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

#### BrandonB

##### New Member
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!

#### danadak

##### Well-Known Member
Most Helpful Member
Is the N/O switch, the node it is affecting, ground that node or pull it up to Vcc/Vdd ?

Sounds like remote trigger input is high Z and need either a pullup or pull down resistor.
So when wire is connected, with switch open, it does not, due to pickup, trigger the remote.

Regards, Dana.

#### BrandonB

##### New Member
Thank you for your reply.

Honestly, I'm not sure about VCC and VCC.

On the N/O switch on the smart device:
2 legs connect to the ground plane
2 legs (appear to) connect to the backup battery + (this seems strange to me but I only know basic electronics)

#### Pommie

##### Well-Known Member
Most Helpful Member
It could be that the remote bell just needs one of it's terminals connected to ground and you were unlucky enough to get the wires the wrong way around. Can you check for continuity between the two bell terminals and the bell ground?
Can you also check for continuity between the two push button sides and the smart device ground?
Also, check for continuity to Vcc.

Is there a common ground between the two circuits?

Mike.

#### danadak

##### Well-Known Member
Most Helpful Member
VCC, VDD are typical references to chip power supply pins.

Your switch pinout description does not seem right. Some various
possibilities in internal connections :

Regards, Dana.

#### BrandonB

##### New Member
It could be that the remote bell just needs one of it's terminals connected to ground and you were unlucky enough to get the wires the wrong way around. Can you check for continuity between the two bell terminals and the bell ground?
Can you also check for continuity between the two push button sides and the smart device ground?
Also, check for continuity to Vcc.

Is there a common ground between the two circuits?

Mike.
Thank you for your reply.

I have done some more testing.

I took apart the remote bell and checked it out with my DMM.
Trigger input T is connected to ground.
Trigger input P reads +3.069VDC.
If I short trigger input P to any ground, the remote bell will sound.

I will have to use my DMM to check the smart device tact switch legs.
Also for VCC.

There is not a shared ground between the smart device and the remote bell.
The remote bell is battery operated (6VDC (4x AA)).

#### BrandonB

##### New Member
VCC, VDD are typical references to chip power supply pins.

Your switch pinout description does not seem right. Some various
possibilities in internal connections :

Regards, Dana.
Yeah, the smart device tact switch legs reading the way they did had me confused.
I will recheck and report back.

Thank you for the help!

#### Les Jones

##### Well-Known Member
Most Helpful Member
I assume that the unmarked rectanular box on your diagram is a relay (Which you have not mentioned in your text.) with a 12 volt DC coil. I also assume that you have not disconnected the switch terminal from the logic that it controles in the ubspecified smart device. IF these two assumptions are correct then you are trying to pull this logic input ( Whose logic high state is either 3,3 or 5 volts.) This could very well damage the input of this logic device. Many logic devices have built in protection diodes which limit the positive input voltage to VCC + about 0.6 volts.
This is what I think is happening. When the button is not pressed the bottom end of the relay coil will be clamped to + 3.9 or + 5.6 volts. this means that the coil has 6.4 or 8.1 volts across it. This could be enough to make it pull in. (Wven though it is rated at 12 volts. I suggest that you measure the voltage across the push button switch when NOT pressed with the power to the smart switch on and your added wires NOT connected. This will tell us what the internal logic high level is on the smart device. Schematics of the smart device and the remote bell would be a great help to enable us to solve your problem.

Les.

#### BrandonB

##### New Member
I assume that the unmarked rectanular box on your diagram is a relay (Which you have not mentioned in your text.) with a 12 volt DC coil. I also assume that you have not disconnected the switch terminal from the logic that it controles in the ubspecified smart device. IF these two assumptions are correct then you are trying to pull this logic input ( Whose logic high state is either 3,3 or 5 volts.) This could very well damage the input of this logic device. Many logic devices have built in protection diodes which limit the positive input voltage to VCC + about 0.6 volts.
This is what I think is happening. When the button is not pressed the bottom end of the relay coil will be clamped to + 3.9 or + 5.6 volts. this means that the coil has 6.4 or 8.1 volts across it. This could be enough to make it pull in. (Wven though it is rated at 12 volts. I suggest that you measure the voltage across the push button switch when NOT pressed with the power to the smart switch on and your added wires NOT connected. This will tell us what the internal logic high level is on the smart device. Schematics of the smart device and the remote bell would be a great help to enable us to solve your problem.

Les.
Thank you for your reply.

Yes, the rectangular box on the test diagram is a 12VDC SPDT relay.

No, I have not disconnected the switch from the PCB.
I did not want to risk damaging the device.

I did partially reassemble the smart device and powered it up to get a reading from the tact switch.
The reading between the tact switch legs (terminals) is +3.400VDC (GND on the left legs in the diagram and + on the right legs).

I want to do whatever I can to prevent damaging the smart device, so I am trying to be careful while I'm testing everything.
This is why I have not tested the test relay circuit while the smart device is power on.
When testing with the test relay circuit the relay does not close UNTIL the smart device tact switch is pressed but it does not allow the relay to open again.
On the test relay circuit, I am only using the relay to switch the ground in hopes that it will not affect or impede the logic circuit on the smart device.

I have looked around, I have been unable to find the schematics for either device.
The remote bell is older and I don't think it was a highly sold item.
It also isn't sold any longer.
The smart device was released within the last few months.
Regardless, I haven't found anything.

Thank you for your help!

#### rjenkinsgb

##### Well-Known Member
Most Helpful Member
If you can find the 3.4V (or near that) power on the positive of one of the big capacitors, that stays stable when the button is pressed, you could connect the LED of an optocoupler, plus a series resistor, between that and the button contact.

That would not overload anything or risk damage, and it would give you an isolated transistor "switch" that could be connected across the button contacts of the remote bell.

What make & model is the new smart doorbell unit?

#### BrandonB

##### New Member
If you can find the 3.4V (or near that) power on the positive of one of the big capacitors, that stays stable when the button is pressed, you could connect the LED of an optocoupler, plus a series resistor, between that and the button contact.

That would not overload anything or risk damage, and it would give you an isolated transistor "switch" that could be connected across the button contacts of the remote bell.

What make & model is the new smart doorbell unit?
Thank you for your reply.

That sounds interesting.
I have heard of optos before but never knew what they did.
I will power up the smart device and poke around with my DMM and see what I can find.

The smart device is a:
Reolink Doorbell Camera WiFi (not POE)

Thank you for your help!

#### Les Jones

##### Well-Known Member
Most Helpful Member
I suggest first measuring the current that the trigger input on the remote bell requires. To do this just set your meter to a current range and connect the probes to the trigger input of the remote bell. I suggest setting your meter to the 100 mA range to start with and set it to a lower range if required. This information will be helpful in choosing an optocoupler and the resistor to drive the input LED in the optocoupler. Be VERY CAREFUL not to short anything out with your meter test probes when tracing a place to pick up the + 3.4 volts.

Les.

#### Pommie

##### Well-Known Member
Most Helpful Member
On your smart device you should have a pushbutton contact that is 3.4V when not pushed and gnd when the button is pushed. This means that anything connected between the supply (Capacitor?) and that push button connection will get 3.4V when the push button is pressed. As mentioned above an optocoupler could be used to power the bell. The opto would need a series resistor and the anode connected to the 3.4V supply. Note, a relay could be used but may draw too much current. However, a reed relay might suffice.

Mike.

#### Pommie

##### Well-Known Member
Most Helpful Member
I'm assuming that the push button grounds an input that has a pullup on it (internal or external). The opto or reed relay would be in parallel with the pullup and shouldn't cause a problem. However, if multiple pushbuttons are on the device then they could be arranged in a matrix and the above won't work safely (may damage the device).

Mike.

#### BrandonB

##### New Member
I suggest first measuring the current that the trigger input on the remote bell requires. To do this just set your meter to a current range and connect the probes to the trigger input of the remote bell. I suggest setting your meter to the 100 mA range to start with and set it to a lower range if required. This information will be helpful in choosing an optocoupler and the resistor to drive the input LED in the optocoupler. Be VERY CAREFUL not to short anything out with your meter test probes when tracing a place to pick up the + 3.4 volts.

