using an oscillating current to switch a relay - Page 1
Forum section: electronics / beginners

author: nst197
author_rank: Newbie
timestamp: October 20, 2021, 01:31:42 pm
content: Hi Everyone,My first post here so please go gentle on meI've got a sensor that produces an oscillating current when triggered. The two states are:- not triggered 24V, 60mA- triggered 24V, current oscillates between 60mA and 120mA at around 1-2Hz.I'd like to switch a 24V device when the sensor is triggered. Can anyone suggest an approach for doing this, and whether there's an off the shelf device that could do it?Thanks

author: drdm
author_rank: Contributor
timestamp: October 20, 2021, 01:48:32 pm
content: You can use a sense resistor for sensing the current and turn it into voltage. from there you can use a simple peak detector with a diode, a cap and a resistor to distinguish between ON and OFF. But this is just guesswork. Can you share some details about this sensor?

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 20, 2021, 02:08:01 pm
content: What kind of sensor is it? Do you have make and model? It's strange because how can it change the current if the voltage is fixed at 24V?

author: Badwolf
author_rank: Contributor
timestamp: October 20, 2021, 07:40:36 pm
content: There are relays whose coil can be powered by an alternating signal. I don't have a reference handy, but you should be able to find some on sites like Mouser

author: TimFox
author_rank: Super Contributor
timestamp: October 20, 2021, 08:32:21 pm
content: "AC coil" relays are normally designed for use with 50 or 60 Hz alternating current.The coil construction differs from "DC coil" relays.

author: bdunham7
author_rank: Super Contributor
timestamp: October 20, 2021, 09:03:06 pm
content: I would start withdrdm'sproposal and add two op amps, one to buffer and amplify the voltage from a current sensor and the other as a comparator after the peak detector circuit. This may sound complex, but can be done with just an LM358 dual op amp, a current sense resistor, the diode/capacitor/resistor in the peak detector and then 4 additional resistors to set the gain of the buffer amp and the threshold of the comparator. The LM358 will work on the 24VDC single supply and has enough output to operate a small relay directly.I'd like to hear more about this sensor as I haven't seen one with that particular behavior.

author: nst197
author_rank: Newbie
timestamp: October 22, 2021, 12:01:28 pm
content: Thanks for all the replies everyone.Calling it a sensor maybe incorrect, but it's a device that you can connect to a telephone and it tells you if it's ringing:https://www.algosolutions.com/product/2506-polycom-vvx-ring-detector/?v=79cba1185463It's designed to trigger their own products so they don't publish how it works, what I described is what I've worked out by testing it.Effectively you apply a voltage across the output terminals, and when the phone rings I'm guessing the internal resistance of this thing drops so you get an increase in current. In reality you will also get a change in the voltage too but it's such a small amount I was assuming you couldn't use that as a trigger, and I was guessing it was loosely designed to work like a current loop which I believe is common for industrial sensors.The peak detector approach is what I managed to work out would do the trick myself whch is good news. What I'm not sure about is whether you can buy a device that does that off the shelf without having to make one? I want to use this to link up telephones to external sounders in an industrial setting.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 22, 2021, 01:38:49 pm
content: In an industrial setting I would use a PLC with analog input.

author: themadhippy
author_rank: Super Contributor
timestamp: October 22, 2021, 02:03:08 pm
content: QuoteIf its on the suppliers lines then it needs to be an approved device,and bt/open reach can get very arsey about stuff being connected to there lines that isn't approved.saying thathttps://quasarelectronics.co.uk/kit-files/electronic-kit/8122.pdfmight be worth a look

author: nst197
author_rank: Newbie
timestamp: October 22, 2021, 02:51:20 pm
content: Just to clarify, these are IP phones connected to a corporate network, not BT phones. The device plugs into a headset socket on the base of the phone so it's not actually connecting to the network.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 22, 2021, 05:08:45 pm
content: Is the current AC at 60mA DC or AC untriggered?Any how this should work. Runs on 6 to 40V DC- under £5 to make.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 22, 2021, 08:35:27 pm
content: A current sensor doesn't really care about the voltage, it works at a constant current, with the only limitation being the power supply rails.So you must load the output to ensure it always stays beyond max voltage at the highest output, or the current won't be able to flow.This circuit uses 147ohm load, so the voltage will be 17.64V (120mA) and 8.82V(60mA), which I don't think it'll be a problem for the sensor (yet to know the model).Using a darlington configuration, you can turn on a relay with microamps at the base, so you only need a small capacitor to store the energy from the peak detector.Adjust the potentiometer to set the threshold, I simulated 45% to trigger at 70ma.Instead the 2N2222A, use a BC547C, has much greater gain (400-800 vs 75-300).It doesn't care if using AC or DC, since the peak detector is also a half wave rectifier.I've tested it at 1Hz, holds the relay switched on with max 0.9Vce.Quote from: BeBuLamar on October 22, 2021, 01:38:49 pmDo industrial PLCs work with such current ranges? I only know 0-20 and 4-20mA sensors!Quote from: Terry Bites on October 22, 2021, 05:08:45 pmTerry, 24V relays usually have a fairly high coil impedance, around 1Kohm (we're talking about a simple relay, not a huge 1000amp contactor).Simple math shows that it would only require 24mA (actually, the switching threshold is always lower than the nominal current).So the relay will be always turned on in that circuit. In any case, you have no control over it.

author: ledtester
author_rank: Super Contributor
timestamp: October 24, 2021, 05:00:57 pm
content: Quote from: nst197 on October 22, 2021, 12:01:28 pmHere is the assembly manual for a Velleman Telephone Ring Detector Kit - K8086:https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k8086.pdfPage 9 contains a schematic.