Resettbale alarm

KMoffett said:

Hero,

I think kwame wants the alarm to sound when he looses "mains" power, not battery power....or when the temperature exceeds the high and low limits of 36C or 39C. My guess is that the loss of mains power is all or nothing.

Ken

Click to expand...

KMoffett said:

Hero,

I think kwame wants the alarm to sound when he looses "mains" power, not battery power....or when the temperature exceeds the high and low limits of 36C or 39C. My guess is that the loss of mains power is all or nothing.

Ken

Click to expand...

Hero999 said:

Here's my idea.

Although it may look complicated, it's pretty simple.

The circuit consists of three parts:

1) Window comparator (U1A & U1B)
2) Standard voltage comparator (U1C)
3) Bistable (U1D)

The outputs of the window comparator and the voltage comparator are OR'd so if either the temperature range is exceeded, or the voltage (Vmonitor) drops below about 10V, the bistable will be triggered, sounding the alarm.

There are two switches: a test which will trigger the alarm (this is optional) and a reset which is self explanatory.

Note: ignore the values of R1 to R5 which will depend on which thermistor you decide to use.

U1 should be the LM339 or a similar open collector comparator.

EDIT:
Here's a link to a website which the building blocks of this circuit:
Voltage Comparator Information And Circuits

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Hero999 said:

I don't know about your part of the world but in the UK there's RS Components, Farnell, Rapid electronics, Maplin and many others and in the US there's Digikey, RadioShack and many more.

I've specified fairly standard components: the resistors are all carbon film 1/4W 5% tolerance, the IC is an LM339 quad comparator, Tr1 is a generic PNP transistor such as the BC327 and the sounder can be any piezo alarm sounder providing it doesn't draw more than about 200mA, for higher currents reduce R14, if you need over 500mA or so, use a Darlington pair or a MOSFET.

As I said before, you'll need to select and appropriate thermistor which will determine the values of R1 to R5.

Click to expand...

Hero999 said:

I don't know about your part of the world but in the UK there's RS Components, Farnell, Rapid electronics, Maplin and many others and in the US there's Digikey, RadioShack and many more.

I've specified fairly standard components: the resistors are all carbon film 1/4W 5% tolerance, the IC is an LM339 quad comparator, Tr1 is a generic PNP transistor such as the BC327 and the sounder can be any piezo alarm sounder providing it doesn't draw more than about 200mA, for higher currents reduce R14, if you need over 500mA or so, use a Darlington pair or a MOSFET.

As I said before, you'll need to select and appropriate thermistor which will determine the values of R1 to R5.

Click to expand...