sensor

In article , snipped-for-privacy@ukf.net says... | | I am looking for a Cheap induction type clamp on device (up to 38 | channels need to be monitored so each logger will require 38 of the | devices). The inputs to the logger are + and - with logging taking | place when a voltage is applied or dropped (+ V detected = Bit 1, - V | detected bit 0 change from bit 1 to 0 = change in state). I am happy | to build such devices or to purchase if cost is within reason. | | Any ideas or products/designs circuits welcome. | |

Have you considered adding an Hall Effect switch alongside the coil of each relay, as a means of detecting whether each is energised?

Hmmm... sounds like a tough one.

How much DC current through the contacts? Maybe you could just measure the current in the contact circuit somehow? That could prove the contacts and maybe even some other components?

daestrom

In article , snipped-for-privacy@ukf.net says... | Hi Harry | | | The problem is that each relay is fitted with NC and NO (front & back) | contacts and I need to monitor the state of each, yep I know that the | contacts would change state together and that the coil would be in an | energised or de-energised state but I need to prove each contact | (legal requirement) I don't even think I can get close enough to the | coil to enable detection with a Hall Effect and if increased in | sensitivity there is much greater possibility of interference from | nearby coils. The coils/relays are DC and the current carried by each | contact is DC. | | |

Hmmm, a tricky one....

If you must prove the position of each individual contact, then there are still ways it can be done. Assuming each contact would have a steady DC voltage across it, or that a DC level would change on the contact opening and closing, then an op-amp circuit could be used to detect that changed voltage.

Assuming a reasonable level of current flow and a small voltage drop would not matter, then a small amount of resistance in series with a contact would produce a voltage across that resistor. Even a PCB track may have enough resistance. Test for voltage across it, voltage there indicates the contact is closed. This again could be done with an op- amp.

Another possible way, assuming good isolation between each circuit, would be to feed a small source of AC onto one contact and then see if it comes out from the other. A capacitor on the input to the contact, plus one on the output contact, would filter off the DC component. Either an op-amp or a 555 timer could then be set up to see if the AC is passing through the individual contact set.

If one end of the contacts were common to each other, then feed AC in onto that common. Then just check for AC on the output contacts.

Without a great deal more information, I can only stab in the dark with my suggestions, but I don't wish to do more than suggest generalised solutions or I will be doing the job for you.

Harry I will email you direct later this week to explain in more detail. AC feeding is OUT (the relays are AC imunue but we would not be allowed by RailTrack law to introduce it or anything else into the circuit).

The ONLY available way is to detect the wires which carry on average DC but yes some do carry AC.