How can I trigger a device using a momentary tact switch already on another device?

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BrandonB

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How can I trigger a device using a momentary tact switch already on another device?

I have a smart device that has a N/O momentary tact switch on its PCB.
I would like to use this switch to remotely trigger a remote bell.
The remote bell can be triggered using a momentary switch to connected to its trigger inputs.

When I soldered wires to the smart device tact switch and then connected those wires to the trigger inputs of the remote bell it is triggered as soon as both wires are connected... without the tact switch being pressed and closing the circuit.
The remote bell cannot be triggered again until a wire is disconnected and then reconnected to its trigger inputs.
It is acting as though the tact switch is closed even though it has not been depressed.

When I use my DMM across the SMD legs of the tact switch I get the following:
(The readings were obtained with the smart device disassembled and powered off. There is a button cell onboard to retain settings.)

continuity
open = 2.428
closed (pressed) = 0.002

resistance
open = 5.66 kOhm
closed (pressed) = 0.0 Ohm

smart device tact switch


I have looked into simple mosfet and relay circuits to act as the middle man but I'm not really sure what would be the most reliable.
I have tested the smart device with a relay and it DOES NOT trigger the relay until the tact switch is pressed.
I don't think it would be a good idea to send any additional voltage through the smart device tact switch as I do not want to interfere with its original use or fry something in the smart device.
Is there a diode or something that I could add to the wires I connected to the smart device tact switch to prevent the remote doorbell from being triggered until the tact switch has been pressed?

Other details
The smart device has its own power supply (24VAC).
The remote bell has its own power supply (6VDC).
The devices are about 50 feet apart.

I did some further testing with a 12VDC power supply, a 12VDC mini SPST relay, and the smart device... I don't have the remote bell with me right now so I cannot fully test what I had been working on or what you recommend right now.
When I connect the smart device tact switch to the relay it does not trigger the relay until I press the tact switch.
After I release the smart device tact switch, the relay does not open.

Here is a diagram showing how I have it set up:


What route do you recommend I take?

Is there a way I can add a resistor and a capacitor (or something) that would draw down any voltage that is holding the relay closed?

Thank you all for your help!
 
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BrandonB said:
Would the constant 3.4V points on the SMD parts near the tact switch on the Reolink Video Doorbell (marked in yellow in the most recently posted picture) work with the opto and the resistor like you had mentioned?
Click to expand...
Yes, that should work fine.

One of these is probably suitable, but to be safe you need to find the current taken by the switch on the bell unit.


The LED of that, in series with a resistor to set the current, would connect between the 3.4V supply and the switch output. That would turn the LED on when the switch was pressed and grounded the output pin.

Either a 2k2 or 1k resistor should be appropriate. That would set the LED current at approximately 10mA or 20mA, respectively.

With that exact opto isolator, with a minimum current transfer ratio of 300%, the transistor within it will turn fully on when the LED is powered, as long as the current through the transistor is not too great; up to 3 x higher than the LED current.
 
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I think it would be too risky to try to solder a wire to the point that you showed that had 3.4 volts in post #18.
I don't think Pommie's suggestion of a reed relay in parallel with the push button would work as the pull up resistor for the logic input will probably be 10 K or higher. So the current available would be less than 0.34 mA which is not enough to drive a relay coil. It coud be used to drive the gate of a logic level mosfet such as a 2N7000 or a comparator IC such as an LM393. (If uou used a mosfet you would actulay need to use two to get the switching sence correct.) Both of these solutions would require a power source. you could get this from the smart device or the 6 volt battery on the bell.
I have just read through the posts again and see that you tried connecting the remote bell trigger inputs directly to the push button. As the signal levels on both devices is almost the same on both devices (Low zero on both devices, high 3.4 volts on the smart device and 3.069 on the bell.) I think it should have worked.
As Pommie pointed out in post #4 you may have had the polarity of the connections the wrong way round. The ground side of both devices MUST be together. You have not commented on this point. If you have confirmed that the polarity was correct then you could try adding a small diode (eg an 1N4148.) in series with the series with the T input to the bell with the anode end of the diode to the T input.

