How to amplify a voltage? 2

[nhungr]

Hello,

I have a hall effect sensor that emits a 4mV pulse when a magnet comes into range. I need to plug the sensor's output into a motor's motion controller as a home position sensor for the motor. The motion controller, however, expects a 0 to 5V signal. So I need to amplify the 4mV signal to a 5V signal.

I have tried doing this using a UA741 opamp, followed by an LM311 comparator. Both chips are powered by the same +5V source that I'm taking from one of the motion controller output pins. I've included a circuit diagram in the attached image.

The circuit works perfectly when I haven't got the output connected to the motion controller. It gives me a jump from about 0.2V to 4V, which is perfect. As soon as I connect the output to the motion controller, though, I get a pulse from 0.2V to 3V! I need at least 3.5V for the controller to recognise the pulse.

Any idea why there would be this drop in voltage? Any idea how I could do this amplification in another way to prevent this drop?

Thanks very much.
Nikolai.

ps. I have tried plugging a bipolar 2N2222 transistor on the output of the attached circuit, but I get the same problem!!!

Attached file:

http://files.engineering.com/getfil...71-4a1e-ab04-302aebfe7968&file=AmpCircuit.JPG

 

[OperaHouse]

Use the same circuit, but use a LM339 for both. This is an open collector type and requires a pull up resistor on each output. 10K on the first and 470 to 1K on the second. You should add a little positive feedback to comparator to get a cleaner transition.

 

[nhungr]

Great thanks for the suggestions. I've continued dabbling and found that I hadn't put together my transistor solution. The following attached circuit now works, although there's still a 1V drop when I connect the output to the controller, but this time from 5V to 4V, so that works for me. Note that the pulse is also reversed in this configuration (5V low and 0V high).

 

[Noway2]

Your controller is drawing current from the 5V supply, through the 1K pullup. With a 1k resistor, a 1mA draw will drop the voltage to 4V. If this is a concern, you can do two things: 1- make the pullup resistor smaller (be sure to re-calculate the base current at minimum beta to ensure you drive to saturation), 2 - use a PNP transistor to source the 5V (~4.7 in saturation) instead of sinking. This would also restore the polarity of your pulses.

 

[nhungr]

Aha, yes, that makes sense. Thanks.