Op-Amp Help

The variable input voltage comes from the pulse coil my bike which is 50mv at low rpms and increases upto 30 volts at high rpms. The pulse goes to the PIC microcontroller. But the MCU requires 5 volt dc. So I thought of addin g an op-amp in between the Pulse coil and the PIC MCU. If any body could he lp me with the schematics of it it will be gr8 for me.

I don't think you want an opamp. But maybe a big capacitor to store the charge (and even out the pulses) and then a 5V three terminal regulator after that. (that wastes power at the high end.) The next step would be a more complicated SMPS.

George H.

. The variable input voltage comes from the pulse coil my bike which is 50m v at low rpms and increases upto 30 volts at high rpms. The pulse goes to t he PIC microcontroller. But the MCU requires 5 volt dc. So I thought of add ing an op-amp in between the Pulse coil and the PIC MCU. If any body could help me with the schematics of it it will be gr8 for me.

depends on whether he wants to use the output to power the PIC or use it as an input

-Lasse

You probably want a comparator rather than an opamp since you actually want to turn a variable input analogue pulse from 50mV to 30V (are you sure about that?) into a clean logic pulse at 5v.

The device will need diode protection from input voltages that go outside its supply rails.

I modified my bike generator, added a diode for DC out, and let that charge an under-seat 6V SLA battery that i used for lights. Then a switch to drive the lights either directly from the battery or from the (now) DC generator. Had to replace the battery once roughly after 5 years.

The variable input voltage comes from the pulse coil my bike which is 50mv at low rpms and increases upto 30 volts at high rpms. The pulse goes to the PIC microcontroller. But the MCU requires 5 volt dc. So I thought of addin g an op-amp in between the Pulse coil and the PIC MCU. If any body could he lp me with the schematics of it it will be gr8 for me.

A comparator run on 5v should do it. It'll likely need a resistor from its output to the +5v rail.

NT

t. The variable input voltage comes from the pulse coil my bike which is 50 mv at low rpms and increases upto 30 volts at high rpms. The pulse goes to the PIC microcontroller. But the MCU requires 5 volt dc. So I thought of ad ding an op-amp in between the Pulse coil and the PIC MCU. If any body could help me with the schematics of it it will be gr8 for me.

-Lasse

. The variable input voltage comes from the pulse coil my bike which is 50m v at low rpms and increases upto 30 volts at high rpms. The pulse goes to t he PIC microcontroller. But the MCU requires 5 volt dc. So I thought of add ing an op-amp in between the Pulse coil and the PIC MCU. If any body could help me with the schematics of it it will be gr8 for me.

s output to the +5v rail.

and a pair of limiting diodes on the input, fed thru a resistor

NT

Possibly an English translation problem I was assuming a pedal bicycle. (which is the common usage of bike over here as opposed to motorbike)

volt. The variable input voltage comes from the pulse coil my bike which is 50mv at low rpms and increases upto 30 volts at high rpms. The pulse goes to the PIC microcontroller. But the MCU requires 5 volt dc. So I thought of adding an op-amp in between the Pulse coil and the PIC MCU. If any body co uld help me with the schematics of it it will be gr8 for me.

sensor

I'd think it is a bicycle, my point was that magnetic sensors can have wild ly varying and sometimes quite high output voltages

-Lasse

If you are trying to _power_ your PIC from your bike then you need to listen to George Harold -- and you will probably have to resign yourself to having things just not work until the pulse coil generates more than

If you're trying to use the pulse coil as a sensor, then a comparator as recommended by "meow2222" may be appropriate.

I didnt realise the OP might be wanting a power supply. If so, adding a small rechargeable could keep it alive all the time. All you'd need then would be a charge current limiter from coil to cells, and a V regualtor from cells to pic.

NT

Well, I don't _know_ he does -- his post is ambiguous, and I'm trying to clarify things.

Don't know which PIC it is but most micro controllers already have a comparator. That can be set to trip at 30mV or so, capacitive coupling should be ok, and clamp the pulse so it never exceeds the absolute maximum ratings. The easiest way would be back-to-back diodes which limit the swings to about +/-600mV.

Hi Tim, I thought that if the OP wants power, Robert Baers's diode into a battery would be much better.

George H.

As a speed sensor, just a comparator might work OK, but the classic tachometer circuit is a pseudo-integrator, where the integrator transfer function complements the speed-voltage characteristic of an inductive pickup. That can give a nearly constant-amplitude output over a huge speed range.

Just an RC lowpass can pretty much do that, but won't add any gain. An RC lowpass feeding a comparator with a bit of hysteresis, with 50 mV or more to work with, should be OK.

If this is a bicycle sensor a poor man's hysteresis via a vacant port pin should suffice. Of course most uC circuit designers are always short on port pins.

If he's got a micro with a built-in comparitor, togling the pull-up bit might be enough.

The variable input voltage comes from the pulse coil my bike which is 50mv at low rpms and increases upto 30 volts at high rpms. The pulse goes to the PIC microcontroller. But the MCU requires 5 volt dc. So I thought of addin g an op-amp in between the Pulse coil and the PIC MCU. If any body could he lp me with the schematics of it it will be gr8 for me.

Hello Sir, The op-amp is not going to do anything. As a pulse signal is time-varying, it must be first rectified to get DC and filtered(smoothed), and then stabilized or regulated with a simple linear voltage regulator

-- e.g., 7805 Hope that helps.