help me about 555

The 7555 is a lower power version.

Apart from that, the 555 hits the nail on the head for most low cost designs (ie consumer electronics) applications I ever come across.

What are you looking for in the newer chip that the 555 does not do?

Bill Naylor Electronworks.co.uk - Electronic kits for fun and education

There's also an ultra low voltage type - The ZSCT555 from Zetex which works down to 0.9V.

I came across a 555 chip with an on board programmable divider ! This increased the range and accuracy !

Part of a Timer I disassembled about 25 years ago don't remember the Chip # ?

Yukio YANO

A divider would increase the range and resolution, but not the accuracy, which is determined by the oscillator (R, C, threshold variance, etc). If the oscillator is 5 % accurate to generate a 50 ms pulse, it is also 5 % accurate to generate a 5 minute pulse.

Best,

His homework.

Cheers! Rich

The XR-2240. Put a ripple counter in the same package as an RC timer.

But, the thing about the 555 was that it was general enough to find lots of use. You could use it as a monostable, or an astable, you could use it as a latch, you could use it as a driver when more current was needed, you could use it for all kinds of things. Because while it was designed as a timer, it brought enough out to the pins to make it versatile. It came out at a time when CMOS was barely available, and TTL was king, yet here was a device that could do some logic functions yet run over a wider voltage range than TTL.

It was succesful, because of that versatility. The early datasheet and application note were full of things you could use it for that wasn't about "timing". And as soon as it was out of the gate, other people cooked up things to use it for.

The success brought other IC timers. Walter Jung's IC Timer Cookbook from about 1977 is full of various things you could do, and the also rans that came along in the wake of the 555. Quad timers like the 558. Things like the XR-2240. "Precision timers", the numbers I can't remember.

Yet those also rans are vague to most, because they never had the success of the 555. They undoubtedly were good for what they did, but they didn't do much beyond what they were good for. It was much easier to stock the 555, and add a counter or whatever, than to have all those different types of IC timers around, which much of the time wouldn't have a use.

Michael

But, if you've got a really long time period, it's far easier to adjust a clock running much faster than waiting all that time to see if the output matches the desired time. You can adjust and see the results almost immediately, whereas if you've got a 555 with a 5 minute cycle, you have to wait five minutes between any adjustment.

Plus, it's easier to get better tolerance capacitors in small values than cheap electrolytics, which is what a lot of people would end up using if they needed long time periods. Likewise the resistors become reasonably valued, rather than tens of megohms where external factors may add resistance in parallel to the timing resistor.

It's not that the XR-2240 makes things more accurate, but the arrangement makes it easier. Of course, you can do the same thing with a 555 and a separate divider.

Michael

hi every body I think you are right may be my explaination make the subject more obvious I'm gonna make a moisture capacitive sensor for a specific oilseed ... the moisture will be checked by the capacity that will change respect to the dielectric beetween the plates(the oil seed) I use this capacitive as the C in unstable multivibrator to make square waves with diffrent T that a micro going to read it and so on but my problem is :because of the C range (50 to 100 pf) I have to use very big R to have a better T and in that way it can't feed the current for 555 to be in the ideal range of work so it's my complete problem is ther any ic for me is there any way to sove this problem

thank you all!

Well there came a nice LM3905 with some neat features, then they got rid of it.

greg

Use a dual 555 (556) system and gate the outputs as mixer effect. you set one at a standard with a null pot and the other will be attached to the capacitor plates.

With that, you can select a much higher range of frequency to work with and product a lower freq offset. etc..

"

Hi

Here are my thoughts: Charge the capacitance with a constant current source. From the equation:

i = c * dv/dt

you will get a constant change in voltage with time.

500uA will charge a capacitance of 100pF to 5V in 1us.

You could use the micro to drive a FET to short the voltage across the capacitance, so you know you are starting with 0V. Release the FET and start the micro timing. Set up a precision comparator to trip at 5V and stop the clock. The current source can be set up with moderately precision components. the timing of the 1us is a bit tight with, say, a PIC, but you can mess with the values to pick a time of your convenience.

Rgds

Bill Naylor

- electronic kits of education and fun