View Full Version : can crystal be use like this for 40khz

07-13-2006, 10:09 AM

07-13-2006, 11:26 AM
40 khz crystals are not easily available, maybe 32.768 khz xtals from clocks and watches.

Any configuration can be used, provided the output PP waveform does not exceed the receiving device max limit.

Any crystal osc. is stable, when compared to RC or LC configuarations, but quite expensive. To achieve even more accuracy, a higher Freq crystal is used and dividers are used to reduce it to the wanted freq.

For eg. $MHz crystal oscillator with a 1/100 divider chain will produce a better stable signal than a crystal osc with a 40KHz crystal.

For the purpose of Ultrasonic txrx, a 555 will do nicely ( with polycarbonate or polyester, greencap capacitors for the RC time constant . NOT Ceramic disc!!!, ) unless you REALLY need accurate an stable sig.

Ceramic capacitors are good ONLY as bypass caps. Not for Timing. They are rated for +20% to -60% temp coeff. You can make a simple and wonderful temp sensor out of it; the freq will change by "blowing" on the capacitor!!

11-26-2006, 12:59 AM
use 555 to generate a 400khz or 800khz and then divide it to get 40khz that way u will get better precision in less cost..

11-26-2006, 10:40 AM
555 wont work at 800KHz.
4040, 4060 may be used with appropriate crystals for 2^n division.

4MHz xtal /100 using 2 74LS90/ 93 or 2 4017 ics

But these methods simply complicate things.

11-27-2006, 01:03 PM
why not use a uC for 40Khz.. i always use one , Maybe because my programming is stronger than my electronics :).
ok its way more exp ... but just an idea !!!

11-27-2006, 04:34 PM
for sonar, you might need to fine tune your oscillator with your ultrasonic transducer for maximum efficiency. So it will be better to use 555 for ultrasonic purposes. however, if you want to modulate IR with 40 khz, crystal can be used. If you can solder SMDs, 555 smds can save some space.

Alternatively, as Vikas mentioned, a low pincount controller (like 10F220) can be used to generate the frequency. however, you require programming, and external crystal(not req if you need low frequencies) for this.

After trying all these, one might resort to the classic 555 for freq<50-100khz :wink: