my explaination - Electronic Circuits Projects Diagrams Free

bananasiong · 22nd May 2006, 02:16 AM

hi all, sorry for creating a new thread because nobody reply there: http://www.electro-tech-online.com/e...d-dc-ac-4.html

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..

indecided · 22nd May 2006, 03:16 AM

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...

akg · 22nd May 2006, 09:07 AM

Quote:

Originally Posted by bananasiong

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?

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

audioguru · 22nd May 2006, 12:30 PM

Quote:

Originally Posted by bananasiong

the 100K resistor creates a gain of a few hundred thousand

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.

Quote:

If a 0.1uF capacitor is added to ground at pin 3 then it won't oscillate.

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.

Quote:

what's the different between a comparator and operational amplifier? can a comparator be an op-amp and vice versa?

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.

bananasiong · 22nd May 2006, 04:29 PM

Quote:

No. The comparator has a gain of a few hundred thousand

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

Quote:

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.

where is the comparator's input current?

Quote:

The 0.1uF capacitor is a dead short to signals so coupling from the output won't occur.

what is a dead short?

Quote:

At 10kHz, the comparator has a gain of a few hundred thousand.

only for LM393?

Quote:

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.

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

thanks for your helping..

audioguru · 22nd May 2006, 08:05 PM

Quote:

Originally Posted by bananasiong

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

where is the comparator's input current?

what is a dead short?

only for LM393?

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

thanks for your helping..

Look at the datasheet for the LM393 dual comparator IC.

bananasiong · 23rd May 2006, 04:26 PM

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...??

audioguru · 23rd May 2006, 05:25 PM

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.

akg · 24th May 2006, 06:37 AM

Quote:

Originally Posted by bananasiong

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?

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

bananasiong · 25th May 2006, 05:28 AM

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

bananasiong · 25th May 2006, 05:29 PM

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?

audioguru · 25th May 2006, 09:19 PM

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.

bananasiong · 26th May 2006, 02:03 AM

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??

audioguru · 26th May 2006, 02:17 AM

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.

bananasiong · 26th May 2006, 04:01 PM

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?

Quote:

The 100k resistor provides a positive input offset voltage of a few millivolts due to the comparator's input current in it

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*

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22nd May 2006, 02:16 AM	(permalink)
bananasiong	my explaination hi all, sorry for creating a new thread because nobody reply there: http://www.electro-tech-online.com/e...d-dc-ac-4.html 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..

22nd May 2006, 03:16 AM	(permalink)
indecided	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...

23rd May 2006, 04:26 PM	(permalink)
bananasiong	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 by bananasiong; 23rd May 2006 at 04:36 PM.

23rd May 2006, 05:25 PM	(permalink)
audioguru	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. __________________ Uncle $crooge

25th May 2006, 05:28 AM	(permalink)
bananasiong	seriously i don't know what about the bias and offset. what should i do? learn them right here?

25th May 2006, 05:29 PM	(permalink)
bananasiong	about the circuit 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?

25th May 2006, 09:19 PM	(permalink)
audioguru	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. __________________ Uncle $crooge

26th May 2006, 02:03 AM	(permalink)
bananasiong	when it receives signal 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??

26th May 2006, 02:17 AM	(permalink)
audioguru	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. __________________ Uncle $crooge