"The Ding of Death"
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Perhaps I can sell it on ebay as “untested”
Things were going well today. I was working on my clever telephone and I had the code doing just about what I wanted with the hardware. So I thought I’d pop the whole thing back in the phone to make it easier to work on.
I shoehorned the Pi Zero and the power supplies and MOSFET controllers into the case and it seemed OK. Then I discovered that some of the hardware interfaces (notably the switch that is triggered when you lift the handset) didn’t work. So I took the lid off and started tracing circuits. At one point I must have let one of the GPIO input wires touch a terminal on the telephone bell coil. At this point a rather unfortunate sequence of events occurred:
The bell terminals are close to ground voltage level, because the bell coils have low resistance. Touching the bell terminal with the wire caused the GPIO input to be pulled low, making the software think that the handset switch has been triggered.
The phone program rings the bell when the handset changes state. This is because when you lift the handset on an old phone you hear a “ding”. So turns on the bell coil to start the ding. This sends 35 volts to the coil……
This 35 volts goes up the GPIO input and blows away the Raspberry PI. Completely dead. Could be used as a heater but not much more.
The good news (if there is any) is that I happen to have got a spare device in stock just in case something like this happens, so I just have to wire it into the circuit. I’ve also put all my software on GitHub already, and so I’ve only lost around 20 minutes worth of coding.
I’m going to redesign the hardware connections to reduce the chances of this happening and insulate the bell terminals properly. A thing like this is only a problem if you don’t learn anything from it……
