# my explaination

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, May 21, 2006.

1. ### bananasiongNew Member

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i'm going to do presentation with this circuit 2 weeks later. if anything goes wrong, correct me ok?

The 555 timer produces a 10kHz(approximately) squarewave output at pin 3 with the combination of R1, R2 and C1. The 'antenna' will produce electromagnetic field on and off in 10kHz.

the resonant frequency of the tank circuit is 10kHz. the 100K resistor creates a gain of a few hundred thousand which is so high that it probably amplifies its own noise and any noise that is around. It might even oscillate.
If a 0.1uF capacitor is added to ground at pin 3 then it won't oscillate. When the signal is received then the output of the receiver should alternate between 0V and Vcc at the frequency of the input. With a 5V supply then the output's min current of only 6mA should be able to saturate good with 5mA from a 1k load.

are these correct? i copied them from urs explaination. is there anything to be explained with this circuit?

edit:
1. what is the purpose of the 0.01uF capacitor from pin5 to GND of the 555?
2. why the 0.1uF capacitor from pin3 to GND at the LM393 can stop it to amplify other noise and its own noise and stop oscilating?
3. what's the different between a comparator and operational amplifier? can a comparator be an op-amp and vice versa?

thanks for helping..

2. ### indecidedMember

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From my understanding, a comparator compares voltages or currents to see which one is larger and switches to the larger one.

An op-amp can be USED as a comparator, wired without a feedback path.

I would think of an op-amp as a building block and a comparator (function) as a sub-set of an op-amp. Correct me if i'm wrong...

3. ### akgNew Member

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1. the pin 5 of 555 is a control voltage , which we can use for a variable triggering.the 0.1uf is for stablility.
2.it forms a short-ckt for hight freq noise , which damps the oscillations.
3.op-amp is the building block of comparators, or opamp without negative feedback can act as comparators

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5. ### audioguruWell-Known MemberMost Helpful Member

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No. The comparator has a gain of a few hundred thousand. The 100k resistor provides a positive input offset voltage of a few millivolts due to the comparator's input current in it. Therefore without a signal the comparator's output is high.

Only a few pF of stray capacitance between wires on the pcb will couple the comparator's output to the input pin 3 and since they are the same phase then the positive feedback makes oscillation. The 0.1uF capacitor is a dead short to signals so coupling from the output won't occur.

A comparator can switch very quickly, but cannot have negative feedback added to reduce its gain like an opamp can have. Because an opamp can have negative feedback then its speed is slowed down very much to prevent it from oscillating.
At 10kHz, the comparator has a gain of a few hundred thousand. A similar opamp (LM324, LM358) has a gain of a few hundred thousand at DC and at very low frequencies but has a gain of only 50 to 100 at 10kHz. Its output can't swing that fast anyway.

6. ### bananasiongNew Member

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the comparator has the gain of a few hundred thousand its own?

where is the comparator's input current?

only for LM393?

can u reexplain these in simpler form? sorry for my poor understanding.

7. ### audioguruWell-Known MemberMost Helpful Member

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Look at the datasheet for the LM393 dual comparator IC.

8. ### bananasiongNew Member

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there are a lot of voltage and current from the datasheet
http://www.national.com/pf/LM/LM393.html
low input bias current
low input offset current
output current
maximum input bias current
offset voltage
i really don't know who r they.. can anyone tell me?

and what is a dead short?

a lot of datasheets don't show that the LM393 has the internal transistor...??

Last edited: May 23, 2006
9. ### audioguruWell-Known MemberMost Helpful Member

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The voltages and currents listed on the datasheet for the LM393 dual comparator are the same type of spec's that are listed for all opamps and comparators. When you learn about opamps then you will know what they are.

A dead short is zero ohms. The 0.1uF capacitor at pin 3 has a very low impedance to ground so it doesn't act like an antenna for interference and pickup from the output.

Of course the LM393 dual comparator has transistors. Opamps also have transistors and ther datasheet shows a schematic of the circuit.

You didn't link to the datasheet. The datasheet has a lot more detailed information on it than the sheet you posted.

10. ### akgNew Member

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u would better understand these , if u study about op-amp principles/theory

11. ### bananasiongNew Member

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seriously i don't know what about the bias and offset. what should i do? learn them right here?

12. ### bananasiongNew Member

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another question, the receiver 'antenna', is it the wire placed between the tank circuit and pin 2? or it is the coil of the tank circuit?

13. ### audioguruWell-Known MemberMost Helpful Member

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The system uses magnetic waves, not radio waves, so doesn't use antennas. The receiver's coil picks up magnetic waves from the transmitting loop (coil) around the room.

14. ### bananasiongNew Member

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when it detects squarewave electromagnetic field, the tank circuit gives a very low voltage to pin 2 when on and 0 when off. The comparator compare the voltage between pin 2 and pin 3. when on, the output of the comparator is switched 0, and when off, the output is Vcc.

am i right??

15. ### audioguruWell-Known MemberMost Helpful Member

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When it receives a signal, the tank has a weak signal that goes positive and negative. When it is negative then the comparator's output is high.

16. ### bananasiongNew Member

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it goes positive and negative? i thought on and off? so, when positive, the output is low, right?
why the tank circuit can detect the electromagnetic field? is there any other type of circuit can do this?

why does it provide voltage to pin 3? the other end is connected to GND, where does the voltage come from??

*i just want to understand, everything works well, sorry for asking so much*

17. ### audioguruWell-Known MemberMost Helpful Member

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The tank doesn't go on and off, it has an AC signal in it from the magnetic AC field it picks-up from the loop coil around the room. Since one end of the coil is connected to ground then the other end goes positive and negative a little.
Yes, the output of the comparator goes low when its inverting input has a higher voltage than its non-inverting input.

Because it is an inductor without a shield. The inductor acts like the secondary winding of a transformer with the loop coil around the room as the primary winding.

The inputs of the LM393 comparator are the bases of PNP transistors. Their input bias current is 25nA to 250nA and creates a small positive voltage drop across a 100k input resistor to ground.

18. ### bananasiongNew Member

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without a shield? u mean the coil is only a thick wire that turned for a couple of rounds? but i'm using a 10mH inductor (i think it is with shield) (i couldn't get 27mH) and i have increased the capacitance 2.7 times so that i get the resonant frequency almost 10kHz.

*what's the value of the coil which is without a shield? it can be any value? i don't know about that, i have just seen them before.
what's the different between with shield and without shield?

19. ### audioguruWell-Known MemberMost Helpful Member

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The inductor has many turns of wire.
The receiver's inductor won't pickup much magnetic signal if it has a shield. An aluminum or plastic cover won't block the magnetic signal but a steel one will.

Did you tune the frequency of the transmitter to be at the peak frequency of the receiver's tuned circuit?

20. ### bananasiongNew Member

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yes. both tx and rx frequency also almost 10kHz.
i think the 10mH inductor which i'm using is a ceramic type, it looks like a big big resistor with brown color. i just want to know whether there is any other way or not to increase the sensitivity and which part of the receiver is the most sensitive to the signal.

does this type of coil have any value?

Last edited: May 26, 2006

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