My first effort at soldering SMD was to attach an LMC7215 SOIC8 to a breakout board. (That is shown in another thread.) When I tried the breakout board in this circuit http://discovercircuits.com/PDF-FILES/AUDIODETECT1.pdf , I couldn't get the circuit to work. (I think I correctly transposed the pin numbers to my SOIC8.)

Thinking that perhaps I damaged the op-amp with too much heat, I built the attached test circuit, but I don't really know if the circuit proves anything or not. The LED lights, but does that ensure that the LMC7215 is good?

Thanks.

 

The Audio Detector circuit output is a sink output.
Not a +/- output as it is labeled. Your chip might be fine.
Connect: 3V+ to 220 res -> LED -> Drain on FET. ( Led cathode to FET)

 

Now, I am even more baffled. In the test circuit schematic in post 1, the output is oscillating from rail to rail at 33.3 kHz. Is the op-amp fried?

 

Edit: oops thought this was an opamp, its a comparator.

The + input is biased slightly V+. It might be an oscillator circuit.
Try changing your test circuit. Remove R3, remove R4, ground the - input.
Led should be on.
Then make R1 much larger to turn off the LED.

 

Edit: oops thought this was an opamp, its a comparator.

The + input is biased slightly V+. It might be an oscillator circuit.
Try changing your test circuit. Remove R3, remove R4, ground the - input.
Led should be on.
Then make R1 much larger to turn off the LED.

Thanks for your help. As you can tell, I am lost.

I removed R3 and R4 and attached pin 2 to the common ground. LED is still lit. Pin 6 is at about +2 V. Oscillation has stopped.

ETA: Just reread your post. I will increase R1.

ETA: Replaced R1 with a 1M pot, and was able to turn LED on and off with pot. So, I guess that means the IC is good?

 

Yes, the IC seems good.

The Audio Detector circuit should work if you connect the LED as suggested.

 

Yes, the IC seems good.

The Audio Detector circuit should work if you connect the LED as suggested.

Thanks for your help.

When I breadboarded the circuit, I didn't actually use a MOSFET. I was just looking for a positive voltage from pin 6 that would gate the MOSFET on. Are you saying that the LMC7215 will actually need to sink current to gate the MOSFET on?

 

So, I'm new to surface mount. What did you use for soldering equipment ?

 

So, I'm new to surface mount. What did you use for soldering equipment ?

Just what I already had: a Hakko 936 with a 900-T-B tip and .032" solder. I needed smaller solder and some flux, and I might have done better with my 1.2mm spade tip. Of course, the SOIC8 is pretty easy as SMDs go.

 

No, The IC would supply a positive voltage to the FET gate. This turns on the FET to sink current. Think of the FET as a switch in this circuit.

You may need the FET or a transistor with a small base resistor to drive the LED. The IC may not have enough current capability at that supply voltage. Check the datasheet

 

No, The IC would supply a positive voltage to the FET gate. This turns on the FET to sink current. Think of the FET as a switch in this circuit.

You may need the FET or a transistor with a small base resistor to drive the LED. The IC may not have enough current capability at that supply voltage. Check the datasheet

That's what I thought. I must have wired the breadboard wrong; I'll rebuild it more carefully now that I know I have a good LMC7215.