The first thing to do was to test the microphone on the iPhone speakers (connected to the op-amp output before the 10uF cap) and it worked fine. I changed the 10uF capacitor to a 0.1uF to give a quicker response when powered up and reduced the boost to 26x for testing. The configuration above boosts the microphone’s output (which is AC coupled) by 100x and the 10K/10K divider on the non-inverting side keeps the opamp’s output at half of VCC so that when sounds are produced there will be an AC waveform (e.g with VCC/2 being 2.5V, we could get a waveform that goes 1.5V to 3.5V) just before the 220uF capacitor.Īfter doing a bit of researching and testing, I’m using a 1.5K resistor for the iPhone microphone which is what I’ve heard that’s what the iPhone uses to power it. I firstly tried a non-inverting opamp configuration but it didn’t turn out to be as good as the inverting one above. I found an LM358 op-amp inverting circuit which we’ll use to start off with (shown above). Now it’s back to making a boost circuit for the microphone, I looked around for a low cost op-amp and found the LMV358 which is just like the LM358 and it’s rail to rail. I had the Ti SN74LVC1G3157 SPDT switch on hand and it should do the trick, just hook up 3 of these for the mic and speakers. It turns out that the microphone was a little hard to hear so after removing the 1.5K resistor it sounded better but it wasn’t loud enough unless you place the mic close to your mouth.Īfter speaking with a colleague, the idea came about of being able to switch between an iPhone and the work desk phone easily, it shouldn’t be a problem since we can an analog switch to do this. The first step was to build a Snom 4P4C to TRRS cable, I used a 220 ohm resistor for protection to find the left/right speakers and then used a 1.5K resistor for the microphone. There are some headset/headphones options available, the Snom uses 4P4C / RJ9/RJ10 connectors but I’ve always got my iPhone headphones on, so I’d like be able to use them on the Snom phone. At work, we use Snom phones and we take quite a number of calls every day, when you use the handset and need to type it’s a bit difficult.
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