Low Voltage Alarm 2

[elfvenlord]

Hi everyone.I have come across this basic low voltage alarm but im not too sure on how it works.

I know that when the first transistor is saturated the second one switches on the buzzer,but how does the current to the base of the first transistor increase as the voltage of the battery drops.I know it has something to do with the zener diode.Could anyone kindly help explain how the first transistor works.Thanks Allot.

 

[Russlk]

The first transistor (the one connected to the zener) is normally turned on, holding the second transistor off. When the voltage drops below about 4 volts, the first transistor turns off allowing the 2nd transistor to turn on and sound the buzzer.

 

[John Sorensen]

Why did you start a new topic?

My explanation in the original post and Russlk's agree. If you had questions, you could have replied in the original post.

j.

 

[elfvenlord]

Hi

Sorry john I checked out the original post yesterday but nobody had replied yet so I thought nobody had seen it.So I started a new one,we must of crossed paths.But anyway thanks allot for your reply guys.
I kinda see the light now and Im definitly gonna take your advice john and experiment with the zener diode.Thanks again.

 

[chemelec]

The Biggest problem with that circuit and most others is they draw a lot of current, Pulling down your battery even faster.

There are Special IC's that are designed for low battery detection. They draw Very Low Power and run a Pulsed LED, also at low power.

Gary

 

[elfvenlord]

Thanks Gary,I know what you mean.I have found another similar circuit that uses a voltage comparator,which I posted the other day.You might of seen it.I think that one uses less current than this one.

 

[Nigel Goodwin]

Thanks Gary,I know what you mean.I have found another similar circuit that uses a voltage comparator,which I posted the other day.You might of seen it.I think that one uses less current than this one.

I don't think it really matters much, the circuit above probably only takes about 1mA or so (when not activated) - that's a pretty small consumption compared to the rest of your RC gear.

 

[John Sorensen]

The Biggest problem with that circuit and most others is they draw a lot of current, Pulling down your battery even faster.

I've always thought that was weird that the low battery indicators light up (or buzzes) when the battery gets low. I mean, isn't that when you want to conserve power? I've seen some circuits that an LED flashes intermittantly on a low battery and draws nearly no power.

Why did you start a new topic?

The other thread was hoppin' that's why I thought it was weird you started another one :wink:

Good luck with your experiments (I did this same experiment, that I described to you, and thought it was very interesting to see how zeners actually work, which is quite different from the ideal zener model.).

j.[/list]