12V power supply failure alarm

neildawes · 08-02-2010, 10:24 AM

Hi,

I need a simple alarm ( buzzer) to operate when the 12V CPU fan in my incubator stops working.

A simple solderable circuit diagram and shopping list would be great. I guess I need a relay circuit and the buzzer to have a separate power supply but I need help with the layout details.

Cheers,
Neil

zpm · 08-02-2010, 11:31 AM

Plug the incubator into a UPS. They have built in alarms and will keep the incubator running during short term power failures.

VBT · 08-02-2010, 11:47 AM

I think what your looking for is a battery monitor circuit. Here's one that should be adaptable to your needs. You would only need the relay to operate an alarm instead of disconnecting the circuit on this one.

Yoyizit · 08-02-2010, 12:22 PM

Quote:

Originally Posted by neildawes

when the 12V CPU fan in my incubator stops working.

I'd go with a photodiode and emitter that puts light through the blades. When the light/dark pattern stops the alarm sounds.

Another way would be a fast responding temp. detector, using a very low thermal-time-constant thermistor or temp. sensing diode.

Or a small microphone next to the upstream side of the blades; no noise = alarm.

mpoulton · 08-02-2010, 03:40 PM

Quote:

Originally Posted by Yoyizit

I'd go with a photodiode and emitter that puts light through the blades. When the light/dark pattern stops the alarm sounds.

Another way would be a fast responding temp. detector, using a very low thermal-time-constant thermistor or temp. sensing diode.

Or a small microphone next to the upstream side of the blades; no noise = alarm.

Most CPU fans have three wires, and the yellow one provides an AC signal that indicates rotation speed. There's no need for external sensors, just read the AC voltage on this wire.

neildawes · 08-02-2010, 06:52 PM

Hi and thankyou for your replies,

However I am still after a simple circuit that I can wire and solder together.
The circuit should sense that the power supply to the fan is not working and cause a buzzer alarm to sound.

Something along the concept in the diagram below.

Many thanks,

Neil

Yoyizit · 08-02-2010, 07:53 PM

Quote:

Originally Posted by neildawes

Something along the concept in the diagram below.

Pick a 9 vdc buzzer
http://www.hosfelt.com/contents/en-us/d475.html
pick a relay with a 12 vdc coil and double throw contacts
http://www.hosfelt.com/contents/en-us/d111.html
wire the buzzer in series with the 9v battery in series with the normally closed contacts of the relay.

neildawes · 08-02-2010, 10:04 PM

There is an error in the diagram above , Sorry,

Neil