Parasitic power supply for Pi Zero W Doorbell?

[Mark Benson]

Anyone have any idea how to extract enough power from a wired (light up) doorbell in a parasitic fashion, to power a pi? Neil are you still lurking on the message board? Wondered if you might have seen a similar approach anywhere?

The transformer is 16vac rms.

I know I could tap directly off the transformer and save myself a whole heap of pain, but was trying to minimize the need to run extra cable.

Currents as measured in the open circuit state (30mA) and the closed circuit state (950mA).

Basic doorbell circuit looks something like this: (Note: AC is the power supply and the transformer represents the solenoid/chime. The doorbell button has a switch with a lamp across it. The lamp is 55 ohms.)

[John Morrison]

Hi Mark

What are you trying to do? Detect the bell ringing or something more? Have you thought about wrapping an induction wire around?

J. 

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[Mark Benson]

Hi John,

The aim is to detect the bell push and power the pi zero just by splicing into the circuit or across the button.

Detecting the button press will probably be the easy bit. (There is a big voltage drop). Keeping the pi powered while the button is pressed for any length of time is another issue!

With the pi powered from a separate power supply I can detect the button press with an opto isolator.

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[mindthegoat]

Does it have to be a pi? As you draw more current from the circuit its
likely the bell will.start to ring. Something with lower power would be
preferable. You could use the quiesent current to charge a battery that
could then power your circuit when the button was pushed. Run the
circuit in a low power mode most of the time, the battery would be able
to supply heavier currents when required.

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[Mark Benson]

I chose the pi zero w because of the camera. I want to be able to see who is at the door (so far it's just an email with a photo).

I haven't measured the pi zero w current consumption with the camera and WiFi running (will try that tomorrow) but I'm not expecting it to exceed 300mA @ 5v.

I like the idea of using the quiescent current to charge a battery (which would solve the problem of power if the button is held down too long) but assuming the pi is powered down, it would take too long to boot up in time and take a picture unless it has a sleep state?

I was messing with a power resistor voltage divider to try and tap off enough power to supply a DC/DC converter to power the pi (more efficient than a linear regulator) but have only been able to get enough power when the button is pressed or conversely not enough power to ring the bell properly. Either way I don't like that approach as it's wasting power.

Was wondering about a buck converter... Maybe that could step up the available voltage?

[Mark Benson]

Hi Rob!

Sorry, thought that last message was from John!

How's life treating you?

Have you found the time to work on projects or is running a campsite all consuming?

Are you going to Maker Central next year?

On 28 December 2017 21:31:57 GMT+00:00, mindthegoat <rob...@mindthegoat.plus.com> wrote: