Okay, so as part of my ultrasonic wind gauge I need to drive the ultrasonic transducers at 40kHz. I've looked at several ways of doing this but I'd just about got it boiled down to using a 555 (either in 1 or 2 resister guise) when a thought popped into my head about using a 40kHz crystal instead.

What I can't work out is the reason to use a 555 rather than a crystal. The only one I can think of is that I would be able to tune a 555, but given there will be 4 transducers tuning the oscillator would be pointless (unless they all happened to tune to the same frequency).

What else am I missing?

 

One main reason is what accuracy do you need, and if drift will be a problem, for accuracy and minimum drift the crystal would be the choice.
Max.

 

if you were using a crystal clock for anything else on the wind guage, you could probably devide it down to the 40 kHz without having to buy another crystal.

 

A 555 uses three internal resistors matched to about 1% in a voltage divider string, and compares the trip points of two comparators to an external voltage divider string made up of Rs and a C. So most of the oscillator frequency accuracy is based on the quality of the external components. If you match the temperature coefficients of the Rs and C, you can get good-enough accuracy and stability for many indoor applications.

For a weather station with the electronics outside, you have to consider the effects of such a wide temperature range operating environment. If the component tolerance numbers at the temp extremes are within your error budget, then ok. If not, a high freq xtal and a divider would be the traditional approach.

ak

 

I'm assuming by "crystal" you mean a complete crystal oscillator module. If just a crystal do the 555, it's simpler to get working.

The 555 may have the edge on how much it can drive, especially the non-CMOS versions. Otherwise both should have a way to enable the output if you need it off.

 

I would use a cheap 8pin micro AND a xtal, then generate the "40 kHz" in code. That gives excellent freq stability and also gives the ability to tune the frequency.

Total parts count and costs similar to the 555 solution which will drift terribly in frequency.

 

LTC6907 1% drift up to 70°c.
Max.

 

Thanks all, that's a lot of food for thought. I think I will have a play, but the 555 is looking like less of a favourite at the moment.

 

Not sure if this helps at all but We used this and it was stable for us at 200khz.