I've got a smoke detector that I'm feeding 9V to but through a JFET. The detector is receiving the 9V but chirps as if there is a low battery. Removing the JFET from the circuit stops the chirping.
Anyone have any ideas on why?
Cheers.
taintedsushi:
Anyone have any ideas on why?
Not without knowing HOW and WHY you did this.
Post a diagram, with parts values.
Leo..
I want to be able to turn off a smoke detector via a signal from the arduino.
I have a 2n5457 JFET. It's hooked up as (crappily) pictured in the attachment.
Thanks.
That jfet could drop ~2.5volt and limits current to ~3mA.
No wonder your smoke alarm chirps.
Could use a small mosfet, like the 2N7000, with a high value resistor (>100k) to +batt to keep it 'on'.
And a second transistor between gate and ground to actively turn it off (I assume with an Arduino pin).
Why do you want to disable a smoke alarm.
Leo..
Thanks Leo.
That makes sense. I am trying to minimize the drain on the smoke detector battery but perhaps I need to supply power to the arduino separately in order to accomplish what I want.
My understanding is that a MOSFET will require constant voltage at the base in order to keep the system active. My plan was to have a JFET to keep the system active and the arduino mostly idle.
Could I simply use a transistor instead of a MOSFET for this as well?
So you're mains-powering an Arduino to save the smoke alarm battery.
It might be better to mains-power the smoke alarm with a small 9volt regulated supply.
mains<>battery switchover could be done with a 2-diode 'OR'. With schotky diodes (to minimise volt drop).
If you want to go the fet/switch way, the gate (not base) of a mosfet does not draw any current in any state.
The base of a bjt transistor does.
I assume you want the smoke alarm to work when the Arduino is off.
Therefore you need a second transistor (could be a mosfet too) to ground the gate (turn off) the switch mosfet.
Leo..
A mosfet is going to use much less power than a simple bjt.
Thanks for those suggestions Leo. Very helpful!
Cheers.
Hi,
How long does a smokedetector battery last.
I replace mine everytime daylight savings kicks in and out.
Batteries are cheap compared to lives.
How do you know your smokedetector will be happy with pulsed supply, no matter what the frequency or duty cycle.
Tom.. 
I replaced my JFET with a MOSFET and used a signal from the Arduino to turn on the MOSFET and allow 9 volts to the smoke detector. The detector seems to be OK with this situation. Because I don't have a drain from the gate, even if the Arduino is disconnected, the smoke detector does not lose power which is what I wanted to happen.
Am I correct to understand that as long as I have voltage running from drain to source, that connection will be stable?
MOSFETs are pretty amazing.
*** To answer my own question, it would appear that the voltage does slowly drop over time. Disconnected from the gate voltage, it was maybe an hour or so before the detector was no longer getting enough juice and started to chirp at me. Reattaching the gate fixed it.
Hi,
How often do you turn the detector ON for and how long OFF?
Have you tested it with smoke.
How much battery life do you expect to get?
Tom...
Hey Tom.
I was hoping to be able to turn it off and then automatically reactivate after 15 minutes (enough time for the air to clear from cooking). I have not tested it with smoke yet. My intention was to have it remotely deactivated via bluetooth but it's looking like power is going to be a problem and my battery life will be pretty crappy. I may need to get my alarms hooked into the mains before this is really feasible (it also makes using a JFET much easier).
I do want to make this project work so I'm not quittin' yet.