Hi All!

I want to know when my battery is at 14V. I can use a comparator using a reference voltage to get a change in state on the output when 14V is reached. I need to convert that change in state to a 1 second pulse to drive a mosfet. What circuit do I use? Maybe there is One that will give me a 1 sec pulse when ever there is a change in state from low to high or high to low??

Thanks!

Joster

 

Use an opamp comparator to detect the voltage, and feed the op amp into a 555 astable to get the pulses.

 

ok thanks for responding!

would the astable 555 not just keep giving pulses? I only want a pulse if the output of the comparator changes state. So I need a pulse which is activated by either a rising edge or a falling edge...

 

The op amp would give a pulse out on detection, if you want to extend it then use a 555 monostable .

 

A monostable multivibrator is known as a "one shot".

The 555 normally triggers on a negative going edge. I'm trying to recall how to get a state-change pulse. Using another 555 is the brute force solution. I think there's a more elegant approach.

 

ok...but the battery is going to hit 14V and keep climbing up until I turn the charger off so the output of the op amp will be high once the battery gets to 14V and then remain high...see what I mean?

 

ok...but the battery is going to hit 14V and keep climbing up until I turn the charger off so the output of the op amp will be high once the battery gets to 14V and then remain high...see what I mean?

So whats the mosfet for then?

 

I am connecting it to the input of my alarm system. when the input of the alarm system sees a short I get an email that by battery is charged. So really I just need to short the two terminals of the alarm input together... I was thinking of using a relay for that...hense the mosfet or bjt...

 

ok so use the comparator to turn off the charger using a relay, as well as triggering the alarm then.

 

If you use the monostable, it will give you a single pulse out.

 

now there is an idea! i think thats the winner!

I dont think the monostable is edge triggered is it?

 

yes its negative edge triggered, take pin 2 to 0v to trigger

 

I think you'll need an input capacitor to couple your signal. See the circuit on the right in the following link:



http://iccomponentsmall.blogspot.com/2012/08/how-to-build-555-timer-mono-stable-one.html

EDIT: Link Corrected.

 

I need a solid bat ref but a 10V zener across the battery doesn't work like i thought it would...I need a solid vref for the op amp and want to do it from the bat. the bat will be anywhere from 11v to 14v but i need 10v vef. tried two 5v zeners and current limiting resistor...tried without the resistor still no luck...am i going to have to power it off another supply??? hopefully not

thanks

 

What didn't work?

 

Never use a zener without a current limiting resistor.

 

i thought that i could get a solid 10v reference voltage across the battery with the two 5v zeners and a resistor?

 

What did you get? What was the value of the resistor? Why make us guess?

 

the resistor is 6.8k

voltages measured are in pic

 

Connect the components in this order: Resistor, Zener, Zener. Like this

V+
R
Z
Z
gnd.

Now take the reference voltage between the resistor and the top zener. You might try a lower value resistor too. Try 300-1k