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DC Dave 21st April 2004 07:47 PM

Current sensing circuit design help
I am not sure if such a circuit is possible for the average DIYer but here is what I am looking for.

I need a circuit that can tell when a device (such as a preamp or processor or whatever) is drawing current (or an increase in current) from the wall plug and switch a relay.

So an electronic device would plug into this circuit, that is plugged into the wall. When the electronic device is turned on a relay is closed and power is sent to other electronic devices. When the electronic device is turned off the relay is opened cutting the power to the other electronic devices.

Does anyone have any ideas? Also does a product like this exist. As I was writing I thought maybe there was already something similar to this like a powerstrip or something.

Magura 21st April 2004 07:52 PM

I have seen something like that, NATURALLY...I cant remember where or what it was called. :(

It was used to switch on a bunch of robotics in a certain order.


sam9 21st April 2004 08:27 PM

go to and look at Project 38. I built one of these a couple of years ago and it works fine.

However, I found an even simpler way to accomplish the same thing. If the unit that you designate as "master" has a switched power outlet on the back (many do), connect a 12Vdc wall transformer to it. You will need an extension cord. When the the "master" unit is switched on the transformer provides 12Vdc that can be used to trigger a relay in another piece of gear. I built an amp with a relay to turn it on when a 12Vdc voltage is present and found I could control it as described. This only works if the unit to be controlled has provision for such a trigger.

DC Dave 21st April 2004 08:41 PM

Project 38 is a good idea but will not work for me as designed. My application cannot depend on audio signals for the switching. Using the switched outlet on HiFi gear is another idea I also thought of, but the gear I want to use does not have such outlets.

I have found some info searching google that looks promising. Specifically here:

I am going to contact them and see if their product will fit my application. No need to reinvent the wheel.


bigparsnip 21st April 2004 08:53 PM

I do believe that I have also already seen a comercial product that will do this. I will try nad have a look about for you to see if I can find it at all and let you know.

Edit: I found the one that I was thinking of, if you go to the website below and search for L01AQ you should go straight to it. the only thing is that this is designed to turn on when a PC is drawing power from the main socket, so a pre amp may not be enough of a draw to activate the other sockets (it does not state the switch on current).


DC Dave 21st April 2004 09:09 PM

Nice find. That is what I was looking for. I kinda figured that there was such a product like that already. Thanks for looking for me.

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