MIDI Keyboard Controller, Part 4

So, I was supposed to be continuing on with the MIDI keyboard, but I've had to press the big red button. Turns out I want this to be a USB keyboard now (still MIDI, but plug right into the PC). So that means a new microcontroller, somewhat new layout, using USB power, and a bunch of firmware redesign. I'm going to do a PCB spin this time around to hopefully speed up prototyping. Now its the USB-MIDI Keyboard Controller!

The Disalarmer! (Part 2)

I'm just going to dive right in. Be sure to have read Part 1, and maybe even have it open for reference. So continuing the discussion where we left off, I mentioned there is a potential issue of feeding 5V into the 3,3V power rail.

Now normally this wouldn't be a huge deal. Perhaps isn't even in this case. When one of the buttons is pushed, current flows into the 1K resistor, thru ESD1 and into the 3.3V power rail. So you'd expect that current (without D1 or R2) to to be something like:  $\frac{V_{B+}-V_{ESD1}-V_{CC}}{1K} = \frac{5V-.3V-3.3V}{1K} = 1.4mA$. Not a lot, right? But that 3.3V supply isn't magic. Let's take a look at the output of our regulator:

It only lets current into the circuit, it doesn't let any out. Those two resistors between the output and ground measure out to about $125K\Omega$. Without D1 & R2, I was finding the 3.3V rail would rise above 4V whenever a button was pressed. The only current paths were back into the regulator and whatever draw the microcontroller had.

The circuit without D1 & R2

So by adding a 3.3V zener and a small resistor (so it wouldn't really turn on in normal operation), I'm able to keep the voltage from rising too much. Interestingly though, the fact that the voltage was above the "safe" operating voltage may not have been too dangerous for the MCU. The 1K resistor between the button and the input limits any current into the circuit to 5mA, and that's if the microcontroller tried to short anything to ground.

Also, there was probably an easier solution had I not already soldered things and had better tools to undo my work: just use diodes on the outputs instead of  resistors (I was expecting active low, not high). They'd block the 5V from feeding into the 3.3V circuit and ultimately reduce idle current (the zener draws a couple mA all the time, so sleeping the micro doesn't even buy much). The only drawback might be the diode drop means you're only supply 2.6V to the input of the clock's processor, which is expecting 5V, and I don't know the logic type of those inputs.

But anyhow, that's not a super exciting discussion. It is however, all that's needed for this project to work. There's a little bit of firmware involved:

•  It has to monitor the UART for a "snooze\r\n" or a "off\r\n" from the bluetooth module and act accordingly. Throw away all other messages.
• It has a timer set for about 9 minutes (the sleep time of the clock is 8 minutes). This allows disabling the alarm between snoozes. Whenever the buzzer goes, the timer is reset.
• After the timer is up, turn off the bluetooth and go to sleep.

That's almost the last thing. We need a phone app. While I've used the various incarnations of the Android studio or SDK's in the past, I actually did this one in Visual Studio. Nothing too fancy. A giant "snooze" button that's easy to hit. A smaller "off" button that requires a long press to activate, and a sort of "reset" button. The app stores the address of a paired bluetooth device so there is no need to fumble thru choices everytime you launch it. It recalls the stored device and opens a socket, no questions asked. Should the need to use a new paired address arise, then the "reset" button just clears it, and the app will then ask.

At this time, I've had this working in the alarm clock for a few months now, pretty much trouble free. Not the most important project I've ever done, or necessary, or all that worthwhile, but still a lot of fun. It was a small and compact project that spanned embedded to Android. Here's a little clip of it in action:

(the app loads slow because.... well I was filming, so something had to not work normally. The blinking on the alarm clock shows that the alarm is still active, after long pressing the "off" button, the blinking stops as the alarm is disabled)