Hi.

This may be off topic, for that I'm sorry.

I read quite a bit about the 555 timer. When it comes to monostable mode, what will be best. High value resistor and low value capacitor, or low value resistor and high value capacitor?

This site says high resistor....
http://home.cogeco.ca/~rpaisley4/LM555.html

NOTE: The leakage currents of electrolytic capacitors will affect the actual output results of the timers. To compensate for leakage it is often better to use a higher value capacitor and lower value resistances in the timer circuits.

This site says low resistor...
http://freespace.virgin.net/matt.waite/resource/handy/pinouts/555/index.htm

As a rule of thumb, it is best to choose resistor values above 1K and below 1Meg. Large electrolytics should be avoided in favour of large value R's to reduce power consumption and timing inaccuracies due to leakage.

Or is it just me, not understanding...?

 

Depends what your goal is, accuracy or battery life. That is what they were talking about (I think). If you need low current then you want a big resistor, but then the capacitor drift makes more difference. You want accuracy then a smaller resistance will be your thing.

Personally I favor large resistors and good capacitors.

When you are designing something your first goal is to set targets.

So, what are you trying to do?

 

Generally I use 47k to 1M & 100pF to 100uF, also successfully used 1M & 10,000uF together to see how long of a timing cycle could be achived-2.5h, but do not expect to find many high val caps with verry high leakage resistance.

 

Hi. Thanks for reply.

So, what are you trying to do?

First of all I'm trying to understand.

It is for my 320 pcs LED circuit. My circuit does not have to be excatly on the hundreds of a second. Less than 10 minutes.

But now it makes more sense to me. Thanks again.

Jens