This post is in response to requests from the review I posted on Amazon. I wanted to use my Gemini system unchanged but add on the Ring Retrofit to give monitoring access over the internet. The Gemini system uses end-of-line (EOL) resistors on the system unit itself and therefore was not compatible directly with the Ring Retrofit. In addition it had a fire alarm channel that was also not directly compatible. The voltage changes on the Gemini sensor terminals were, however, straightforward. When a contact sensor was closed, the voltage on the input was zero. When the sensor was open, the voltage would rise to 12V, i.e., the system voltage on the Gemini. The fire alarm channel was zero volts with no fire and 7.5V when the fire alarm was triggered. This is all the info needed to build a comparator board to interface between the Gemini system and the Ring Retrofit. I have uploaded two files: 1) the schematic and 2) the .brd file that can be submitted to Oshpark (or another foundry) to make a circuit board. The design is overly complicated with twice as many resistors as needed. That is because I wanted to make the board very general so that I could apply signals to either inverting or noninverting inputs. Unfortunately that could make it confusing to the novice builder. For a system with positive voltage on the sensors, the sensor signals are applied to the noninverting (+) lines of the comparators. On the schematic, there are “resistors” R1-R14 and R31-32 connected to the inputs and they come in pairs, two for each input. Connect only one low value resistor (i.e., a short) to R1, R3, R6, R8, R9, R11, R13 and R32. Do not install anything on the second resistor spot. Do not install any voltage-dividing resistors to the (+) noninverting side of the comparator. That is, do not install resistors at locations R15, R17, R33, R35, R38, R40, R20, R22, R23, R25, R41, R43, R45, R47, R28 and R30. For the negative (inverting) side of the comparator, you need to install resistors. Let’s assume that your system voltage is 12V and your contact sensor signal goes from 0V (closed) to 12V (open). Then you want to use a divide by 2 voltage divider to give 6V on the inverting (-) input to the comparator. That way, with 0V on the (+) side, the comparator will turn on (and look like a closed switch to the RIng) and with 12V on the (+) side, the comparator will turn off (look like an open switch to the Ring). To do this you need to use equal value resistors in the voltage divider (let’s say 10kohms, although the exact value isn’t critical as long as they are equal). For example, R16 and R34 form the voltage divider on the (-) side of first comparator of U2, R18 and R36 form the voltage divider for the second comparator and so on., and each resistor is 10kOhms. For my fire alarm example, where the sensor voltage goes from 0V to 7.5V, we want the comparator to switch at a voltage intermediate between these values, or approximately 4V. Then we need a divide by 3 voltage divider, again using two resistors. If RH is the resistor connected to system voltage (JP10) and RL is the resistor connected to ground, then the voltage output from the divider is V * RL/(RH+RL), where V is the voltage at JP10. If we choose RL = 5K and RH = 10K then the output is V/3, exactly what we want. You do the same sort of thing if you want a different switching point. There was a question about where the comparator would load down the sensor voltage signal and the answer is no, since the comparator has a high input impedance. Also, note that there are resistor positions on the bottom of the cirsuit board as well as the top. R49 - R56 are the “EOL” resistors to make the Ring think that it is looking at real sensors. I used 2Kohms. If you are going to buy resistors, all of the pads are for “0805” size resistors. Use a very fine tip soldering iron with a blob of solder on the end and hold the part down with tweezers or a pointed tool while you touch the solder to the resistor end and the pad simultaneously. The comparators are relatively large and easy to solder. Position the part, hold it in place with a pointed tool, and then solder one leg. Rotate to the opposite side leg and then do that one to fix the part in place. THen solder the remaining legs. I recommend that you fire the board up first on the bench and make sure everything switches properly before installing on your system. And, of course, I accept no responsibility if you screw things up.
Postscript: the system won’t let me attach the .brd file to make a circuit board with. Ridiculous.