I'm trying to build Sparkfun's sound sensor and testing the first stage only. Here's the first stage:
If I scope before C2 and at Pin 1, I get the following when I scream at it:
where Ch1 is the mic input and Ch2 is the opamp output. As you can see, the gain is just 1x where it should actually be 100x.
Here's my breadboard:
Am I missing something here? Shouldn't I get a 100x gain? I'm using a LM324 (with 5V) rather than a LMV324, but I think they're essentially the same.
Thanks
Best Answer
As shown in the photos you have neither power supply pin connected.
Most ICs work very badly in that state :-).
If there is no gnd or VCC on IC then Vout ~= Vin as it is a fancy piece of wire with a 10k to ground and supply. In that case what you see is about what you'd expect. .
Note that pin4 is Vcc and pin 11 is ground.
Not being rail to rail is fine. Maybe increase R5 to about 12K to move DC point away from top rail but not essential.
It's not 100% certain what your stage-off connections go to but if clip at top right is scope-ground it's apparently connected to V+. If psu has same ground they may get annoyed.
It's potentially OK to do but you seem to have Mic R1 in gnd side not Vcc side. This is OK as long as Mic polarity is OK> Is Mic polarity OK?
It's not obvious what your scope shows. Y scale is 50 mV/div and CH2 probe is 10X. That may mean 50 mV/div or 500 mV deep-ending on Rigol philosophy. I've never used a Rigol. CH1 is presumable 10x as well but I think you and it say not.
LM324 input common mode range is about gnd to Vcc-1.5 V so here = about 0 to 3.5V Vin DC point as shown is 2.5V so you have 1v of input swing positively which should be plenty. Vout swings near but not to Vcc - see data sheet.
That should do for now.
Try powering it and report back.
Checklist:
What are the mystery leads in the photos connected to?
Is there ground and Vcc ON THE IC?
Is CH1 set to 10x ?
What is the scope vertical display actual eg 50 mV/divn or 500 mV/divn?
Is the mic polarity OK
Is the mic resistor in the ground lead and did you intend that.
If clip at top right is scope-ground it's apparently connected to V+. If psu has same ground they may get annoyed.