I knew it was too easy

Time to brain storm again.

 

See here is your problem. Notice in the diagram, both inputs to the XOR are same frequency but different phase so XOR outputs a false detection.

In order for this to even have a hope of working, you need to use the same oscillator for your TX and RX, see modified image of your original drawing.

I do not know how to calculate doppler shift, I think it is V= (deltawavelength/wavelength)x speed of light

V is velocity but I am not sure of the units. meter/sec maybe?

Attached Images

	XOR.gif (9.4 KB, 5 views)
	ultrasonic%20alarm copy.gif (9.4 KB, 5 views)

 

yeah I drew it up by hand with one 555 timer outputing the 40kHz signal to both the transmitter and XOR but decided to draw it in paint with two 555 timers just for clarity.

So if I have the same 555 timer provide the signal for both it should work?

 

The XOR itself will almost always have a voltage on its output. You have detected a moving object when that voltage changes. You need a band pass filter at the output of the XOR so you only respond to frequencies between about 20 Hz and 500 Hz. I explained this earlier. If you read it and still don't understand, please ask questions.

I also explained how to calculate Doppler shift. If I need to further clarify, please ask questions.

The speed of light has nothing to do with the Doppler shift of sound.

 

I understood your side of it but i was wondering if I could do this without the filter. I thought the XOR gate only outputed high when inputs differed and remained low when inputs were equal. I know low does not mean 0V but if the difference between low and high is reasonable enough, then i can just add a resistor so that the low voltage is dropped enough so that it does not "activate" the transistor while the high voltage is still large enough after the drop to activate the transistor.

Correct me if I am wrong in anything I have said. I do not mean to dismiss the information you have given me, I am just merely playing around with the circuit to gain a better understanding of what is possible and what is not. I really appreciate all of your input. It has been very helpful.

 

Without a filter, the transistor will turn on and turn off exactly like the gate does.

__________________
Uncle $crooge

 

I think that is what he was doing. No need to get snooty.

And as I stated, I was not sure about the formula. I found it on the net, it stated C as being speed of light, as wavelength formula is lambda x freq = C (speed of light) I can see the relationship.
Since Doppler works in the electro magnetic spectrum as well as sound. I am sure there may be several ways to calc doppler shift.

 

Isn't the doppler shift calculated as follows:

f_prime = f_0 (1/(1+v/c)) where v is speed of the person and c is speed of sound?

 

c is the speed of light if your are dealing with light and speed of sound if you are dealing with sound... i think

 

Okay I understand that. And I am glad you brought this up (although I was going to bring it up after I sorted all of this preliminary understanding stuff first).

How do I keep the alarm on once it is triggered instead of just going on and off with the gate (as audioguru said). Is that what the filter is for?

I am leaving work now so I won't be on until later tonight but I am loving this discussion

 

I see where you need more explanation. The XOR is fast and outputs the instantaneous state of the two signals. If the two signals are a different frequency, the waveform at the XOR output will vary between 0 (when the two signals are in phase) and 1 (when they are out of phase). At other times, the output will be 40 KHz of varying duty cycle.

The rate at which this variation occurs is the difference between the frequencies.

A low pass filter on the output of the XOR (at about 500 Hz) will stop the 40 kHz, leaving you with a sine wave equal to the difference between the frequencies.

If you then apply it to a high pass at about 20 Hz, then it will reject slowly occurring changes in the environment.

After the filtering, you have a sine wave that should contain only object movements. Apply it to a Schmitt trigger and you have something a micro can use.

I didn't look up the Doppler formula, just worked it out on a napkin, so yours is a lot better. Be sure to account for the two paths; I think that each would have its own shift.

Thanks for being more patient than I was.

 

The output of the Schmitt trigger is a digital signal at a frequency related to the speed of the moving object. I've assumed that you'll want to apply it to a PIC so that you can make some sense of it.

You'll want to reduce false triggers (don't trigger on a lone pulse), latch an alarm condition, drive your indicators, stop the alarm (maybe a coded keypad), etc.

 

Not to disparage what my esteem colleague Mneary has suggested, but I have issues with this whole concept, as I see it being a little more difficult than you may anticipate, as you have many things to take into account.

Would it not be simpler to simply detect the loss of your transmitted signal at the receive end as opposed to measuring shifts in frequency. Perhaps an IR light transmitter, and IR receive.

For example: Signal present, all is well and no alarm. Signal momentarily gone, circuit replies hey what gives? Sound alarm.

I think this would be a much easier approach.

 

They make baby wandering alarms that detect a loss of signal:
1) When the baby wanders 1m behind metal shelves.
2) When a group of people come by.
3) When the battery runs down.

__________________
Uncle $crooge

 

Yes, there are many simpler ways... some are very good. I guess I got carried away with the Doppler idea.