Author Topic: Sound generator for alarm clock repair  (Read 508 times)

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Offline MartinnTopic starter

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Sound generator for alarm clock repair
« on: July 09, 2020, 05:28:48 pm »
Hi all,
my Braun analog RC alarm clock just died - alarm doesn't sound anymore. With great effort I was able to take it apart and found that all contacts, switches, wires (which I had thought would have been the problem) were in perfect condition. I think one of the glob top chips must be defective (radio and clock part still work, just sound and light don't work anymore), which means - this thing goes to the trash bin.  :(
Now I have to say I quite liked this clock. It looks nice (for a piece of plastic), was not super cheap and worked well so far. So I thought: Both the alarm "enable" (on top of the clock) and alarm "trigger" switches are purely mechanical and I could just wire them in series and switch on some sound generator with them. Now here comes the problem
- we only have 1 AA cell (voltage say 1-1.6 V)
- there is very little space
and on top I dont want it to blare at full steam from the start, but begin with gentle short beeps, getting louder and longer continuously (as the original alarm did before it died).
So on top of the 1 V supply and build volume limit, there needs to be some amplitude pattern. Speaker (still works) is dynamic, but as usual of helmholtz resonator style with probably large peak at it's resonance, so no point trying to play music.
First I thought of some transistor oscillator circuits, but this would get pretty complex. The best (but least elegant) I can see right now is a charge pump or switchmode voltage converter to some > 2 V (Iq is no issue as it it switched off 99.99% of the day) and a small microcontroller with DAC (or maybe PWM) analog output (eg STM32F3). Sound could either be in a ROM table or synthesized (sine with some kind of envelope). However this feels like a huge overkill for a (well not so) primitive beep generator.
Any better ideas? Maybe low power small media players one could use? Preferably single cell powered?

Thanks, Martin
« Last Edit: July 09, 2020, 05:30:58 pm by Martinn »
 


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