Les.
 
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I am pretty comfortable with soldering SMD components.
Are you only warning against doing this because of the tight proximity to the other components in the area?

I really would've liked to make this work without a relay or any other components but I have been unable to figure it out with the basic knowledge that I have.
I thought of using a relay because I assumed it would isolate the voltage from the tact switch away from the trigger inputs... which it does, but it does not allow the relay to open after it has been closed.

Yes, I forgot to mention on the most recent reply that I think I have tried all combinations of connecting the wires I soldered to the Reolink Video Doorbell tact switch to the trigger inputs of the iChime Doorbell.

The Reolink Video Doorbell and the iChime Doorbell do not share a ground for their power sources.
Their grounds are not connected until I connect the leg of the Reolink Video Doorbell tact switch and the iChime Doorbell trigger input - T (ground).

I will look to see if I have one of those diodes on hand and see if that does anything.

Thank you!
 
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I was assuming that you had very limited experience with electronics in general as you could not trace the etch on the board back to a larger component where it would be easy to make a connection. There is no technical reason for not soldering to the SMD device. Finding thin enough wire to make the connection might be a problem.

Les.
 
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I guess I have skills greater than my electronics technical knowledge.
I just don't understand electronics logic and only have basic knowledge of components to use for a project I am working on.
Usually, I just Google around for something someone else has done to complete a similar project.
Then I just tweak it for what I am working on.
This time I wasn't getting anywhere and I decided to ask for help.

The larger components are on the other side of the PCB, specifically, the larger SMD capacitors.
I believe all of those capacitors are reading 22VDC on my DMM.
The SMD components that are near the tact switch on the Reolink Video Doorbell are easy to access and I have thin enough diameter wire on hand.
 
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Les Jones said:
...you could try adding a small diode (eg an 1N1418.) in series with the series with the T input to the bell with the anode end of the diode to the T input.
Click to expand...

I do not have any 1N1418 diodes on hand.

Since I do not have any Zener diodes on hand, here's what I do have:
rectifier diodes
fast recovery diodes
ultra fast recovery diodes
super fast recovery diodes
schottky diodes
fast switching diodes
high speed switching diodes
Could any of them work?

Just for the heck of it, I tried a 1N4007 rectifier diode (I had tried this before) and it did not work.
When both wires are connected at both ends, the iChime Doorbell is not automatically triggered... that is good.
When the tact switch is pressed on the Reolink Video Doorbell the iChime Doorbell still does not trigger... that is not good.
Here is how I connected everything:

Obviously, if I flip around the 1N4007 rectifier diode and face the anode toward the tact switch (GND) on the Reolink Video Doorbell the current will not be blocked and the iChime Doorbell is automatically triggered and then cannot be triggered again until 1 wire is disconnected, then the process repeats.
 
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I had made two errors in post #22. I had typed 1N1418 when i meant to type 1N4148. (I had noticed this and corrected it a short time ago.) I also typed T input when I should have typed P input. Using a 1N4007 would work just as well as a 1N4148. If my theory had been correct then it should have worked with the diode in the ground wire but with the cathode end to the T input. As you have tried reversing the diode there is something wrong with my theory. I would be interested to know the voltage between the T and P terminals on the bell before the button is pressed, while the button is pressed and after the button is released. This is either with the diode in the black wire with the cathode to the T input or with the diode in the red wire with it's anode connected to the P terminal.

Les.
 
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I did have a 1N4148 fast switching diode on hand but it did not work (just as you suspected).

I went back through everything with my DMM and recorded the readings I got and depicted them the best I could.
All the readings were taken from trigger input P (+) (red DMM probe) and trigger input T (GND) (black DMM probe) on the remote bell (iChime Doorbell).

