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Topic: the right capacitor? (Read 1 time) previous topic - next topic


I've not used a capacitor in any of my experiments/projects yet and wondered if introducing one here would help conserve battery life.

I plan to install a basic 'alert' system in my shed which will tell me if the door is opened through the night. I have purchased a magnetic door switch from flee-bay and hacked this into the switch of a wireless door-bell transmitter.

When the door is opened, the reed switch closes the circuit and the battery sends current through the  RF transmitter and I'm abruptly woken by Green Sleeves chimes   :smiley-eek-blue:
The problem, If I have the door open for any length of time, I imagine the battery will drain pretty quickly since the circuit will be closed constantly.

I was wondering, if I swap out the reed switch for one that closes the circuit while the shed door is closed, the battery could perhaps charge a capacitor? On opening the door, the switch would open and the capacitor would discharge via the RF transmitter. This would happen only once per door opening. Would this work? or would the capacitor cause constant drain?

My challenges:

Where do I position the capacitor to ensure it discharges via the RF transmitter and not directly into the battery.
How do I work out the type and size of capacitor I need? The battery that powers the doorbell is 12v(23A). Does the picture of the circuit give any clues?

I've attached an image to help clarify...


You'd need something to disconnect the capacitor from the battery when you transmit.  Capacitors also have a discharge curve opposite from what you get from a battery...  The voltage drops quickly at first and then levels-off as approaches zero.  Capacitors can be used as batteries if you have very-low current and/or lots of "extra" voltage and an efficient voltage regulator.

With a MOSFET (or transistor, or relay) and some timing in your sketch (look at the LED blinking examples) you can turn-off the transmitter after a pre-determined amount of time.


Thank you for you reply Doug.

Do you mean for me to use the discharge from the capacitor as the switch/gate in a MOSFET? If so can you offer any advise on size of capacitor and resistance on the part of the circuit that would 'switch on' the transistor?


But then if you missed the alarm when the door opened (visiting the pub?) you wouldn't know when you got home whether the door was open or closed would you?


Hi, I'd walk past the shed and hopefully notice. This is where the arduino will come in later. Eventually I intend to hack the receiver, im thinking possibly sms message, or hooking up to a camera of sorts...

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