Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Relays: what kind should i use?

Status
Not open for further replies.

Hælvis

New Member
This should be simple enough, but I've never used mechanical relays for anything, so I need a bit of input.

It's a 12V circuit. A DPDT switch starts a blinking LED using a car relay (or 555, this is unimportant). A momentary switch is supposed to turn the led on full time using a relay and I need the relay to go back to the off position when the DPDT is switched to "off" ("on2"). Also, I don't want power to any coil when the DPDT is in the "off" position.

I bet this is really easy, but I have very little experience with relays.

Note also, the momentary will get it's power from same source as the blinking LED. (12V 1.5-4Hz square wave)

Edit: It's very important that a second press on the momentary does NOT switch off the relay!
 

MikeMl

Well-Known Member
Most Helpful Member
Welcome

I'm a little confused about the switches.

Is one a DPDT toggle switch that stays (mechanically) in the last position the handle was moved to?

Is the other switch a momentary push-button? Is it SPST? NO or NC?

Is the role of the relay to create a "latching" circuit which is SET by pushing the momentary switch, and CLEARED by moving the handle of the toggle switch to the OFF position? What is supposed to happen if the momentary switch is pushed with the toggle switch in the OFF position? Is it necessary to put the Toggle switch into the ON position for the LED to blink? Is the relay supposed to make the LED light up steady?
 

Hælvis

New Member
Is one a DPDT toggle switch that stays (mechanically) in the last position the handle was moved to? Yes.

Is the other switch a momentary push-button? Is it SPST? NO or NC? Momentary push-button NO

Is the role of the relay to create a "latching" circuit which is SET by pushing the momentary switch, and CLEARED by moving the handle of the toggle switch to the OFF position? Yes, using the signal for the LED via the push-button. The toggle switch may be ON-ON or ON-OFF. I will buy whatever the circuit requires.

What is supposed to happen if the momentary switch is pushed with the toggle switch in the OFF position? Nothing, there won't be any power to it.

Is it necessary to put the Toggle switch into the ON position for the LED to blink? Yes.

Is the relay supposed to make the LED light up steady? Yes.

Thank you for welcoming me and getting back to me on this! :)
 

MikeMl

Well-Known Member
Most Helpful Member
How about this:

Relay1.jpg


The Flasher is like a two-terminal thermal flasher like in a car's turn signal. The diode keeps the flasher from energizing the relay coil until the MOM button is pushed. When the MOM button is pushed, the relay coil is powered, pulling in the armature and its own contact. The relay contacts short out the flasher, applying steady power to the LED. The relay coil stays powered even when the MOM button is released. Everything resets when the toggle switch is opened.
 

Hælvis

New Member
Cool!

Yes, that would work fine! Was hoping I could avoid the diode, but all my doodles without one lie crumpled on the floor! :D

Thank you so much for this, it looks great! Very tidy! Except the momentary needs to be in paralell with the D2. Don't redraw it, though! What program did you use, by the way?

Again, thank you!
 

MikeMl

Well-Known Member
Most Helpful Member
... Except the momentary needs to be in paralell with the D2. Don't redraw it, though! What program did you use, by the way?!

No, the diode is in the right place to bypass the flasher at the instant the MOM button is pushed. The relay contacts snap shut ms after the MOM is pushed, leaving power applied to both the relay coil and the anode of the diode after the MOM button is released.

I used the graphics circuit editor from LTSpice to create the schematic.
 

Hælvis

New Member
I aggree that it works beautifully, but for this application and the physical position of the switch and the LED, I need to be able to use the square wave to the LED to trigger the relay. It's a box connected with a three core cable (one earth, one feed for the LED and a return from the momentary)... ;)

No, the diode is in the right place to bypass the flasher at the instant the MOM button is pushed. The relay contacts snap shut ms after the MOM is pushed, leaving power applied to both the relay coil and the anode of the diode after the MOM button is released.

I used the graphics circuit editor from LTSpice to create the schematic.
 

eTech

Well-Known Member
How about this:

View attachment 81389

The Flasher is like a two-terminal thermal flasher like in a car's turn signal. The diode keeps the flasher from energizing the relay coil until the MOM button is pushed. When the MOM button is pushed, the relay coil is powered, pulling in the armature and its own contact. The relay contacts short out the flasher, applying steady power to the LED. The relay coil stays powered even when the MOM button is released. Everything resets when the toggle switch is opened.

