SPDT Relays to create momentary closure when a load is switched on, then a different momentary closure

Thread Starter

tedmunds

Joined Nov 23, 2023
6
I am planning to purchase a TV lift stand that can be operated by wireless or wired remote control. I want to automate the lift so that it goes up when the TV is turned on and down when the TV is turned off. Unlike some models, there is no 12 V trigger port on the lift. But the supplier suggests that the wired remote can be hacked/replaced with some circuitry to accomplish the automation.

What is needed: When the TV is turned on, a momentary (a few ms?) contact closure needs to be created between the remote's COM and UP pins. Then when the TV is turned off, a momentary contact closure needs to be created between the remote's COM and DOWN pins.

My idea: Use either the TV's USB output voltage as a trigger, or a smart power bar (like Smart Power Bar on Amazon) (where the TV's load turns the other outlets on the bar on/off) with a basic transformer as the trigger. Then I need to build a device/circuit that will translate the trigger's leading edge into one momentary closure and the the trailing edge into a different momentary closure.

This seems similar to some other discussions I have seen on the forum (for example this one), but I got the idea from this relay diagram that this task can be accomplished using just relays and basic components.

I sketched a circuit in LTspice, using the DPDT relay models from the hardware design forum. When I run the simulator, the output seems to give what I want (when the TV-linked power source turns on there is a momentary connection of the U1 (UP) relay, and when that power source turns off there is a momentary connection of the U3 (DOWN) relay).

My questions:
1. Is this circuit actually a reasonable/safe solution?
2. Are there any changes improvements that would be appropriate (or necessary for safety)?
3. How can I actually go about building it? This is my first attempt at electronic circuit building since Physics labs 20+ years ago. Where can I source parts (I live in Canada - is mouser.ca or leeselectronic.com likely to have what I need)? What actual parts should be used for the relays/diode (I assume capacitors/resistors are easy enough)? What should I use to hold the parts together (I have basic soldering skills/equipment)? What kind of enclosure should I use to safely contain the project?

Thank you for any advice/assistance.

(This is also my first thread post, so I also welcome any suggestions regarding the presentation of my questions.)
 

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Thread Starter

tedmunds

Joined Nov 23, 2023
6
Thanks for your reply! I couldn't figure out a way to keep the DOWN contact disconnected while the TV power is on without the third relay. Is there a way to avoid that?

Also, I thought of another question: does the U2 relay provide enough resistance on its own to prevent a short circuit on the usb/alternative power source? Does it need a load in series?
 

sghioto

Joined Dec 31, 2017
5,369
U2 is fine and won't load the USB supply.
In the 3 relay version the diode and resistor can be eliminated.
Can you repost that schematic in a jpeg or png format?
 

Thread Starter

tedmunds

Joined Nov 23, 2023
6
U2 is fine and won't load the USB supply.
In the 3 relay version the diode and resistor can be eliminated.
Can you repost that schematic in a jpeg or png format?
In the simulator, taking out the diode caused the UP connection to be closed again when the TV powers off. I think it behaved badly without the bleed-off resistor as well. I'll check it again.
 

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sghioto

Joined Dec 31, 2017
5,369
Version using one relay. C1 and C2 provide the momentary pulse required. The resistors and diodes provide a discharge path for the capacitors when the relay contacts are not connected.
1700785120517.png
 

MisterBill2

Joined Jan 23, 2018
17,814
What the first drawing looks like is what the TV lift system's external connections should be to provide the wired remote control functions. It does not appear that those are outputs that can operate relays. So the first question is about that: Is the connector shown the control for the TV lift system?? That is what is stated on that drawing.
That implies that a different scheme is required. Some signal is required to tell when the TV is switched on. At that point the soution becomes quite similar to what worked for controlling the projection screen for a video projector setup. that was solved on this forum perhaps two years ago.
For this application it does require a SPDT relay to operate when the TV is on, and to release when it is switched off. Also required are two capacitors, and possibly two resistors, and a connection to the TV lift system control's 5 volt supply common.
The operation would be that when the TV is switched on, the relay operates, and the +5 volts is applied to ine side of a capacitor, the other side is connected to the "UP" connector pin. When the set is switched off,the relay releases, and the 5 volts is applied to one side of a different capacitor, whose other side is connected to the "DOWN" connector pin. I suggest starting with capacitor values of 0.47 microfarad, although 0.1 microfarad may be adequate. The challenge may be in obtaining the signal for the TV being on or off.
There is a simple solution for that if it is required.
 

sghioto

Joined Dec 31, 2017
5,369
That schematic I show in post #7 is highly dependent on the UP - DN input requirements of the lift mechanism.
May require additional components.
 

MisterBill2

Joined Jan 23, 2018
17,814
That scheme I described in post#8 worked for a projection screen application a while back. The one critical thing is the capacitor polarity, which depends on the polarity of the control voltage. That is not so very complex to figure out.
 

Thread Starter

tedmunds

Joined Nov 23, 2023
6
How will the hidden TV turn-on if its infrared sensor is not exposed to obey the remote ? I think it would have to rise fully first to be able to receive then, the remote signal.
I'm planning to turn on the TV through its smart features (e.g., Google TV), or IR through a screen door, so that should be ok.
 

Thread Starter

tedmunds

Joined Nov 23, 2023
6
What the first drawing looks like is what the TV lift system's external connections should be to provide the wired remote control functions. It does not appear that those are outputs that can operate relays. So the first question is about that: Is the connector shown the control for the TV lift system??
Yes, that's the port for the wired remote connection. I don't know the details of the controller's internals - I just guessed at it being +5 V on COM so that I could simulate in LTspice to check whether the relays were closing the COM-UP/COM-DOWN connections at the right time. I agree that it would be unwise to rely on that voltage to activate a relay - for that I'm planning on using the TV's USB (or a smart power bar).

For this application it does require a SPDT relay to operate when the TV is on, and to release when it is switched off. Also required are two capacitors, and possibly two resistors, and a connection to the TV lift system control's 5 volt supply common.
The operation would be that when the TV is switched on, the relay operates, and the +5 volts is applied to ine side of a capacitor, the other side is connected to the "UP" connector pin. When the set is switched off,the relay releases, and the 5 volts is applied to one side of a different capacitor, whose other side is connected to the "DOWN" connector pin.
That sounds similar to what sghioto posted in post #7, so that seems like a good solution (provided I can verify whether COM is +5V and UP/DOWN need to be pulled up, or vice-versa).

Are there suggestions for how I should actually construct the circuit? Ideally it would be a self-contained (enclosed) unit with a USB connection (for connecting to the TV) or standard 5V power jack input (for connecting to a wall-outlet transformer on a smart bar), and a cable for connecting to the wired-remote port.

Thanks!
 

MisterBill2

Joined Jan 23, 2018
17,814
OK, so the controller is sort of unknown. It might be that the voltage source for that wired remote is "zero", and that the push-buttons apply a "low-true" sort of input. My scheme will only work with DC, though.
Sensing if the TV is switched off or on can also be achieved using a reed switch with a few turns of wire around it, forming a current operated reed relay. That trick is at least 30 years old.
An interesting alternative would be to switch on the TV when it is raised, as the watching while it is in the hidden position is rather useless.
Remote control of up and down may be much simpler as the choices are fewer.
 
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