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Unusual problem here… I need a cheap device that can emit a click or chirp every 25±5 seconds, for at least 26 hours.

Basically, I need a metronome but the $5 devices all begin at 30 beats per minute, while I need only about 3 beats per minute.

I could easily program a computer to do this, but unfortunately, I need a self-powered device that fits on a platform that is 25 cm × 7 cm (10" × 2½"), and that isn't so expensive that it'd be a major loss, should it disappear.

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A battery operated bluetooth speaker on the odd platform where you can't get power. Then inside, plugged in if need be, something that can play to the speaker. This could easily be a phone since it isn't going to fall off the platform.

As for making the noise, you could record yourself saying "beep" every 25 seconds for a few minutes, then play it on loop. Or whatever plan you had for getting a computer to do it. As long as it's playing to the speaker that is out on the platform.

Also, you only need two beats a minute, but would whatever you're trying to do still work if you got more beats than that? There would never be more than 25 seconds of silence, right? Or is the silence as important as the noise? Because if being noisy more often is ok, go back to the metronome plan.

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First I thought of using a

  • smartphone app like "Metronome Beats"

that I found on my phone. But loosing a phone might be too expensive. The app allows a minimum of one click per minute when set to only one click per beat.

Then I thought of some kind of recording module. The first I found was the

  • Whadda WSAH8094 EXTENDED RECORD / PLAYBACK MODULE for up to 8 minutes playback (I have not read anything about a minimum so far).

for about 32 Euro at a German electronics dealer named conrad.de. There would still need to be a battery and speaker clamped to it, and maybe a case if the environment could be damp.

Through this I even found the

  • Whadda WSAH195 VOICE RECORDING/PLAYBACK MODULE for anything between 30 and 90 seconds. And the
  • Whadda WPM449 ISD1820 VOICE RECORD/PLAY MODULE for up to 20 seconds

at a website called velleman.eu and expect those to be cheaper, but have not looked into prices. This doesn't look good for a 25 second interval, but maybe I've given a good hint for something similar.

And lastly there is the manufacturer's website at whadda.com that I did not know it existed until you asked.

I liked your question and I am curious what the purpose of your sound application is. Hope this helps.

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This suggestion is seriously on the DIY side of things, but has quite a bit of practicality to it. The typical microwave oven with a turntable within will have a motor that operates at six revolutions per minute, ten seconds per rotation.

The requirement of 25 seconds means a 2.5 to 1 ratio. I have dismantled a few discarded ovens and found one type with a motor that operates on 20vac, and a different type that runs on 110vac / 60Hz. I do not know what one might find in European countries (50 Hz), but I suspect the RPM will be 5/6ths or 6/5ths of the USA versions. This means a different ratio to accomplish the objective.

The motors will likely have a D-shaped shaft end. It is relatively simple to use a product known as hand-moldable plastic to create an adaptor. This would allow the attachment of, for example, a twenty tooth gear or a twenty millimeter diameter pulley. The match for the twenty tooth gear would be a fifty tooth gear, providing the two point five ratio. The pulley would, of course, be fifty millimeters for the same reason.

On one point of the output "wheel," you could have a rigid "finger" to strike a semi-flexible surface, causing a click. You could have a stationary pin which would cause the finger to flex back a bit, then strike another surface, perhaps a bell as it continues to rotate.

If you have a 3D printer or know of a makerspace in your area, the manufactured objects and even the mounting assembly would be relatively easy to create or have created.

The motors are smaller than 7 cm and about half as deep, with mounting ears for easy attachment. Gears or pulleys can be 3D printed or cut from 3 mm plywood by hand, scroll saw, laser cutter and mounted with the relatively common skateboard bearing. Gears are meshed, pulleys would require a belt, which can be an off-the-shelf O-ring. All of this would fit easily within your constraints.

It will certainly have a home-made appearance, perhaps reducing "attractiveness" for theft.

Image below created by software licensed to me:

2.5 to 1 gearing

50 tooth to 20 tooth

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  • Clever, but I imagine that powering the device would be an issue. Wouldn't a constant 20 V alternating current for 26 hours require batteries and an inverter that couldn't fit on the platform, not to mention these parts alone being expensive?
    – Adám
    Dec 20, 2022 at 7:18
  • Unfortunately I overlooked the self-powered aspect. Powering the motor would be a complication that disqualifies my suggestion. The other parts, salvaged and hand-made would have a trivial cost.
    – fred_dot_u
    Dec 20, 2022 at 11:19
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Simple sound hack:

For a few dollars ($25 - $30) buy a battery-operated voice recorder (new or used) for the task.

They will operate for a while with new batteries and are completely self-contained.

The downside is that you will have to take the time to create your click track for playback. Once done, though, it's ready for multiple use.

Good luck.

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You need an "event timer relay," an inexpensive device you can easily program to close a relay after a specified amount of time, with a repeat feature: https://www.mcmaster.com/timers/wide-range-socket-mount-timer-relays/

There are different models that give different time interval scales, kinds of switches (NO, NC), whether "one-shot" or "repeat," and number of different circuits.

You simply set up this switch between a power source and a sound source.

I haven't bought one of these in ten years, and I see the price has more than doubled. But they are very reliable and dependable, and programmable.

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You can make a small, inexpensive, accurate integrated-circuit repeating timer/oscillator that operates on 9V battery power.

You would construct the circuit on a small breadboard using a 555 IC timer chip. Select the values to give you the time interval you wish and the amount of delay.

This tutorial site https://www.build-electronic-circuits.com/555-timer/ has all the necessary directions and links to a timer calculator for component values selection (resistors/capacitor)

Good luck.

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