Electronics → Re: Low Voltage Illuminated Switch

Okay, So I had some additional thoughts on how to do this and i found an online circuit simulator, that all i can really do is draw out the circuit.

Simply put, my end goal is to Close Pair of Dry Contacts from upstream device. I want to do that two ways:

a) manually, and have an indicator when manually closed from a switch. (Switch 1)
b) from a controller and have a indicator when done by the controller. (Switch 2).

Does the attached picture make sense?

if done by (a) relay 1 will close the dry contacts.

if done by (b) relay 2 will close the dry contacts.

in both cases, their respective LED illuminates to indicate which one caused the dry contacts to be closed. if both are true, then both LEDs are illuminated.

What you posted will work, but if I was doing it I'd use steering diodes and a single relay. Diodes are cheaper/smaller then relays. Plus if system is commanded by both control circuits power consumption is only a single relay.

Bottom line it all depends on what you have laying around in your junk box.

I did not look up the data sheets for the LEDs. If they are rated for 12V great, if not don't forget to add a series resistor or you will convert then to DEDs (dark emitting diodes).

/tom

oh wow. that's awesome. thanks very much.

i have all the parts in my junk box for both scenarios, but i'm going to try yours as it's more efficient and i might learn something from doing it that way.

(as i've claimed, i'm an amateur, so this might be a dumb question) how do i determine the right diode to use? i see you spec'd out one.

The series diode needs to be able to carry the current needed to operate the relay and the LEDs and have an operating voltage in excess of the supply. A general purpose diode like the 1N400x family or if the relay current is low a signal diode such as the 1N4148 will be fine. If this was a high speed switching circuit diode choice would be more complicated.

As good practice you should also add a inverse diode across the relay coil. Inductors generate a voltage spike when the circuit is opened. Depending on the type of LED you use, some do not take kindly to high reverse voltage, plus it is a source of electrical noise.

/tom

excellent. thanks again. i really appreciate it.

As tschmidt said, Don't forget the series current limiting resistors for the LEDs...

For 12v, I'd start around 680 ohms, 1/4w (flameproof if possible), 510 or 470 ohms if the LED needs >20ma.

Thanks for that reminder. I will ensure I put them on there.

I've got loads of DEDs if anyone is interesting in taking them off my hands, just pay postage and handling.

No thanks, I've been producing them since the 70's. Ya think I'd learn.

Tom,

Do you think the reverse voltage from the relay coil will damage the LED? The LED spec is for 5V.

Not sure I ever tried it.

Probably not, the wired OR steering diodes will be reversed biased, but 1N400x are fairly slow. Since this is not a cost sensitive production item and I have zillions of 1N400x and 1N4148 diodes sticking one in the circuit is easier than figuring out if it really needs one.

Depending on the relay the OP is using it may already have one built in.

/tom

I had some time last night and I was able to build a working POC with the help that you gave me. I even added a diode across the relay. I was able to hear a noticeable delay when opening the switch and hearing the relay disconnect. Very interesting.

Thank you very much for the assistance.

Attached are some pictures, two LEDs, two SWITCHES (not all visible) and one relay.

I only had 1N5408 diodes in my drawer, they are monsters. But work.

I"m going to try to finalize this on a PCB board this weekend and get it into a small project box. Alternatively, I was thinking of just soldiering and heat sinking the components directly inline with the wire. Would that be unwise?

Although I've tinkered with resisters and LEDs and other components in the past, I never really understood what a diode did. So I certainly learned something here. I appreciate the time you spent to explain.

What kind of delay you seeing, .25-.5 sec?

Maybe on the shorter end of your range.

Speed for my application is not an issue.

yeah, typical. I picked the longer one because you said you noticed it.:) It's just the diode. The inductor (relay) is discharging through the diode.

Ahh, excellent point. I have a warped sense of time. Everything i do only takes 15 minutes which drives my wife crazy.

Glad everything is working.

Now that you have one project under your belt time to start thinking about the next one.

/tom