Short circuit generator

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konczakp
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Short circuit generator

Post by konczakp » Wed Aug 09, 2017 1:13 pm

As on the picture below I have a 12V device (Base) which is using 2 wires for powering and communicating with receivers (R1, R2, Rx) Line is always high and now only Base is able to start communication by pulling Low power line for very short amount of time (about 12 - 16 us). In this way the Base is able to send High and Low to the receivers. Receivers somehow are able to respond. I wanted to change for example Receiver R3 to my maple mini and start the communication in the same way as the Base is doing. To be able to communicate with other receivers. I think i need to do short circuit for a very short time (12us) to generate Low state on the power line. Does anyone know how to do this? Maple mini is a 3.3v and the line is 12V. I couldn't find any switch working as fast as I need ...
Selection_002.jpg
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RogerClark
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Re: Short circuit generator

Post by RogerClark » Wed Aug 09, 2017 9:30 pm

Why not use a FET

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Pito
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Re: Short circuit generator

Post by Pito » Thu Aug 10, 2017 8:44 am

I do not understand what you want to do.
Shall the MapleM be the initiator (the Base) shorting the 12V??
Or, the MapleM shall be the R3 - receiving the signal via pulse on 12V??

When shorting the 12V you need 2 switches most probably:
1. to switch the +12V rail off the power source (ie. a high-side pmosfet), and after some small delay
2. to short the +12V against GND (ie. nmosfet) to generate the precise 12us pulse, and after some small delay then
3. to switch +12V back on.
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konczakp
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Re: Short circuit generator

Post by konczakp » Thu Aug 10, 2017 11:03 am

I want to replace R3 but I want to be able to act as a BASE. I'm already able to receive signals via a 12V rail. In other words the BASE and me can init the communication with other Receivers. The problem is I cannot switch off the base and power the rail from other source. BASE must stay online and power the rail (it has a short circuit protection :)) The easiest way would be to use a SSR switch to short circuit but I couldn't find as fast as I need. I only used MOSFET for dimming or dc motor driver but I don't know if it is possible to use it in a way I want. I think MOSFET need to have some resistance to switch it on and when applying R then there is no short circuit. Or maybe I'm wrong ? If You have any idea could You also draw a simple circuit for the connection ?

EDIT: Maybe It would be easier if I would say that this is an intercom simillar to this one:
IioHAS_3000_Magistrala.JPG
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ahull
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Re: Short circuit generator

Post by ahull » Thu Aug 10, 2017 12:54 pm

How short is short?

In other word, do you need to crowbar the 12V with a few milliohms of short, or would a short of a few ohms, or tens of ohms be more appropriate?

The reason I ask, is that a dead short will provide a huge spike of energy, as the current will be high, but a few ohms, or tens of ohms will limit the current and therefore the switch required can be less robust. Does the system in question specify what a "short" is?

BTW most pulse dialling doesn't rely on shorting, but rather on loop disconnect of a current loop. https://en.wikipedia.org/wiki/Pulse_dialing
It is possible in these systems to dial by "flashing the hook" at the correct rate to simulate the rotary dialler, this requires a little practice, but once learned, you can achieve pretty consistent results simply by clicking the hook switch at the correct rate.

EIT: You might also like to take a look at this.. -> https://www.sparkfun.com/tutorials/51

... and this ...

Image

.. and this ...

https://www.google.co.uk/search?q=pulse ... 581#imgrc=_
Last edited by ahull on Thu Aug 10, 2017 1:12 pm, edited 1 time in total.
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konczakp
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Re: Short circuit generator

Post by konczakp » Thu Aug 10, 2017 1:11 pm

ahull wrote:
Thu Aug 10, 2017 12:54 pm
BTW most pulse dialling doesn't rely on shorting, but rather on loop disconnect of a current loop. https://en.wikipedia.org/wiki/Pulse_dialing
It is possible in these systems to dial by "flashing the hook" at the correct rate to simulate the rotary dialler, this requires a little practice, but once learned, you can achieve pretty consistent results simply by clicking the hook switch at the correct rate.
Maybe this is how it works. But I'm unable to click the hook with 16us 21 times :) :) What circuit do I need for that ?
To be honest I took saleae logic analyzer and connected to those 2 wires by a voltage divider and I recorded all pulses. I don't know how it works but I'd like to do the same in reverse. To be able to dial from receiver.

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ahull
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Re: Short circuit generator

Post by ahull » Thu Aug 10, 2017 1:32 pm

konczakp wrote:
Thu Aug 10, 2017 1:11 pm
ahull wrote:
Thu Aug 10, 2017 12:54 pm
BTW most pulse dialling doesn't rely on shorting, but rather on loop disconnect of a current loop. https://en.wikipedia.org/wiki/Pulse_dialing
It is possible in these systems to dial by "flashing the hook" at the correct rate to simulate the rotary dialler, this requires a little practice, but once learned, you can achieve pretty consistent results simply by clicking the hook switch at the correct rate.
Maybe this is how it works. But I'm unable to click the hook with 16us 21 times :) :) What circuit do I need for that ?
To be honest I took saleae logic analyzer and connected to those 2 wires by a voltage divider and I recorded all pulses. I don't know how it works but I'd like to do the same in reverse. To be able to dial from receiver.
Do you have some example images of the signalling from your logic analyser?

Are you sure the line voltage of the system runs at 12V DC? Often phone (PBX) systems run at a notional +50V DC (more commonly +48V DC), and rely on current sensing rather than voltage sensing as this is far more reliable over long cable lengths. https://pbxbook.com/other/trunks.html

Image

There is of course nothing to suggest that your system works this way, other than the fact that this is a very common standard.

Do you have the manufacturer, model number and spec and/or schematics of the intercom?
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ahull
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Re: Short circuit generator

Post by ahull » Thu Aug 10, 2017 1:54 pm

If your system works in a manner similar to the above schematic, then you could hook in to it with a suitable load, and relay, and pulse the relay at the required rate. You may either power your system parasitically from the line (more difficult to implement, easier to install), or run it from a separate isolated supply. Pulsing a relay is pretty easy.
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Pito
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Re: Short circuit generator

Post by Pito » Thu Aug 10, 2017 6:11 pm

There is not such relay to pulse at 16us duration/rate..
What is the current drawn by the R1..Rx phone?
Does the Base switch to zero Volt? It could be the Base switches from 12V to 6V (for example) - still enough voltage to power the phones and the phone can easily read the pulses..
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konczakp
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Re: Short circuit generator

Post by konczakp » Thu Aug 10, 2017 7:14 pm

I have attached the captured data via saleae (remove txt extension). Base is proel 1803 and the receiver is proel pc255. I don't have any schematics. When nothing is on the line there is 12-14V.
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domofon.logicdata.txt
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