555 alarm circuit

Thread Starter

magnet18

Joined Dec 22, 2010
1,227
OK, I got the parts in, but when I hooked the speaker up with the 100 ohm pot as a voltage divider like bill had posted, it worked pretty well, but when I turned it up, the frequency of the signal itself began to increase...
I can live with it, but I would really like to know if anyone knows why or if it can be fixed.
 

Wendy

Joined Mar 24, 2008
23,415
I suspect it has a lot more to do with impedance. When you have the volume turned up the 555 is loaded with the speaker (8Ω) and the capacitor (call it 2Ω), and the pot. On the AC front it is heavily loaded. The DC analysis is a different story, but I bet the chip is getting a little warm.

Now for some RMS numbers. The waveform is a square wave (assuming 50%), so the RMS voltage is ½ whatever Vcc is. If it is a 6V RMS signal loading 10Ω (I am ignoring reactance for the moment, these are off the cuff numbers) then the speaker is getting 600mw. The instant currents are pretty high, which is what Wookie was alluding to earlier, but sustainable. Could be wrong about that, but I haven't blown a 555 chip using that scheme yet.

I saw something on the datasheet about free running frequency, is this related?
I don't know.
 

CDRIVE

Joined Jul 1, 2008
2,219
Why use an 8Ω speaker when little 30Ω speakers are now common and cheap?

Edit: Check your Vcc. You can bet that this heavy load is reflecting back to it. Make sure your you have bypass caps and a good regulator.
 

Thread Starter

magnet18

Joined Dec 22, 2010
1,227
Alright, thanks, the piezo version worked fine (lil quiet for me, I'm a heavy sleeper) so I figured it had to do with the super low impedance, I just wasn't sure how that would make the frequency increase.
I'll find a higher ohmage speaker tomorrow, I just pulled one from an old phone that should work nicely.

@CD, it's rated for 12A, it's holding up alright.
 
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