House doorbell button to double as a sound board trigger

Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
This is a separate from another thread I have on this device:

https://cdn-learn.adafruit.com/downloads/pdf/adafruit-audio-fx-sound-board.pdf

It takes up to 5v inputs to trigger the pins for sound--3.3V preferred. One lead of your trigger hits the pin; the other connects to the board's ground.

The house doorbell would make a good trigger, but it's around 20VAC (I've not tested it but I see the transformer back in the wall).

The thought was: a rectifier and then step down the DC to 3.3v, on the input leg from the doorbell to the board.

Would a half-wave rectifier be sufficient for this or a full wave? I ask because it only need 125ms of trigger time on the board to activate a pin.
 

MrChips

Joined Oct 2, 2009
19,768
Your doorbell works on a different circuit (24VAC) from your FX (5VDC).
Use a mechanical relay or an opto-coupler to trigger the FX from the doorbell.
 

Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
Your doorbell works on a different circuit (24VAC) from your FX (5VDC).
Use a mechanical relay or an opto-coupler to trigger the FX from the doorbell.
Right, that's why I was looking at rectifying then stepping down to 3.3v. That's a lot of work, and I'm still not sure the wiring would be possible.

What type of mechanical relay could I put in that doorbell button (if that's what you meant)? Did you mean, a 24VAC relay that would close the DC circuit?. Never even heard of an optocoupler but i'm looking at them now.
 

MaxHeadRoom

Joined Jul 18, 2013
19,461
I would look at an opto also, something like the 4n35, you would only need a small bridge rectifier on the 24vac signal and the output would go direct to your micro with a pull up resistor.
The 24v input from the bridge would just require a series resitor into the 4n35 opto LED (60ma MAX)

Max.
 

Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
I would look at an opto also, something like the 4n35, you would only need a small bridge rectifier on the 24vac signal and the output would go direct to your micro with a pull up resistor.
The 24v input from the bridge would just require a series resitor into the 4n35 opto LED (60ma MAX)

Max.
How small a rectifier? Never built one.

You're saying rectify the doorbell, then a series resistor would be suitable for the opto?

Why a pull-up?
 

MaxHeadRoom

Joined Jul 18, 2013
19,461
A miniature bridge good for the 40ma or so for the Opto LED in the diagram the battery would be the AC and bridge, transpose the P.B. and resistor so R1 is on pin 1 and the P.B. would be the door bell push.
the pull up on pin 5 is usually needed to satisfy the input requirements of any semi or micro device.
Max.


upload_2017-9-30_13-27-37.jpeg
 

Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
I've tried looking at the pic you uploaded; saved or not it won't enlarge enough to read the details. Is the pull up resistor really needed? I can't find anything in their documentation that recommends it.

I'm having trouble deciding on the capacitor and diodes for the bridge. It's a 24VAC doorbell, 60hz, singlephase residential.

The input voltage on the trigger pins of the adafruit board should strive for 3.3v but not more than 5.

So I need to make 24VAC become 3.3v DC, or make 24VAC become some other DC and step it down. Then I can explore the opto route--though I have zero experience with those.
 

MrChips

Joined Oct 2, 2009
19,768
I've tried looking at the pic you uploaded; saved or not it won't enlarge enough to read the details. Is the pull up resistor really needed? I can't find anything in their documentation that recommends it.

I'm having trouble deciding on the capacitor and diodes for the bridge. It's a 24VAC doorbell, 60hz, singlephase residential.

The input voltage on the trigger pins of the adafruit board should strive for 3.3v but not more than 5.

So I need to make 24VAC become 3.3v DC, or make 24VAC become some other DC and step it down. Then I can explore the opto route--though I have zero experience with those.
Yes, you need a pullup resistor, in the range 1k to 10kΩ. 4k7Ω will do fine.

You do not need to make 24VAC become 3.3VDC.
You do not need a bridge rectifier nor a capacitor.

Connect a 1N4001 rectifier in series with a 4k7Ω resistor to drive the LED of the opto-coupler.
 

philba

Joined Aug 17, 2017
960
I agree with the opto. I would also add a capacitor across the opto inputs (probably 10uF) because the rectified AC will look like a series of pulses to the FX board and it's not clear how it would behave. An innocuous result could be to create a 60hz hum while the doorbell was pressed. Perhaps worse things could happen. The 4K7 resistor and 10uF cap would create a low pass filter with about a 3 hz cutoff, effectively eliminating the pulses. I'd use a 50V cap as you will get 33.6V peak pulses.

[edit] You might want to reduce the cap size a bit to increase responsiveness. maybe a 4.7uF would be better. You could also cut down the resistor to 3.3K. 4.7uF and 3.3K would give a 10 hz cutoff. [/edit]
4n35 doorbell.png
 
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Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
I've tried looking at the pic you uploaded; saved or not it won't enlarge enough to read the details. Is the pull up resistor really needed? I can't find anything in their documentation that recommends it.

I'm having trouble deciding on the capacitor and diodes for the bridge. It's a 24VAC doorbell, 60hz, singlephase residential.

The input voltage on the trigger pins of the adafruit board should strive for 3.3v but not more than 5.

So I need to make 24VAC become 3.3v DC, or make 24VAC become some other DC and step it down. Then I can explore the opto route--though I have zero experience with those.
 

Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
*update*

Got the fx board powered, but I'm having trouble figuring out a way to wire it. wiring each terminal of the pushbutton is just parallel to my fx board...it's powered full-time.

I need a way to wire it in series i think to only get power to my opto when the button is pushed....unless i'm overlooking something.
 

philba

Joined Aug 17, 2017
960
I sent you an idea to avoid rewiring the doorbell but spent a little more time on the model. Here it is. I don't have a 4N35 model so I used a 27 but don't believe there will be much difference. The voltage input on the left comes from your doorbell button switch. When the button is not pressed, it is powering the circuit and keeping the 100 uF cap charged. When the button is pressed, the voltage falls to zero and turns on the PNP. The cap provides enough power to run the opto's LED for around 100 mS. R3 is the pullup resistor on the FX board. Note that my experience with the 4N35 LED is that it's good to around 4 mA so the simulation which shows it working down to less than 1 mA is probably wrong - could be because the 4N27 that I used. Using 4 mA as the cutoff point, it's more like 50 mS which should still be enough to trigger the FX board. Increase the cap if it's not enough. And, of course, adjust R1 if the voltage is different from 12V.
pnp doorbell.png
 
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