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Thread: Monitoring external systems - a solution

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    Automated Home Sr Member mcockerell's Avatar
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    Default Monitoring external systems - a solution

    For some time I having been looking for a way to interface 3rd party systems into Idratek to allow me to monitor the operation of the heating, cooling and ventilation systems in the house (and ultimately control them better) and I came across a small chip that seemed ideal for the job - the HCPL-3700, which is described as an AC/DC to Logic Interface Optocoupler.

    Karam kindly provided me with the specification for a typical Idratek digital input and earlier this week I finally found time to make up my first prototype - see photo below. My initial requirement is to monitor AC mains feeds, but as you will see very few components are required (just 2 resistors and a capacitor) as the IC takes care of most things. For a DC version just 2 resistors are needed (no filter capacitor required) - in both versions the resistors determine the voltage trigger level, which in the prototype I set to approximately 160V RMS.

    Yesterday afternoon I hooked it up to a spare input on an Idratek module for the first time and am pleased to say that it works perfectly. I powered it from the nominal 12V Idranet supply, so it actually only needs 3 connectons on the low voltage side - Gnd, +12V, Output->Digital input.

    I now plan to build a set of these; at the moment I have 3 circuits to monitor, all AC mains (solar pump, brine pump, boiler enable). Each will be mounted in a small plastic box close to my main Idratek panels, with 2-core mains cable running back to the circuits to be monitored. For anyone that is interested the component cost of a single unit is around 6.50; the chip, capacitor and high-spec resistors make up 3.95 of this and the connectors, board and box account for the rest - I obtained everything from RS.

    I hope that this is of interest to others - if it is I can recommend the application notes for the device.

    Martin
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    Automated Home Legend chris_j_hunter's Avatar
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    well done ... wonder if getting the supply voltage down to say 5V would be OK & save significant power consumption over time, perhaps using the spare pair of the Idranet Cat-5e cable ?

    Chris

    NB: best we'd done, in looking for something like this, were some of the Arduino & PicAxe project boards, or maybe something from Cebek & Velleman - all of which would have been overkill, and much more complicated / expensive / power-consuming !
    Last edited by chris_j_hunter; 7th March 2010 at 04:34 PM. Reason: typo's

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    Automated Home Sr Member mcockerell's Avatar
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    I did consider running off 5V, or even 3.3V, but the IC is rated for a maximum supply voltage of 18V and the data sheet states a maximum supply current of just 4mA so this seemed to be adding unnecessary complexity - it might be worth considering if there were a whole bank of them mounted together. I've just checked my prototype and it draws just 2.5mA when active (mains applied) and no measurable current (< 1uA) when inactive, so it consumes less than 40mW (given that the nominal 12V supply voltage is around 15V).

    Martin

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    Automated Home Legend chris_j_hunter's Avatar
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    great, many thanks ...

    Chris


    BTW, found the maker's datasheet quite easily, but had a bit of trouble finding a useful applications sheet (that could help me choose component values etc) - best I found were these :

    http://www.fairchildsemi.com/ds/HC/HCPL3700.pdf
    http://www.datasheetcatalog.org/data.../HCPL-3700.pdf
    Last edited by chris_j_hunter; 7th March 2010 at 11:51 AM.

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    Automated Home Legend Paul_B's Avatar
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    M****erell,

    This looks very interesting. But as I'm a noobie when it comes to electronics what are you monitoring and how is this displayed in Cortex?

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    Automated Home Sr Member mcockerell's Avatar
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    Quote Originally Posted by chris_j_hunter View Post
    BTW, found the maker's datasheet quite easily, but had a bit of trouble finding a useful applications sheet (that could help me choose component values etc)
    Here is a link to the original manufacturer's website for the datasheet and application notes - the one that I found most useful is AN-1004 which explains the principles and helps choose values for the components. For mains sensing I used a pair of 27K 2W resistors and a 22uF 16V electrolytic.

    http://avagotech.com/pages/en/optoco...tor/hcpl-3700/

    Hope this helps.

    Martin
    Last edited by mcockerell; 7th March 2010 at 04:28 PM.

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    Automated Home Sr Member mcockerell's Avatar
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    Quote Originally Posted by Paul_B View Post
    This looks very interesting. But as I'm a noobie when it comes to electronics what are you monitoring and how is this displayed in Cortex?
    I'm monitoring, for example, when the pump in the solar heating system is switched on; the most direct way of achieving this is to measure the voltage across the pump itself - no voltage = off, full mains voltage = on. The output from the sensing circuit is high when no voltage detected, pulled low when the threshold voltage level has been exceeded. The output is connected directly to a digital input on an Idratek module and can be used in the same way as any other digital input in Cortex, for example a switch that takes a digital input to 0V when it is closed. At present I am simply using Cortex to log when each of the external sytems is activated so that I can observe how the various systems are interacting - I am considering using the LEDs on a QBI to show the state of those that I am particularly interested in.

    Martin

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    Automated Home Legend chris_j_hunter's Avatar
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    ah, that's better, thanks ...

    Chris

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    Automated Home Legend chris_j_hunter's Avatar
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    wonder if Karam's tempted to add this trick / device to update an existing module, or even maybe create a new module with it - it's very simple & there might be many uses ! ?

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    Automated Home Legend Paul_B's Avatar
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    It's all very quiet on the Idratek / Cortex front atm. Next big thing must be a new version of Cortex the current version has been around for over a year now

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