I’ve seen some similar threads but none that quite seem to ask the same question: I have a device that’s USB powered and I want to build a dry contact relay into it (I’m going to bury the relay inside the plastic shell of this device). I want a dry contact to close a switch, which is easy to find (Fortrez, Fibaro, Qubino). The hard part I’d appreciate some help with is finding a module that can run from 5VDC. Everything I’ve seen so far requires 12VDC, 24VAC or 120VAC (I’m in the US) … well, the Fortrez seems to be 9VDC, but doesn’t seem to run off 5.
Unfortunately, that doesn’t quiiiiite work for me because that’s not a plain (dumb) 5 Volts DC, which is what’s trivially available on any plain USB bus, including a USB-C charger that’s failing to negotiate any other Voltage levels (as I understand it USB-C will attempt to negotiate a Power Delivery profile at various stepped-up Voltages; start at about Page 13 here for more info ). In my case I have an old school USB device that’s got 5 Volts and that’s it!
My application: I’m attempting to elegantly modify an existing USB-powered device by embedding a Zwave or Zigbee control inside the existing molded plastic. The device is specifically “dumb” old school 5VDC USB, so that’s all I have to work with power-wise. Using more modern “smarter” USB-C, which can provide higher Voltages, does not help solve my problem.
My current solution is to simply relocate my smarthome control: instead of embedding within the plastic molded case I’m going to use a standard socket to control the existing USB power supply. This is less elegant for a variety of style reasons in my case that aren’t important here. The most important question was a baseline technical one: are there Zwave or Zigbee controls that work off 5VDC? (apparently: no)
A related workaround could be to find a battery powered Zwave or Zigbee control that’s using a Lithium 3.3v cell and regulate an existing 5v supply down then solder it to the batter terminals. I don’t want to go down that path right now but if there were battery controlled relays that would be noteworth here… are there?
One of the most challenging engineering tasks when designing network devices is getting the heat flow right in order to protect the radio. In the price ranges that we’re looking at, these are pretty inexpensive devices. They are definitely vulnerable to damage from overheating or even just uneven heating within the specified range.
I just wouldn’t embed any device of this type inside another powered device, even if you can get the signal out.
I’m not saying it’s necessarily a safety issue, but I do think you will reduce the quality of the transmission and the lifespan of the radio device.
Actually, I know of two devices that might fit the bill, and both use Zigbee. The first is @iharyadi’s awesome little environment sensor that I believe may have the option to control a digital output pin connected to a standard dry contact relay. The second is @drandyhaas’ very cool Mona Lisa board, which I am certain can control a few digital output pins.
Neither of these are truly commercial off the shelf offerings, but they are pretty cool devices, and both are fairly small. Both are powered by 5VDC.
I completely understand. Trust me heat is not a concern here - this is a large device (many cubic feet, mostly hollow, tons of space to fit a tiny RF device) which generates less than 1watt of waste (it’s OEM power supply is a 250mA @ 5VDC supply).