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I have an old Russian clock, that you wind up every day, but has a AA battery for the alarm. It somehow creates sound and vibration by resonating a metal piece. However, recently this broke. And my idea is to replace it with a small Piezo buzzer and some kind of an oscillator. Since I'm not an expert in this part of electronics, I only have a basic model in my head:

  • A capacitor charges to a voltage
  • A transistor is saturated enough by this voltage to pull the cap to ground and discharge it
  • Capacitor discharges and the whole cycle repeats
  • Piezo is connected to the oscillating pin and ground.

Is this possible, or are there any other alternatives using common components only?

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  • \$\begingroup\$ There a piezo buzzers with built-in oscillators available for as low as $2. \$\endgroup\$ – Janka Feb 18 '18 at 10:33
  • \$\begingroup\$ @Janka those are rated 3-24V. Since this is an alarm, it needs to be loud so running at half the minimum voltage is not ideal IMO \$\endgroup\$ – DaniFoldi Feb 18 '18 at 10:38
  • \$\begingroup\$ All piezos need a high voltage. That's the very reason your device doesn't use a piezo but a mechanical buzzer. But these are available widely, too. \$\endgroup\$ – Janka Feb 18 '18 at 10:40
  • \$\begingroup\$ Is there an electrical contact that closes for the duration of the alarm period? \$\endgroup\$ – Andy aka Feb 18 '18 at 10:47
  • \$\begingroup\$ @Andyaka that is correct \$\endgroup\$ – DaniFoldi Feb 19 '18 at 19:15
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A transistor is saturated enough by this voltage to pull the cap to ground and discharge it

The problem with that is that an NPN transistor does not switch on/off abruptly. If it did your solution would work. There are Unijunction transistors which do just this.

But with an NPN it will discharge the capacitor to such a voltage where there's a balance, it will discharge it slowly such that the transistor's input voltage is such that it will discharge at the rate (slowly) I just mentioned. So: no oscillation! It will just sit there, consume current and nothing else.

It is possible to make a one transistor NPN oscillator if you add a transformer, the well known "Joule thief" circuit (for making an LED light up on a single 1.5 V battery) is an example of this:

enter image description here

Note how there is a (small) transformer there. It takes care of the phase inversion needed to make this oscillate. This circuit could be adapted to drive a piezo element instead of an LED.

Design challenges will be:

  • making this oscillate at an audible frequency (as is it works at around 100 kHz)

  • The LED limits the Vce of the NPN, a piezo element will not limit the voltage so maybe a zener diode is needed there.

  • getting the transformer right will require some trial and error.

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