Hi

Won't diode D2 cause a voltage drop that will make the LED alternate between dim and bright?

eT
 

eTech

Well-Known Member
I aggree that it works beautifully, but for this application and the physical position of the switch and the LED, I need to be able to use the square wave to the LED to trigger the relay. It's a box connected with a three core cable (one earth, one feed for the LED and a return from the momentary)... ;)
Hi

Is the LED signal really a square wave, or positive going pulse train?

eT
 

MikeMl

Well-Known Member
Most Helpful Member
Hi

Won't diode D2 cause a voltage drop that will make the LED alternate between dim and bright?

eT

The voltage across the LED-resistor combo will be 12V if the flasher turns it on, and will be 12-0.65V when the relay picks up. Assuming the forward voltage drop of the LED is 2.2V, the current through the LED would change from (12-2.2)/470 to (12-0.65-2.2)/470; or from 20.85mA to 19.47mA, which would be barely perceptible to the eye.
 

Hælvis

New Member
Hi

Won't diode D2 cause a voltage drop that will make the LED alternate between dim and bright?

eT
I plan to use an adjustable automotive LED indicator blinking unit, and that's solid state, so there's probably som voltage drop there as well. Or I could add a resistor in series with the flasher so that it's dimmer than via the relay/D2.

Anyway, it doesn't really matter. The functionality remains the same. ;)
 

Hælvis

New Member
Hi

Is the LED signal really a square wave, or positive going pulse train?

eT
Not really proper square wave, since it's that automotive blinker thing. However, if that doesn't work, I'll be running a 555 circuit, and that's decidedly as square a wave you get for next to nothing.
 

eTech

Well-Known Member
The voltage across the LED-resistor combo will be 12V if the flasher turns it on, and will be 12-0.65V when the relay picks up. Assuming the forward voltage drop of the LED is 2.2V, the current through the LED would change from (12-2.2)/470 to (12-0.65-2.2)/470; or from 20.85mA to 19.47mA, which would be barely perceptible to the eye.

Ah!...Ok

I thought the OP said that the only energy available was a 4 Hz square wave to energize
the relay. That's .125 s on and .125 s off.

Anyway thanks...

eT
 
Last edited:

alec_t

Well-Known Member
Most Helpful Member
What current does an automotive flasher require to operate these days? In the good old days the (significant) current through a filament bulb was required. Will the LED current be enough?
 

MikeMl

Well-Known Member
Most Helpful Member
What current does an automotive flasher require to operate these days? In the good old days the (significant) current through a filament bulb was required. Will the LED current be enough?
Not for the thermal flasher, maybe for the newer solid-state flashers. One could always use an old-fashioned lamp.
Haelvis was planning to use a 555.
 

alec_t

Well-Known Member
Most Helpful Member
Here's how you could do it using a 555. Closing the SPDT switch starts the LED flashing at ~2Hz (rate can be changed). Q1 and Q2 form a latch to keep the LED on (non-flashing) when the momentary switch (button) is pressed. The latch resets when the SPDT switch opens.
SwitchedFlasher.gif
 

Hælvis

New Member
Ah!...Ok

I thought the OP said that the only energy available was a 4 Hz square wave to energize
the relay. That's .125 s on and .125 s off.

Anyway thanks...

eT

Only to trigger the relay, but then it'll have a feedback loop to keep itself in the on position.

What current does an automotive flasher require to operate these days? In the good old days the (significant) current through a filament bulb was required. Will the LED current be enough?

The automotive flasher is solid state, so that should be OK, even if I don't have the specs. The current through the momentary will be drawn from the flasher directly, not in series with the LED, as in the circuit diagram MikeML drew.

Here's how you could do it using a 555. Closing the SPDT switch starts the LED flashing at ~2Hz (rate can be changed). Q1 and Q2 form a latch to keep the LED on (non-flashing) when the momentary switch (button) is pressed. The latch resets when the SPDT switch opens.
View attachment 81413

Thank you for taking your time to draw this, but I've decided against the 555 and will build it with the flasher. It reduces the complexity of the build significantly, and adds to the robustness of the system. Also, if I can find the right socket, It can easily be replaced should it burn out.
 

Hælvis

New Member
MikeML: The 1N4002 looks like a very good choice for it's position. Do you think it would make a good rectifying bridge too? I like the idea of making it polarity insensitive, so that any idiot with any 12V power supply (over 1.5A?) could hook it up any way they wanted. (Have you noticed that some wall warts are negative center pin? And the universal ones, you need really good eyes to see what way is what with the prongs and the plug...). And don't get me started on car batteries or cigarette plugs! ;)

Or maybe RS407 for simplicity?
 
Last edited:

Hælvis

New Member
I was trying to find a quality relay (that's also in the list of relay drawings in PCB creator) and I came across Omron G5LE-1A 12DC. To me, this looks like something sturdy for a fair price, and it switches up to 125V DC, so I'm thinking this would be a solid deal.

What do you guys think?

Also, should I incorporate a resistor for the feedback loop so there's less chance that the coil will burn?, I mean, it's only maintaining the position? Or am I overthinking this?
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top