Les.
As far as the remote bell is concerned, it is an:
iChime Doorbell
It is currently powered by 6VDC (4x AA batteries). (It does not require a doorbell transformer, but can be powered off of one if you choose... I just use batteries.)

To trigger the doorbell (remote bell), all that has to be done is to short Trigger Input - T (Ground) and Trigger Input - P (+3.069VDC).
Trigger Input - T (Ground) and Trigger Input - P (+3.069VDC) are to be connected to either side of a doorbell switch (or any momentary switch) and it will trigger when the button is depressed.
When I consider this, I would think this is 0 volts because a momentary switch provides no voltage only completes the circuit from one trigger input to the other trigger input... but I am guessing that this thought process is incorrect and shows my ignorance to the way less basic electronics logic circuits work.

Please explain if you can.

When I use my DMM and probe Trigger Input - T (Ground) and Trigger Input - P (+3.069VDC) I am able to trigger the bell by using almost every setting.
When selecting continuity, I touch the black probe to Trigger Input - P (+3.069VDC) and the red probe to Trigger Input - T (Ground) and the bell will be triggered.
When selecting tone continuity, I touch the black probe to Trigger Input - P (+3.069VDC) and the red probe to Trigger Input - T (Ground) and the bell will be triggered.
When selecting resistance, I touch the black probe to Trigger Input - P (+3.069VDC) and the red probe to Trigger Input - T (Ground) and the bell will be triggered.
...etc.
If I reverse the probes for the above probings, the bell is not triggered.

Maybe I am doing something wrong.
I do not have my good DMM handy, just a pocket DMM.
AUTOOL Mini VC921 3/4 DMM AD/DC Multimeter Pocket Digital Multimeter Frequency Tester 4000 Counts

Thank you for your help!

#### BrandonB

##### New Member
On your smart device you should have a pushbutton contact that is 3.4V when not pushed and gnd when the button is pushed. This means that anything connected between the supply (Capacitor?) and that push button connection will get 3.4V when the push button is pressed. As mentioned above an optocoupler could be used to power the bell. The opto would need a series resistor and the anode connected to the 3.4V supply. Note, a relay could be used but may draw too much current. However, a reed relay might suffice.

Mike.
I will probe around for the voltages on capacitors that are close to the Reolink Video Doorbell tact switch.

Those reed relays are super expensive... $20.00! #### BrandonB ##### New Member I'm assuming that the push button grounds an input that has a pullup on it (internal or external). The opto or reed relay would be in parallel with the pullup and shouldn't cause a problem. However, if multiple pushbuttons are on the device then they could be arranged in a matrix and the above won't work safely (may damage the device). Mike. There are only 2 tact switches on the Reolink Video Doorbell. 1 for the doorbell button (the one I am trying to trigger the iChime Doorbell with). 1 for resetting the configuration of the Reolink Video Doorbell. #### BrandonB ##### New Member On your smart device you should have a pushbutton contact that is 3.4V when not pushed and gnd when the button is pushed. This means that anything connected between the supply (Capacitor?) and that push button connection will get 3.4V when the push button is pressed. As mentioned above an optocoupler could be used to power the bell. The opto would need a series resistor and the anode connected to the 3.4V supply. Note, a relay could be used but may draw too much current. However, a reed relay might suffice. Mike. Okay, I did some probing with my DMM to search for constant 3.4V points when the Reolink Video Doorbell tact button is pressed. I was unable to find any 3.4V points on the large SMD capacitors on the opposite side of the tact switch. I did find 3x constant 3.4V points on some SMD parts near the tact switch. Here are the points that I found (marked in yellow). #### Pommie ##### Well-Known Member Most Helpful Member Those reed relays are super expensive...$20.00!
Sorry, didn't look at the price. Try looking at a local supplier to see what's available.

Mike.
Edit, here's a cheaper one with a coil resistance of 500Ω so only 6mA current required.

#### BrandonB

##### New Member
If you can find the 3.4V (or near that) power on the positive of one of the big capacitors, that stays stable when the button is pressed, you could connect the LED of an optocoupler, plus a series resistor, between that and the button contact.

That would not overload anything or risk damage, and it would give you an isolated transistor "switch" that could be connected across the button contacts of the remote bell.

What make & model is the new smart doorbell unit?

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?

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