Kerry Wong's LC meter - Issues with schmitt trigger side.

[GabYoung92]

Hi all!

I've been prototyping Kerry's LC meter from http://www.kerrywong.com/2010/10/16/avr-lc-meter-with-frequency-measurement/
Prototyping the "LC Meter - Oscillator" circuit on a breadboard gives a nice output from the first OP-Amp with the input shorted (L/C and Ground).  My issue now is the second output which is labelled to go to pin5.

Of the attached image: CH1 is the first output, CH2 is the Schmitt trigger output.

Would anyone have an idea of what may be going wrong?

- I am using a LM319

[PA0PBZ]

It is an open collector output, so if you leave the arduino out you will need something else to pull up the output.
1-10K to VCC should do the trick.

[GabYoung92]

The image attached uses a 1K pull-up resistor.  If I attach the output to an arduino's #5 pin, I still get the same as my first post.  I'm thinking I could use the pullup resistor as well, but is the waveform clean enough to do this? 

[Thor-Arne]

I don't trust that spike on the blue line.

Perhaps the cause is the wiring on the breadboard, considered solder it on a strip board?

[GabYoung92]

With the blue trace connected to the arduino, it's reading 100kHz higher than it should be... 

Linking the output to a 74HC14 gives a nice output and is reading correctly.

[PA0PBZ]

Can you show how you used the LM319 in the circuit? I have the feeling that something is wrong in your schematic.

[AlphZeta]

Hi @GabYoung92,

Sorry I forgot to include the pull up resistor in the schematics, and also the value of the feedback resistor should be 100K instead of 10K for the schmitt trigger stage. I will update the post and put a note there.

Of course, you could just use 74HC14 instead.