buzzer when ground removed
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=121251
Printed Date: May 11, 2025 at 1:23 AM
Topic: buzzer when ground removed
Posted By: justinthurn
Subject: buzzer when ground removed
Date Posted: April 09, 2010 at 3:53 PM
I need to wire up a relay to sound a buzzer when a ground is removed. The ground is normally there, but when it is removed and the wire is neutral, i need a buzzer to sound. I am searching for something but havent found it yet. any help is appreciated.
Replies:
Posted By: justinthurn
Date Posted: April 09, 2010 at 4:32 PM
Posted By: icearrow6
Date Posted: April 09, 2010 at 5:31 PM
The above will work, if the buzzer needs a +12v when the ground is removed.
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Posted By: oldspark
Date Posted: April 10, 2010 at 4:15 AM
FYI:
If the buzzer is a low current device, you can connect one end to GND and it's +ve end to the grounding switch and through a resistor to +12V (etc).
The resistor is big enough to not cause excessive drain (or heat) when the switch is grounded (where resistor current is V/R - eg, 12V divided by R Ohms and power/heat is V**2/R = VxV/R) but small enough to allow sufficient current through the buzzer.
The switch connects the buzzer's +ve end to GND, hence off. (And connects the resistor from +12V to GND.)
The switch opens and hence allows the buzzer +ve to be pulled up to +12V.
(EG - a 15mA buzzer (3-16V etc) could use a 470 Ohm 1/2W resistor. The buzzer voltage (switch open) at 12V would be 5V (7V is dropped across the 470R resistor).
The switch-closed resistor dissipation at 15V is 0.47W.)
BUT - if that resistor/buzzer effects other circuits that use that switch, it may not suit.
And if the buzzer doesn't work well at 5V....
Or if the buzzer is of higher current, then smaller resistors meaning more heat are required - in which case relays etc are better.
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