Make a pre-amp

A project log for External mic for phone timegrapher app

Checking the timing of a mechanical watch using a phone app is easy, but you need an external microphone.

RobGRobG 11/27/2022 at 15:570 Comments

Piezo transducers like the one we're using have a very high impedance output and therefore need a high-impedance input preamplifier in order to drive line inputs (or headphones). Also, the sounds we're trying to capture are very quiet so we also need some gain.

(see files for the LTSpice model)

I built this basic circuit on veroboard and put it in a metal box. I actually used a TLC271CP opamp, but I don't think it matters much. It has a gain of 1000x, and a high-pass to try to cut mains hum and lower harmonics, without really affecting the 'tick tock' sounds.

I can't stress enough the need for screened cable on the input, and a metal box, to keep out mains hum - nothing else comes close!

I tried connecting this circuit to my Motorola smart phone (via a 4-pin 'TRRS' plug) and it kinda worked, but was defeated by poor software! The phone kept insisting it used the built-in mic every now and then, and I had to keep unplugging and replugging to force it to use the external mic instead. Very annoying, so in the end I gave up and used a 3-pin 'TRS' plug, aka regular headphone jack, to plug into my PC. Besides, the TG software is much better than any of the Android apps I tried. More later...