroller as a watchdog, in fact that "small microcontroller" is one of the cogs. Works for me and in my Tachyon Forth system one cog is normally running all the background linked timers etc as well as a watchdog. Once an application starts the watchdog it will start running and if it doesn't get retriggered in time it will normally reboot the system or some other predefined action.