Here you go:


 
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I now think that the switching is not just a simple DC voltage change. I think that when the button is NOT pressed there are pulses on the side of the switch that is not at ground level. The smart device seems to detect that the bell is connected as it seems to change level when the bell is first connected. As it does this even with the diode in circuit it must be detecting a high level when it is expecting a low level. (With the diode in circuit the bell could not drive the non- grounded side of the switch low. As there is a difference of about 0,5 volts between the results with and without the diode in circuit it must be the smart device holding the level low. I like the way you presented the results. It was a great help to understand what was happening. I think the 3.4 volts is the average level and that the logic high of the pulses are greater than 3.5 volts. I think you will need an oscilloscope to investigate ant further.

Les.
 
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Les Jones said:
I don't think Pommie's suggestion of a reed relay in parallel with the push button would work as the pull up resistor for the logic input will probably be 10 K or higher.
Click to expand...

Regardless, I would prefer to complete this project without relays of any kind... I was just testing to see if that would work.

I would also like to stick with smaller components.
I have SMD components left over from other projects.

I like the idea of using an opto now that I have a little bit of an idea about what they can do.
My Amazon.com order for them should arrive tomorrow afternoon.
 
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rjenkinsgb said:
Either a 2k2 or 1k resistor should be appropriate. That would set the LED current at approximately 10mA or 20mA, respectively.
Click to expand...
A 1k resistor would give a current of 3.4mA and a 2k2 resistor just 1.5mA. For 10 mA you need ~300Ω and for 20mA ~150Ω. if you take the Vf (assumed 2V) of the LED into account then you need ~140Ω or ~70Ω. Or was you assuming a higher Vcc?

I'm sure it was just a typo but wanted to let the OP know.

Mike.
 
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Thank you, my diagrams with the simulated DMMs was the best thing I could think to depict what I saw without typing a convoluted list of stuff in paragraph form.
I'm very visual when trying to learn new things and it helps me to work things out that way.

Well, I did have a very basic oscilloscope that I assembled from a kit a few years ago.
I will look for it when I get to the office in the morning.

What would I need to look for when using the oscilloscope?

While disconnected from each other...
probe trigger input P (+) and trigger input T (GND) on the remote bell (iChime Doorbell)?
and...
probe the tact switch legs on the Reolink Video Doorbell?
 
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Pommie said:
A 1k resistor would give a current of 3.4mA and a 2k2 resistor just 1.5mA. For 10 mA you need ~300Ω and for 20mA ~150Ω. if you take the Vf (assumed 2V) of the LED into account then you need ~140Ω or ~70Ω. Or was you assuming a higher Vcc?
Click to expand...
Doh.. They are a decade off.
That's Pre-Coffee mental arithmetic for you.

The LED Vf is given as 1.2V, or 1.4V max so around 2 - 2.2V across the resistor. ~220 Ohms for 10mA and 100 Ohms for 20mA.
 
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Re post #33

For the smart device you would connect the scope probe ground to the ground side of the push button and the probe tip to the other connection on the push button. Set the scope for negative edge triggering. Trigger mode to auto. 2 volts per division. I have no idea what the pulse frequency will be so you will need to try different sweep rates. When you have a reasonable display on the scope you may need to set the trigger mode to normal and adjust the trigger level

For the bell connect the probe ground to T and the probe tip to P. Use the same scope settings as used on the smart device.

I think you are more likely to see pulses on the smart device than the bell.
 
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Thank you for the tips!

I had to dig the oscilloscope kit out of my electronics bin and I realized that I never even finished building the kit or tested it yet.
Anyway, I finished building it, went through all the testing procedures, calibrated it per the build guide, and played around with it for a while to try to figure out the interface.

I performed a test capture using your recommendations for the oscilloscope.
The scope probes were connected to trigger input P (+) and trigger input T (GND) on the remote bell (iChime Doorbell).
What you see happening in the screenshot is...
- the remote bell (iChime Doorbell) was standalone
- the scope probes were connected to trigger input P (+) and trigger input T (GND) on the remote bell (iChime Doorbell)
- the 3V line is the voltage from trigger input P (+) on the remote bell (iChime Doorbell)
- the drop is when I touched a wire (shorted) between trigger input P (+) and trigger input T (GND) on the remote bell (iChime Doorbell) in order to make the bell chime

Here is what I got:


I know the scope is basically a toy, but would this capture be similar to what we are looking for?
Or would it need to be zoomed in more?

