Parking Car ElectroMagnetic Sensor

[giusepped]

Yes I know. In fact if I put dual voltage (+Vdd and -Vdd) the circuit oscillates.
I would buld one with single supply.
Regards

 

[jpanhalt]

If all you want is a single-supply oscillator, I would consider using inverters. You can get a single Schmitt to oscillate, but I usually use just 3 CMOS inverters. See: AN-118 fromFairchild Semiconductors.

https://www.fairchildsemi.com/an/AN/AN-118.pdf#page=1

John

 

[giusepped]

I prefer to use the opamp because I would have stable oscillations.
I found some single supply voltage opamps, like LM386 and LT1006.
Find the attached circuit.
Now, the problem is this.
If the object is near the sensor, the capacitance will increase, maybe to some % of farad, and the frequency of the oscillator will decrease toward a a dc value. If the object i far, in theory the capacitance should be 0. As the object disappear the frequency will increase.
At the limit, it should be infinity, but I think that a dc value will appear.
So, in order to detect the object, I thnik to add a counter of pulses.
Is it good?

 

[Nigel Goodwin]

I prefer to use the opamp because I would have stable oscillations.

Logic gate oscillators are probably just as stable as opamp ones.

 

[ayodejiprecious]

pls i need your help on my project title "dual beam people counter" both theory and circuit diagram

 

[Nigel Goodwin]

pls i need your help on my project title "dual beam people counter" both theory and circuit diagram

Perhaps you should try doing your own school work!.

 

[jpanhalt]

So, in order to detect the object, I thnik to add a counter of pulses.
Is it good?

You can do it with a frequency counter. You still need to get from the frequency counter to a logic signal.

What I did was use the tone decoder chip which set a "no object" frequency. Then as an object appeared, the tone shifted and the output state of the chip changed. That was my logic signal. I believe the LM567 chip is listed as obsolete, but it is still in good in the US. There may be substitute chips available or use a microcontroller. John

 

[giusepped]

Somep roblem

I did this circuit(in simulation).
But I really do not understand why the output is not 0-9v and instead it is 1.8-7.5 range.
I forgot something?
Any reply appreciated
G

 

[jpanhalt]

The LT1057 does not go rail to rail. See:
https://pdf1.alldatasheet.com/datasheet-pdf/view/70281/LINER/LT1057.html

With a +/-15 volt supply, the typical swing is +/-13 V. In other words, it only gets to within 1.5 to 2 volts of each rail, which is about what you seem to be seeing. John

 

[giusepped]

The LT1057 does not go rail to rail. See:
https://pdf1.alldatasheet.com/datasheet-pdf/view/70281/LINER/LT1057.html

With a +/-15 volt supply, the typical swing is +/-13 V. In other words, it only gets to within 1.5 to 2 volts of each rail, which is about what you seem to be seeing. John

I completely did not know it! Thank you.
therefore, it is better to use a comparator? Like LM111?
(I would learn opamp more )

 

[jpanhalt]

I guess the real queston is why you need it to go rail to rail. What's wrong with what you have, at least as a start. You could then focus on the rest of the circuit and come back to the oscillator, if you like the results from the capaciflector. I would also sugggest reconsidering the logic-based oscillator at 12V, single supply. John

 

[giusepped]

Final

I will build the following circuit, which seems to work (in simulation)
Goodluck (to me)

 

[giusepped]

I guess the real queston is why you need it to go rail to rail. What's wrong with what you have, at least as a start. You could then focus on the rest of the circuit and come back to the oscillator, if you like the results from the capaciflector. I would also sugggest reconsidering the logic-based oscillator at 12V, single supply. John

Because I would build also a PIC which accepts the oscillator output, but this circuit for a second step. PIC accepts TTL logic values, 0/5V.

 

[jpanhalt]

PIC accepts TTL logic values, 0/5V.

True, but a PIC can accept higher voltages as well. Search other threads on this forum about interfacing higher logic voltages to a PIC. There have been several, and the interface can be as simple as a single resistor.

The following link is just one example.

https://www.electro-tech-online.com/threads/interfacing-24v.27754/?highlight=input+voltage

John

 

[giusepped]

Ok, I'll do it.
The truth is that my local store does not have all the components too!

Anyway, I'am near the prototype. One more thing.
The frequency of the oscillator increases as the object disappear, and decreases as the object is approaching. since I would test the circuit by my ear (see the LM386 in the previous circuit) I was wondering if there is a technique to "convert" the frequency.

I was thinking to a VCO...

 

[jpanhalt]

Ok, I'll do it.
Anyway, I'am near the prototype. One more thing.
The frequency of the oscillator increases as the object disappear, and decreases as the object is approaching. since I would test the circuit by my ear (see the LM386 in the previous circuit) I was wondering if there is a technique to "convert" the frequency.

I was thinking to a VCO...

I am not sure how a voltage controlled oscillator (VCO) would help you. What you describe is a frequency to voltage converter. Search on "tachometer". For example, see: https://pdf1.alldatasheet.com/datasheet-pdf/view/103466/NSC/LM2917N-8.html

A simple oscilloscope or a multimeter with frquency display will help you. Although, for proof of concept, the your ear is pretty good at discriminating tones. John

Edit: A simple low-pass filter with passive components may work. See: Horowitz and Hill, 2nd edition, page 618. There are other circuits based on op-amps (ibid, Chapter 9, regarding digital to analog conversion).

 

[giusepped]

NOt yet

I mounted the circuit I posted.
UNfortunately, I cannot hear anything from the speaker (4ohm)
I still do not own an oscilloscope so I cannot say if the oscillator oscillates.
Anyway, I put a led at the output of the voltage follower and it runs.
The multimeter says about 2.2 v there.
I think LM386 does not like square waves...
Anyway, I ordered a used oscilloscope and in a day I should be able to test more accurately.
In the meantime, there is another way?
G

 

[jpanhalt]

I mounted the circuit I posted.
UNfortunately, I cannot hear anything from the speaker (4ohm)

I don't understand how that statement agrees with your earlier statement:

The frequency of the oscillator increases as the object disappear, and decreases as the object is approaching. since I would test the circuit by my ear (see the LM386 in the previous circuit)...

In any event, you will find it much, much easier to troubleshoot with the oscilloscope. Which oscilloscope are you getting? John

 

[giusepped]

What dont you understand?
Did you remember my previous circuit?
I attached the LM386 in order to drive the speaker. Its input will be a square wave, from the oscillator.

Anyway, the oscilloscope is a 100 mhz tektronix (used from yahoo auction)
G

 

[giusepped]

Got working

I made the following circuit and it seems to oscillate.
But I don't understand why.
Moreover, the capaciflector must be circular. I used two shapes, one was a rectangular shape and the second one is circular.
With the rectangular shape, nothing happens at the oscilloscope.
With the circular shape, I can appreciate some 1KHz variations in the rectangular waveform.
Somebody can explain to me why the circuit oscillate?
I know that this is relaxation oscillator, but the capacitance of the sensor is very high and in theory it should not oscillate. I am missing something?