It doesn't go to 16.
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Some thoughts on construction:
1. Daughter card that mounts on keypad.
Power GND and (SDL SCL GND and Interrupt) x 2 the latter for daisy chaining. Call it the I2C connector.
2. The octopus-like cable that breaks out the bell connector (maybe) and the I2C connector.
3. The +5 V and ground connector.
4. A +12 (assuming +12 for the bell) to +5 breadboard converter (purchased)
5. A nice Bell 1 - Off - Bell 2 switch. Maybe an LED that's always co-incident with the bell.
6. The bell supply - whatever that is.
7. What you could do, is use a tiny switchmode converter and power the processor. Then when needed power up the larger power supply for the bell if your concerned about energy efficiency. It does add another output, but it would make it more energy efficient and cost more.
8. Power inlet connector including fuse and power switch. Use a Triac or relay to tun power on for the big supply. You might have to delay the transmission of the first code for 10-50 mS. If there is no more bell codes, then turn off the big supply.
aside: If you can measure the resistance of the bell, that would be very cool. Even if you use a 1.5V battery and the 300 mA scale on a meter, you could read R=0.005/(300e-3) ; Basically saying that you might be able to resolve 5 mV, you could possibly measure to 0.016 ohms.
Remember, that you MUST use an ammeter AND a voltmeter simultaneously to make this measurement. You have to measure the current through the device and the voltage across it to measure a low resistance. That will give us an idea of the peak current, knowing resistance, providing the voltage is known. You can size the power supply based on this peak current or you can add capacitance.
Still toying with the idea of an injector driver, but we need to protect the bell with something.
This reminds me, of writing computer programs based on specs. when someone knows nothing or very little. You show them something and then they add or subtract.
9. A nice engraved legend panel with the codes
e.g.
Code:
1 1 Attention
2 16 Testing
...
F 1-1-1- Whatever it is
I'm trying not to get too carried away, but my methodology is always:
What would I want if I had all the time and money in the world?
What do I need?
It's never really failed me. The reason why it doesn't is it anticipates and that anticipation adds "hooks" in the software/hardware. Those "hooks" prevent re-writes and re-do's.
So, when it comes to the "low-power" mode, the hooks might be:
1. Chose a port for that function and make the cable to support it.
2. Assure that the box has the space to add it.
3. You could purchase a line operated low wattage 5V supply instead of the DC-DC converter.
Is there any benefit of having the power supplies near the bells and/or having a power supply for each bell?