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Op-Amp

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Deepak Kamble

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Hi every one,
I desperately need your help.
I would like to amplify and regulate an input signal. The input voltage is variable i.e 50mv to 30volt ac, The frequency is 0-20000/min which is varying continously. . The pulse will go to the input of a micro-controller. But the problem is the micro-controller need 5 volt to sense the trigger. The applications are many, but due to the input pulse voltage I couldn't proceed further. I Thought of adding an op-amp as an amplifier or a comparator in between the pulse coil and the micro-controller but Its out of the box for me. Will someone help me. Few of my friends suggested me to use op-amp as a comparator or voltage amplifier.
I desperately need your help.

<Mod edit> Repeatedly saying "Plz" will not get you answers any faster, and will only annoy the other members here. Thus, I have removed them from your post.
 
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audioguru

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Use a Cmos rail-to-rail output with a +5V supply. Its maximum output will be +5V.
Set it with two resistors so its voltage gain is 5V/50mV= 100 times.
1) When its input is +50mV or more then its output will be +5V.
2) When its input is +25mV then its output will be about +2.5V.
3) When its input is +5mV then its output will be about +0.5V.

The input offset voltage of the opamp will mess up the levels since your input voltage is so low.
 

Deepak Kamble

New Member
Hi audioguru,
Thankyou for your indeed quick reply.
Will you plz suggest which comparator to use and what about the hysteresis(While surfing I found this term)? To be very honest, I am not that acquainted to electronic terms in that depth, If you could kindly post the schematics it will be really easy for me.
Kindly Waiting for your reply
Cheers
 
Last edited:

Deepak Kamble

New Member
Use a Cmos rail-to-rail output with a +5V supply. Its maximum output will be +5V.
Set it with two resistors so its voltage gain is 5V/50mV= 100 times.
1) When its input is +50mV or more then its output will be +5V.
2) When its input is +25mV then its output will be about +2.5V.
3) When its input is +5mV then its output will be about +0.5V.

The input offset voltage of the opamp will mess up the levels since your input voltage is so low.

Hi Audioguru,
Plz I need your help. I had posted on various forums but no-one replied, So I am a bit frustrated.
I checked in the local market. I found 2 CMOS rail to rail Op-amps.
LMC6482 and LMC6484
Should I go with any of these ?
Some people suggested me to use LM339 or LM324 As voltage comparator.
So which one should I go for. I am not technically sound in the field of electronics.

Waiting for your reply
Cheers
Deepak
 
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audioguru

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A comparator or opamp used as a comparator triggers from a DC level which might be the positive peaks of the input signal. Since the input signal is a frequency then the output of the comparator will switch high for each peak of the signal frequency which you probably do not want.
To detect that a signal is present then the output of the comparator must drive a filter that will slow down its attack and release times which you probably do not want.

An LMC6482 has two opamps inside, an LMC6484 is larger, more expensive and has 4 opamps inside. The unused opamps must be disabled properly.
An LM339 has 4 comparators inside, an LM393 has two comparators inside. An LM324 is too slow to detect frequencies above a few kHz and has 4 opamps inside. Its output high voltage goes to about 1.2V less than its supply voltage.
 

Deepak Kamble

New Member
Hi audioguru,
So which one should I go for. I have purchased the lmc6482 r-r cmos. I have also purchased the lm 339+, 324, 741.
I dont know exactly what the results would be with each of these op-amps, One of the application is the DC- ignition system, when the pulse coil sends a pulse to the microcontroller, it is delayed according to the calculations and further send to the scr I have attached the schematics. So plz suggest me Which one should i go for RAIL TO RAIL CMOS lmc6482 or the regular ones?
Cheers
Deepak
 

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audioguru

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The ignition pulse for a gasoline car engine is usually advanced so that the maximum power of the gasoline burning is when the piston is near top dead center. But I do not know why you want to retard (delay) the timing.
I do not know what sensor produces your low signal level and I do not know if it is a positive DC pulse or if it is AC with a DC average voltage of zero. Maybe you are making a CDI circuit for a little motorcycle?

I guess you did not read the datasheet for the LM324 that shows it with no output above about 2kHz and for the 741 that shows only a 30V supply and no output above about 9kHz.
 

Deepak Kamble

New Member
Basically I am making the cdi for my bike. Single cylinder. I checked the pulse voltage using a multimeter and scope. on one of my bike the pulse voltage at idling rpm was 250mv and as the rpm increased the volte rose to 10-15 volt. So when I am cranking my bike from o rpm to idling rpm the pulse voltage would be 50-90 mv at 400-500 rpm on my friends bike the idling speed voltage was 8v and increased to 30 volts. So I would like to design a universal system for triggering the MCU. The timing at idling speed is retard and as the rpm increases the mcu is programmed to advance the timing. But the mcu requires 4-5 volt pulse. The max Rpm any bike can reach is 15-20k.
 

audioguru

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I know nothing about motorcycles in your country. Here most motorcycles use a 4-stroke engine but maybe yours has a 2-stroke engine?
The maximum ignition frequency of a 4-stroke 20k RPM engine is only 167Hz and for a 2-stroke it is only 333Hz. Then an LM324 or 741 opamp might always retard the timing because they are slow.
Most of your schematics use the opamp with positive feedback that is wrong. An amplifier uses negative feedback.
 

MikeMl

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... I desperately need your help. Plz Plz Plz Plz
What is the minimum pulse width from the crankshaft sensor? What is the duty cycle? Does this change with RPM?

Here is a hack at it using a comparator. Two tests, one at 25mV at low pulse rate, and the other at ~25V at high pulse rate. It has input clamps to protect the comparator inputs.

120.gif

120a.gif
 

alec_t

Well-Known Member
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.... and here's another option, using a CMOS gate (it doesn't have to be an inverter as shown) with its input biased at the logic 0/1 trip point (~half the supply voltage) so that it is sensitive to small input changes (a few mV). D1/D2 clamp high inputs to the supply rails, and R3 limits gate input current.
CMOS-buffer-stage.gif
 

Deepak Kamble

New Member
What is the minimum pulse width from the crankshaft sensor? What is the duty cycle? Does this change with RPM?

Here is a hack at it using a comparator. Two tests, one at 25mV at low pulse rate, and the other at ~25V at high pulse rate. It has input clamps to protect the comparator inputs.

View attachment 90171

View attachment 90172
What capacitor is c1? electrolytic or polyster?

A few info about wht I know about the cdi is as follows. Hope it might be helpful.

Actually its a sine wave from the pulse coil. The negative pulse is removed and only the positive pulse is sent to the MCU. I have also attached the dc-cdi schematics. A diode is placed in between the pickup coil and the pin#10 of mcu. And when the +ve pulse is sent to the mcu, the mcu advances the timing as the rpm increases, i.e it delays the time between the in put pulse from pickup coil and the pulse going to the gate of scr. the gate of the scr is triggered on b4 few degrees i.e b4 the piston reaches the TDC to occur the spark and that too for some degrees only for the spark to occur.
other wise for the rest of the time there is no more pulse send to the gate of the scr during the complete crank rotation and the SCR doesn't conduct then

Also the following website reveals the pulse signal on the scope. I hope it might be required
http://www.transmic.net/en/simulator.htm
http://www.transmic.net/en/16628-v6.htm

It may seem that I am informing too much unnecessarily, I am sorry for that and Sorry If I am using incorrect English.

Cheers
 

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MikeMl

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Since, per the links, there is no dc component in the pulse signal from the pick-up coil, you can leave out C1 in the circuit I posted back in #11.
 

MikeMl

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I meant "dc component" as "dc level". There is no dc level coming out of the pickup coil. You started this conversation claiming that the pickup coil produces 50mV at low rpm. 50mV will not trigger the PIC using the circuit you just posted.
 

audioguru

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Since the input signal is AC with no DC bias, the opamp input and negative feedback should be capacitor-coupled so that it does not amplify its own input offset voltage.
 

MikeMl

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Most Helpful Member
Since the input signal is AC with no DC bias, the opamp input and negative feedback should be capacitor-coupled so that it does not amplify its own input offset voltage.
Not using an opamp; using a comparator, where the trip point is defined as a DC level. No capacitor is needed. Read the previous posts.
 
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