pc1550-interface
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dougkpowers
(possible) voltage issue
I has come to my attention that the keybus data and clock likes are at 12v. Would running these voltages into an arduino analog pin damage it? I am interested in building this, however do not want to release the magic smoke from my arduino. Thanks in advance.
What leads you to believe they are 12v? Are you doing a direct measurement? I am not doing any voltage regulation before connecting to the 1550 to my arduino, so that leads me to believe that it is not 12V. However, I don't recall what the exact voltage is.
I came accross this article, which showed that the voltages were above 5v, I have also probed my system boards to confirm this. I am just curious as to how the arduino can handel these voltages. http://electronics.stackexchange.com/questions/11017/dsc-keybus-protocol
I'm not using a voltage divider and haven't had any problems-- my arduino seems to handle the voltage without problem and it's been chugging along for three years now.
Check out issue #1 before adding a voltage divider as it has been tried before. Specifically, the data line is used both to send data to the keypad and to send data from the keypad back to the main board (so make sure your voltage divider is bi-directional).
If you are really worried about voltage and don't have a bi-directional voltage divider, you could stick to using the PGM terminal and a voltage divider.
Every circuit I have seen uses two 10K resistors. However, using a voltage divider calculator it shows the out voltage to be 6V assuming 12V input voltage and higher than 6V with higher input voltages. On my system aux is 13.8V which would make it 6.9V. Arduino is 5V. I am wanting to do something similar, but to an ESP board which is 3.3V and I am trying to figure out why the 2 10K resistors work for 5V so I can pick values for 3.3V. In addition does the circuit work in both directions? At first I will just want to read values, but eventually I will want to send values as well.
I have something similar to the Sparkfun device @Quiqui64 said he used, but when I asked if it will do 12V to 3.3V the seller said no.
Thanks
On the sparkfun website, the schematic for the converter has been provided and one could build the bi-directional converter. As long as you use a Mosfet with a gate threshold voltage of less than 3.3v and a Drain-to-Source Breakdown Voltage of over 12v you will be able to convert from 3.3v to 12v no problem. A common Mosfet to use would be a IRF1404, the BSS138 used on the sparkfun board would also work for this purpose. Any n-channel Mosfet should work as long as it meets the criteria above and can handle a couple milliamps. Note that a simple voltage divider could be used on the clock line while a bi-directional converter must be used on the data line.
@wiihoffmann Thanks. I just got the board in and it is marked J1 sideways Y so I believe it is a BSS138 and will give it a try.
The board is working fine for me as inputs. I still need to try a analog pin and output.
I've measured the PC1550 signals at ~13.7v on an oscilloscope, it's possible that the Arduino is handling this via the AVR internal ESD protection diodes (https://www.rugged-circuits.com/10-ways-to-destroy-an-arduino) but for esp8266/esp32 this could be an issue.
For an example aside from using a bi-directional level converter, I've added support for PC1500/PC1550 to dscKeybusInterface, using resistor voltage dividers on the inputs for Arduino, esp8266, and esp32. Writing back to the panel is handled with a separate pin that drives an NPN transistor to toggle the data line.
By the way, thanks for the details on the Classic series protocol! It was very useful to get these panels integrated.
