Creating a 3v trigger when a constant 12v signal disappears

Thread Starter

MKPINN

Joined May 8, 2020
5
I have a constant 12v signal which on a brief interruption rings a bell. I wish to provide a signal to a remote bell (which works on 3v ) when the 12v falls to 0v. Please can you suggest a suitable way ? The currents involved are negligible .
 

Bernard

Joined Aug 7, 2008
5,524
Just thinking out loud. Connect a large cap. thru a diode to supply bell power & a 555 timer. When 12 V falls, 555 is triggered, falling 12V output inputs to a DC to DC converter , 3 V output rings bell.
 

Thread Starter

MKPINN

Joined May 8, 2020
5
Many thanks to both of you for swift replies.
The output goes to a remote rf trigger which works on a CR2032.
And thanks for the second suggestion , perhaps i can use the constant 12v to charge the cap and then when the fall in 12v triggers the 555 , use the cap to supply the remote rf trigger with 3v ?
 

Thread Starter

MKPINN

Joined May 8, 2020
5
So you want this circuit to provide a button press on the remote? How long do you need the button pressed? Do you have access to the switch contacts?
Yes please ! I havent exactly timed it but i would estimate about 200ms should do it as a very fast press and release triggers the bell. And i do have access to the switch contacts.
 

BobaMosfet

Joined Jul 1, 2009
1,108
I have a constant 12v signal which on a brief interruption rings a bell. I wish to provide a signal to a remote bell (which works on 3v ) when the 12v falls to 0v. Please can you suggest a suitable way ? The currents involved are negligible .
2N3905 + 4N35
 
Last edited:

MisterBill2

Joined Jan 23, 2018
6,071
I have a constant 12v signal which on a brief interruption rings a bell. I wish to provide a signal to a remote bell (which works on 3v ) when the 12v falls to 0v. Please can you suggest a suitable way ? The currents involved are negligible .
How complicated do you want this circuit to be? Is the 12 volts just on an open wire and then disconnected. Or is it powering something and then a switch is switched off and it drops to zero. How much current is flowing in that 12 volt circuit until the voltage drops to nothing? For some circuits running 12 volts DC I could do it with a diode and a capacitor, but your application may be different.
 

Thread Starter

MKPINN

Joined May 8, 2020
5
Hello all
I finally had access and It turns out i had made a mistake while measuring :( . The circuit is triggered when a 12V , 0.37 ma signal is activated ( Not reduced to 0V as previously stated ) And the remote i need to trigger requires a 3V , 50micro amp signal .

Am i right in saying all i require is a diode and a resistor to bring the voltage down and limit the current ? I would be grateful if you can direct me to the right circuit. Many thanks.
 

MisterBill2

Joined Jan 23, 2018
6,071
If isolation of the common terminals of the circuits is not required then a simple capacitive coupling along with a diode clamping circuit is all that you need. And possibly a series resistor as well as the series capacitor. So a 0.001mFd capacitor and 4 silicon diode in an arrangement where they are in series and conduct to limit the positive amplitude of the pulse.
And you may also need to add a diode in series with the capacitor to avoid a negative pulse being generated when the 12 volt signal is switched off. I had neglected to consider that part of the function.
The common term for such a circuit is "differentiator", since it converts a rising edge into a step change in level.
 

ci139

Joined Jul 11, 2016
1,463
there are two timings involved (that likely must be trialed out ... making suggesting the solution a bit . . . tricky)

  • the OFF time or the minimum OFF time of the 12V side (that should trigger the bell)
  • the minimum ON time of the 3V side (that actually will trigger the bell)

otherwise you could load a small cap by 12V and keep bi mosfet -- one disabling another when the 12V is ON . . . so when 12V fades the mosfet to 3V allows the cap to discharge on the "trigger pin" (bringing it high for a while) ... it might be not the best way to do it
 
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