I would like to make this oscilloscope work for this project but would entertain buying a better beginner scope if necessary.

Here is what the manufacturer says the scope can do:
Analog bandwidth: 0 – 200KHz
Sampling rate: 1Msps max
Sensitivity: 10mV/Div – 5V/Div
Sensitivity error: < 5%
Vertical resolution: 12-bit
Timebase: 10us/Div – 500s/Div
Record length: 1024 points
Built-in 1KHz/3.3V test signal
Waveform frozen (HOLD) function available
Save/recall waveform

Thank you for your help!
 
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The waveform you have captured is typical of that seen when touching wires together or operating a mechanical switch.
This is what I would like you to do now. Connect the the smart device and the bell as in the last picture of post #28 but with a switch in series with the red wire on the bell (anode.) side of the diode . (This will probably give a slightly cleaner transition than touching two wires together.) Connect your scope to the wires from from the push button. with the switch in the off position see if there are any pulses displayed on the scope. Then press the button on the smart device and try to observe if there are any pulses displayed on the scope after the button is released or if there is a delay after releasing the button before the trace on the scope returns to about 3 volts. Next close the switch and try to get the scope to capture the waveform. (I think you have said that this sometimes triggers the bell.) Do this a few times to see if it always triggers the bell..
Your scope looks like it will do what is required. (It is a lot better than my first scope that I built when I was a teenager using a WW2 surplus VCR97 cathode ray tube and valves.)

Les.
 
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Okay, I'm back after some testing and documenting everything over the weekend and then editing and depicting everything today.

I made a mistake when connecting the inline momentary switch, I connected it to the trigger input T (GND) on the remote bell (iChime Doorbell).
Regardless, I think the outcome that I ended up with will still be usable.
If you deem it necessary for proper diagnosis, I can redo select scopings.

One other note that you will probably notice when looking at my diagrams, when I connected the power adapter to the smart device (Reolink Video Doorbell), the remote bell (iChime Doorbell) did not chime until the tact switch was pressed on the smart device (Reolink Video Doorbell).
I never tested it in this manner because I was trying to be careful when I was probing and testing different ways to wire it and the components I was using.
This is still a problem because as soon as power is removed from the smart device (Reolink Video Doorbell) the remote bell (iChime Doorbell) is triggered and cannot be triggered again when pressing the tact switch on the smart device (Reolink Video Doorbell) until it is powered again.

I did some research on ways to cut power faster than the residual power depletes in the transformer of the power adapter in the smart device (Reolink Video Doorbell) after unplugging it or (more specifically) during a power outage.
I was reading about a Schmitt triggers maybe being able to do that but I didn't get very far with my research.
Thoughts?

This is what I came up with, when not using a diode, and with and without powering the smart device (Reolink Video Doorbell):


This is what I came up with, when using a 1N4148 diode, and with and without powering the smart device (Reolink Video Doorbell):


Thank you for all the help and recommendations!
 
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Looking at the waveforms they are displaying what I would expect to see with the pressing of the tact button. There are no unexpedted pulses or any sign of any latch up condition.
when you say that you have to break the connection between the smart device and the bell unit do you mean that break and remake both connections (Red wire and black wire,) before the bell will trigger again from a press of the tact button ? In the next to last picture with the diode in circuit there is no change in level at the point when the momentary switch is released. So I don't understand how the bell unit knows that the connection was broken. Can you clarify if just breaking the red lead connecton and remaking it is enough to get the bell to respond to the tact button again ?

Les.
 
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I'm sorry for the confusion... this is really hard to explain properly.

I have made additional diagrams that I hope will help me make more sense this time.

Here is the first one:


Here is the second one:


How can I make the tact switch on the smart device (Reolink Video Doorbell) work properly even when the device is not powered (plugged in)?

I want to prevent the remote bell (iChime Doorbell) from chiming immediately after power is removed from the smart device (Reolink Video Doorbell) and allow the tact switch to control it properly.
(ie: during a power outage, etc.)

Once again, thank you so much for your help!
 

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