Posted on Feb 7, 2014

The following circuits are presented in a logical order (unlike the way they were designed) from initial power-up to the frills tacked onto the end of the project. A less organized and more stream of consciousness version of the Mark II design can be found here, this is the sanitized and organized version. Some general changes since the Mark I include the switch to 9 volt operation (so I can just use a

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single battery which fits in the 9V battery compartment of the box I decided to use) and the switch to a TLC3704 voltage comparator chip (a very low power CMOS version of an LM339) instead of a LM324 op-amp. One of the (obvious once I finally thought of it) improvements over the Mark I was the change to actually disconnect the batteries when I don`t have the phone in the system, thus saving lots of battery power while I`m not at home and the phone isn`t in the Mark II. To prevent accidental triggering of the detector during the initial power-up period when capacitors are charging and the photo resistor is stabilizing, I needed to make sure nothing drastic happens for a few seconds after the power comes on. This circuit does that by holding the 555 timer reset pin low while powering up. The Mark I and Mark II both operate by noticing when the phone display lights up (which happens on incoming calls). The circuits are fairly similar in both, but I`m using the TLC3704 chip now, and one important change was the addition of resistor R8 to provide some hysteresis in the circuit so it doesn`t accidentally trigger after the photo resistor has already been lit up and is now going back to dark. (I also needed to tweak R8 a little - after everythng was together I found the detector failed intermittently and decided I was getting a little too much feedback. The 390K value here is the final version and works much better than the initial...

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