Buzzer vodoo

Thread Starter

Chillum

Joined Nov 13, 2014
546
Hi,

I'm using a strong bar magnet the size of those white plastic boxes with small magnet inside for the reed switch. First I need to explain, we leave the security gate open during the day, and have a piezo buzzer 555 system with magnetic reed switch to trigger it, to alert us when the gate is opened or closed... and it works beautifully; but the gods do not favour us, every 1 to 2 hours or so we get a false positive, and it can't be the magnet, it's way above specs, and when it was still in breadboard form we wrote off the false positives as bad connection on the trigger capacitor, but now (buzzer7.mees.intuitief) I put it on vero (strip) board, so it can't be a bad connection on the vero board side, and it can't be a bad magnetic connection, so why do the gods not favour us with false positives? Any ideas?

PS buzzer7.pdf is the scematic for this project (my first)
 

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cmartinez

Joined Jan 17, 2007
8,220
Hi,

I'm using a strong bar magnet the size of those white plastic boxes with small magnet inside for the reed switch. First I need to explain, we leave the security gate open during the day, and have a piezo buzzer 555 system with magnetic reed switch to trigger it, to alert us when the gate is opened or closed... and it works beautifully; but the gods do not favour us, every 1 to 2 hours or so we get a false positive, and it can't be the magnet, it's way above specs, and when it was still in breadboard form we wrote off the false positives as bad connection on the trigger capacitor, but now (buzzer7.mees.intuitief) I put it on vero (strip) board, so it can't be a bad connection on the vero board side, and it can't be a bad magnetic connection, so why do the gods not favour us with false positives? Any ideas?

PS buzzer7.pdf is the scematic for this project (my first)
555s are prone to glitches if you don't place a strong capacitor between their Vcc and ground pins. I'd suggest one with a value of at least 10 or 22µF
 

cmartinez

Joined Jan 17, 2007
8,220
BTW, reed switches are quite sensitive to vibration. If your gate is not firmly anchored and the wind (or some other influence) makes it vibrate, then it might trigger false positives. You could replace the reed switch with a hall sensor and that would pretty much solve things, if that's what's happening.
 

Thread Starter

Chillum

Joined Nov 13, 2014
546
BTW, reed switches are quite sensitive to vibration. If your gate is not firmly anchored and the wind (or some other influence) makes it vibrate, then it might trigger false positives. You could replace the reed switch with a hall sensor and that would pretty much solve things, if that's what's happening.
Hall sensor? It's a very strong magnet, I shaked the gate vigorously and no false positive hope its the filter cap, I used 22uF, is that enough?
 

cmartinez

Joined Jan 17, 2007
8,220
Maybe your cap is connected incorrectly. It should be connected between R2 and pin 2, and not between the switch and pin 2.
Check the figure on the right:

Mono-Stable-One-Short-with-555-timer-circuit.jpg

EDIT: You can find the circuit's explanation here.
 
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cmartinez

Joined Jan 17, 2007
8,220
What might be happening is that you're leaving the cap's negative "floating" and it's charging over time with electric static, until it triggers the 555
 

cmartinez

Joined Jan 17, 2007
8,220
BTW, that last circuit toggles the relay, it's just an example.
If you implement these changes, please let me know how it went... I'm curious... :D
 
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Thread Starter

Chillum

Joined Nov 13, 2014
546
Maybe your cap is connected incorrectly. It should be connected between R2 and pin 2, and not between the switch and pin 2.
Check the figure on the right:

View attachment 79904

EDIT: You can find the circuit's explanation here.
but its the same, except for the extra resistors and diode
edit: are you referring to the 01uF cap, the one between the resistors, do I need those?
 

Thread Starter

Chillum

Joined Nov 13, 2014
546
Oh OK, thx!
On the veecad layout the polarity of CsinglePulse is correct, this is how it is on my vero board (buzzer8.1.pdf)... do I still need to move that wire (The polarity was *sigh* a typo, but I wired the polarity correctly on VeeCAD -- veroboard cad for veroboard masks)
 

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cmartinez

Joined Jan 17, 2007
8,220
On the veecad layout the polarity of CsinglePulse is correct, this is how it is on my vero board (buzzer8.1.pdf)... do I still need to move that wire (The polarity was *sigh* a typo, but I wired the polarity correctly on VeeCAD -- veroboard cad for veroboard masks)
Yes you do... I didn't correct the polarity when I posted that last image
 
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