Switching on voltage

Hi there,

I have a device that will set a pin to <1V whenever it’s “not ready” and the voltage will rise to 1-15VDC.
I am measuring this between two pins of that device. Let’s call one the signal pin and the other D-GND (device GND).
Unfortunately I cannot connect D-GND to Arduino GND and simply use the ADC with a Zener protection for measuring. That what I tried but that’s what made my mains protection breaker flip.

My goal is to be able to tell in my Arduino sketch whenever the “signal pin” goes high (=above 1V).
So what would be my best approach?
I was thinking about using the signal pin to trigger a transistor base that would switch a relay which would then connect one of the Arduino pins to ground. But I am very unsure about how to connect the transistor to that signal pin.

Any ideas? :slight_smile:

Best,

Tillman

I would like to know what device that is, and read its specifications for myself.

A device that has a low voltage DC output, and its ground is somehow connected to the mains ? That is very weird. Maybe it is broken.

Do you know what the output voltage is ? Can it really be anything between 1 and 15V ? If it is 15V and it can drive a led, then you can use an opto-coupler.

That what I tried but that’s what made my mains protection breaker flip.

This sounds like a serious problem in one of the power supplies, or the building wiring. Potentially very dangerous to humans.

It's a Hazer.
The unit has a remote control receiver that is connected through a 3 pin XLR.
Between blue and black will be a potential difference of 0V as long as the unit is not ready (it needs to pre heat).
Once it's ready the difference is 1,3VDC.
At that point connecting the third lead (brown) to black will engage the hazer.

The black wire is not "ground compatible" (pardon my layman's language) to the Arduino.
So I am looking for a decoupled way of measuring the voltage between blue and black and once it crosses a certain threshold I would like to set a GPIO to either low or high so that I can react to that event in code.

There is no datasheet or any other documentation available.

Best,

Tillman

We can't answer that. For the brown to black you can use a relay, but I don't know what to think about that 1.3V DC. Is that really 1.3V DC ? How did you measure it ? Can it supply some current ?

For an opto-coupler, at least 2V or 3V is needed. A transistor is not possible, because there is no ground.
There are 1.5V reed relays, but can the 1.3V DC drive the relay ?

The more I think about that 1.3V, the more I think it is unlikely. Perhaps it is an open relay output, or open collector output. Perhaps the black wire is the mains, and you measure mains noise on the blue wire.
I think it is too dangerous to mess with something unknown.

Does it have a brand or type, or any number. Can you make photos of it ?

This is the unit:

I said 1.3 vdc but it actually is 1.5 vdc between blue and black measured using an basic dmm.
Would it be possible to use an op-amp and a 1.5vdc reference voltage as comparator and use that to switch a relay?
Between black and Arduino GND there is about 200mV difference.

It is not clear what the 1.5V DC is, therefor it is too dangerous to use it.

The smoke machine is sold under a few different brands and types. I can not find the manufacturers page.
This is a picture of the inside : http://g02.a.alicdn.com/kf/HTB1ud.9HpXXXXcqXpXXq6xXFXXXR/223543045/HTB1ud.9HpXXXXcqXpXXq6xXFXXXR.jpg

I can see the mains switch, the heater, the bi-metal switch. But I can't see the wire to the XLR, and I wonder what the black and red wires are.

Perhaps you can open it and reverse engineer it to make a schematic.
My only advice at this moment is not to continue with trying to connect something to the XLR plug. There are too many unknown things, and it's too dangerous.

I'm with Koepel - without a decent schematic, you're potentially playing with fire. Especially with cheap Chinese crap like these units.

Take the time to 'reverse-engineer' it and draw up a schematic. You might be able to use the 1.5V at the Hazer end to switch a transistor from a higher internal voltage derived from the Hazer, then use that to power an opto-isolator (all inside the Hazer box), with the opto-isolator output switching the Arduino input. A relay controlled by the Arduino could then be used to safely switch the brown wire to the black to start Hazer operation.

The 1.5V difference on the XLR connector is likely a balanced signal. I'm not sure, but I think this would likely be a RS485 signal. I think DMX uses this sort of connector/signal.

This is what I'd do. Use a RS485 chip (I think one is called MAX485) and monitor the communication between the remote and the device to see if there's some identifier indicating the machine is ready.

DuaneDegn:
The 1.5V difference on the XLR connector is likely a balanced signal. I'm not sure, but I think this would likely be a RS485 signal. I think DMX uses this sort of connector/signal.

This is what I'd do. Use a RS485 chip (I think one is called MAX485) and monitor the communication between the remote and the device to see if there's some identifier indicating the machine is ready.

You might well be right, Duane. I almost suggested that he measure between the black and blue wires with the DMM range on AC, rather than DC, to see if there was more going on than a simple DC voltage.
RS485 to TTL converters are cheap, if it is using that protocol..

Again, reverse-engineering that part of the Hazer circuit would immediately make that clear.

Did you see the inside with the link in my Reply #6?
Is there any TTL logic inside ? a transformer ? something digital at all ?

Koepel:
Did you see the inside with the link in my Reply #6?
Is there any TTL logic inside ? a transformer ? something digital at all ?

I'd missed that. Not much of anything in there. So little, that it makes me wonder how the 1.5VDC is actually derived. It appears that everyting of interest must be in the remote unit. And if so, the 1.5V must have been measured with the remote unit attached. It appears to be more than a remote control - it's the controller.
There must be digital circuitry in that. For the RF remote control, if nothing else. From the look of it, everything is in there.
One thing for sure though, there's no RS485 communication between the main box and the remote unit.

I think that the OP needs to provide more information.

Thses are some of the circuit schematic that I had found on some china site.

As you can see, no isolation of the main circuit.

Thanks for your suggestions guys.
The "remote unit" is a SC2272 circuit to communicate with the remote control using a SC2262.
As soon as the XLR blue wire goes high (1,5vdc when measured against the black wire) the remote key fob will light a green led.
Then pressing any of the two buttons on the remote control will trigger a relay in the remote unit to connect the black and brown wires. That will start the hazer.
While the hazer is active the voltage on the blue wire jumps between 1,5 and 12VDC.

Currently I just use a relay to power the mains to the unit and use a second relay to connect the brown and black wires. After 4 minutes I turn both relays off. This approach works ok but obviously is more a hack than anything else.

So I see two ways to go from here:

  1. find a way to measure the voltage differential (like a DMM does) without the need for a common ground and be able to react to it
  2. use the remote control and connect the Arduino to it to remotely control the remote control :wink: