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I have what I believe to be a fairly simple schematic, as shown below, to take either a PWM signal from an AVR board to play a song/alarm, or radio signal.

I'm connecting both of these with a summing circuit OpAmp. Only one of these signals will be on at a time. The rails on the OpAmp are +5V and ground, so a voltage divider was used to feed in a 2.5 volt signal to the non-inverting output. The output I want to get from the OpAmp to the Audio Amplifier is about 0.15 volts, so I selected the appropriate resistors to achieve that voltage level. Rf= 10kOhm, R1 = 330kOhm.

It was recommended to put a 0.1 uF in series with R_2 to block any DC signal on the AVR line. When I start probing the signal after the capacitor the signal appears to just disappear. I've tried AC and DC coupling with 50 Ohm termination and haven't been able to see any noticeable signal. I have tried the set-up without the CAP and it will play the song as expected.

Am I using the incorrect voltage gain equation? Are my resistor values way off? Is the capacitor recommendation incorrect?

Any suggesetions are greatly appreciated and let me know if I need to provide any additional information.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ The capacitor on the AVR input is to prevent DC flowing out of your circuit since the op-amp '-' input is at 2.5 V DC. You need one on the radio input too. Your schematic is difficult to read. There is a schematic button on the editor toolbar. Put in all the actual component values and the op-amp part number. \$\endgroup\$ – Transistor Nov 3 '18 at 16:44
  • \$\begingroup\$ Discuss how you visualize the PWM "song" working. What's your plan regarding the PWM frequency? How do you imagine converting that to useful sound? \$\endgroup\$ – jonk Nov 3 '18 at 16:45
  • \$\begingroup\$ I'll work on making the schematic better. The PWM "song" was connected to the audio amp without out the capacitor and I could hear the song through the speaker at a reasonable volume. \$\endgroup\$ – manninosi Nov 3 '18 at 16:55
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    \$\begingroup\$ The capacitor needs to also be a non-polarized type. Are you using a ceramic or film capacitor, or are you using an electrolytic cap? \$\endgroup\$ – Hearth Nov 3 '18 at 17:14
  • \$\begingroup\$ I've tried using both, but I will switch it to a ceramic cap. I'm also looking at this site to get an idea of what is going on. Maybe my cut-off frequency is too low using the large resistors? \$\endgroup\$ – manninosi Nov 3 '18 at 17:22
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The problem with your proposed circuit is that there is no DC operating point defined at your opamp inverting input.

You need to make sure that when no input signal is active, then the inverting input of the opamp is biased at 2.5V (like the noninverting input).

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