How to shorten the signal to my doorbell

[Sulfur2996]

My camera surveillance system NVR has two "alarm" connectors which is a N/O switch. I routed wires from/to my doorbell chime so NVR motion detection can ring my unused rear door chime "dong". The front door chime does a different "ding dong" so I can tell the difference.

It works but the problem is the NVR closes the N/O switch for a hard-coded ~60s and the doorbell chime and 20VAC doorbell transformer is humming the whole time and something's going to blow so I don't use it.

I need to add an inline N/C switch (or something) that opens itself after a short time 0.75 to 1.0s to simulate the release of the doorbell button after a normal human button push delay. Can you give me a tip like "build/buy a self resetting 20VAC N/C thingamajig" to point me in the right direction?

Other people online asking for this get no answer.

I'm mostly a SW guy but tinker with HW too.

[tmd63]

Probably want a pulse shortener using a 555 timer.
check falstad circuit simulator java, I believe has has 555 timer circuits there.
Or check this out
https://www.edaboard.com/threads/pulse-shortening-circuit.297254/

[ajb]

There's an easy off-the-shelf solution: a timer relay.  These are standard automation components, and can often be configured for multiple operating modes with a wide timing range.  The mode you'd want is usually called "one shot". 

There are cheaper options than these, but AutomationDirect has a lot of options with very good documentation (not sure if they ship to Canada): https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/timer_relays

These instructions in particular have a nice description of the various configurations, but many/most multifunction timer relays will have similar options: https://cdn.automationdirect.com/static/manuals/fujitimersm/trm10installation.pdf

[Sulfur2996]

I'll try the off the shelf configurable timer relay idea. Had no idea these things existed. Thanks! 

[NiHaoMike]

A simple solution would be a series PTC. Measure the current and select a PTC with a substantially lower trip current and a much higher voltage rating than your circuit voltage. (Repeated operation of the PTC near its voltage rating causes it to degrade.)

[PCB.Wiz]

I need to add an inline N/C switch (or something) that opens itself after a short time 0.75 to 1.0s to simulate the release of the doorbell button after a normal human button push delay. Can you give me a tip like "build/buy a self resetting 20VAC N/C thingamajig" to point me in the right direction?

If you are ok with a PCB module, there are shiploads of what look like cloned timer relay variants from Aliexpress.
Some use Nuvoton N76E003 MCUs so you could completely reconfigure if you wanted to, but the default of trigger -> time relay pulse sounds like what you wanted anyway.

LED models
https://www.aliexpress.com/item/1005006151283961.html
and more recent are LCD models
https://www.aliexpress.com/i/1005004189362865.html

or, you could experiment with a simple series cap / resistors that give a short decaying pulse on the leading edge.
If it is a NO relay drive, a parallel drain resistor across the Cap discharges it.
Needs to be large enough to not trigger the bell by itself, but low enough to give a reasonable retrigger time, tho your fixed 60s may limit that anyway.
 

[Sulfur2996]

To conclude I wound up buying an GRT8-B1 on aliexpress. It works perfectly and the delay to hear the chime without hearing any extra humming turned out to be around 0.25s (analog screw adjustment) so shorter than initially expected.

I'm impressed with the depth of knowledge on these forums. Thanks to all for helping an amateur electronics tinkerer.

[soldar]

Timer Relay Delay Switch Module Adjustable Trigger Cycle Timing Circuit Switch

https://www.ebay.com/itm/294579278113

[Sulfur2996]

Bonus Question:
How to implement a delay configurable hush button on the doorbell unit?

My smoke detectors have a hush button that will silence the alarm for ~7 minutes (for poor cooking skills...)

If a family member is temporarily doing some activity in a location triggering repeated doorbell rings, a hush button on the doorbell unit would be nice to have. A physical on/off toggle switch would be too easy to forget to turn back on as would turning off "Alarm Out" in the NVR settings.

The delay off timer relay is next to the NVR in a hard to reach location. The configurable hush button would be separate and would be on/near the doorbell unit and must handle allowing/suppressing the same 20VAC incoming signal.

[themadhippy]

go ole skool and use a pneumatic time delay switch

[ajb]

If a family member is temporarily doing some activity in a location triggering repeated doorbell rings, a hush button on the doorbell unit would be nice to have. A physical on/off toggle switch would be too easy to forget to turn back on as would turning off "Alarm Out" in the NVR settings.

The delay off timer relay is next to the NVR in a hard to reach location. The configurable hush button would be separate and would be on/near the doorbell unit and must handle allowing/suppressing the same 20VAC incoming signal.

Sure, you could just use a second timer relay.  Put its contacts in series with the one you already have, but instead of using the common and 'normally open' contacts, use the 'normally closed' contact.  That will allow the NVR timer to work normally and trigger the doorbell as long as the hush timer is inactive.  When the hush timer is activated, it will effectively disconnect the NVR timer, preventing it from activating the doorbell.

[Sulfur2996]

Sure, you could just use a second timer relay.  Put its contacts in series with the one you already have, but instead of using the common and 'normally open' contacts, use the 'normally closed' contact.  That will allow the NVR timer to work normally and trigger the doorbell as long as the hush timer is inactive.  When the hush timer is activated, it will effectively disconnect the NVR timer, preventing it from activating the doorbell.

Oh that's an elegant solution! You even drew it up. Thanks!