Help Designing Circuit to Switch Two Subcircuits

[PianoV]

Hello! First time for my on this forum - hoping someone can help me. I know very little about 12VDC. I need to design a circuit board that has three terminals (my electronic vocabulary may be way off.....). Two of the terminals will be switches - one off and one on. When power is applied to the third terminal, the switches will reverse - the one that was on will switch off and the one that was off will switch on. I'm hoping this is really basic. Any ideas? (I don't have a clue where to start, except by asking this question!)

 

[carbonzit]

Your vocabulary is fine. But we need just a little more info before giving you an answer.

When you say you want two switches, do you mean you will have two physical switches, that someone can throw, and you want to reverse the positions of both switches electronically when you apply power to this device?

Or do you mean that you want one terminal to be normally on, the other normally off, and you want this to be reversed when you apply power?

Sometimes, the biggest problem here is getting folks to describe their problem/project adequately. The rest (actually figuring out the solution) is often easy.

 

[Diver300]

A changeover relay should work.

https://www.thetoolboxshop.com/ekmps/shops/adrcomponents/images/0-728-12-durite-12v-20a-30a-mini-changeover-relay-[2]-2141-p.jpg

Connect pin 86 to ground. Connect pin 30 to a 12 V supply.

Pin 87A will be on, so that a load connected between that and ground will be powered.

When you connect 12 V to pin 85, pin 87A will turn off and pin 87 will turn on.

 

[PianoV]

BINGO Diver300! Yes, that indeed sounds like what I want to do.

Carbonzit: The second scenario - "...you want one terminal to be normally on, the other normally off, and you want this to be reversed when you apply power." And it sounds like the changeover relay that Diver300 suggested will do just that.

Now one more question to be sure this will work for my application. The two pins (87A and 87) sound perfect - there will be one LED light connected to those - the LED light has a low power (brightness) lead and a high power lead - the changeover relay will enable me to switch between the two brightness levels when power is applied to pin 85.

My question is with the power that goes to pin 85. Being that the changeover relay is grounded, it would seem to me that I have to be able to turn on and turn off the power to the wire that goes to pin 85. That's NOT what I want to do (or at least not directly - or in the conventional way). The wire that goes to pin 85 I want to always be hot (connected directly to my 12VDC battery). However, I want to be able to turn the juice on and off, by grounding that wire - let me explain further.......

The dual intensity LED that is powered by pins 87A & 87 is to be switched when I turn blinkers (turn signals) on. This is on a bicycle where in cruising mode I have the dual intensity red LED flashing brightly. When I turn the turn signals (right or left) on, I want the dual intensity red LED light to switch from bright flashing to steady (non-flashing) low intensity. The reason is that all you see is confusion when the bright red LED is flashing wildly along with the turn signal - it doesn't really even look like a turn signal - just looks like confusion. That is why when the turn signals are energized, I would like the brightly flashing red LED to switch to the low-power-steady mode.

But the wire that would go to pin 85 is the same wire that is my HOT lead going to my turn signal flasher (hot wire to the flasher). The second wire from the flasher (only two wires - one in and one out) goes to my turn signal switch (that switch is a three-position rocker switch with the neutral position letting no power out from the switch, push it down one way and left turn signals go on and down the other way and right turn signals energize). So the lead to pin 85 would always be live. Instead of wiring pin 86 to a ground, could I instead connect pin 86 to the input (hot) of my turn signal flasher?

If that is not clear, let me try asking another way: If I have an always hot wire connected to pin 85, if I put a volt meter positive lead to pin 86 and the negative lead to the battery negative terminal - it should read 12VDC - correct? So that means pin 86 is always hot (of course, if it is grounded) - correct? So that in turn should mean that if pin 85 has a wire directly from the batter + terminal, if I connect pin 86 to the hot lead of my flasher - when I complete the flasher circuit by turning either the right or left turn signal on, the changeover relay should reverse pins 87A & 87 - and thereby switch my bright red flashing LED to a low-power-steady burn (obviously the bright flashing red LED will be powered through a separate LED strobing control and the low-power-steady-burn red LED lead would go directly to either pin 87 or 87A (whichever one makes it work correctly)).

Sorry for being so long winded, but I hope this helps to clarify things.

If this is going to work for me, I looked at several of changeover relays - they all seem to be in the 10 to 30 amp range capacity range. My LED lights draw WAY less than one amp. Am I loosing anything by using such a large capacity relay? Or does a 20 amp relay work just as well for me as a smaller one? My suspicion is that the larger capacity relay will work just fine - my only loss might be that it will cost a small amount more (not a huge deal in my book). If not, any suggestions on where to find a 1 amp changeover relay in North America? Or maybe a smaller relay would itself draw less power (my guess not anything significant)?

I sure appreciate the input so far. Thanks big time!

 

[Diver300]

Firstly, just use a 20 A relay. Automotive relays are cheap and easy to find. You can get smaller relays, but they won't save you much money, and are more fragile mechanically and electrically. There is absolutely no problem in running just the LED current from the relay.

The rest of the circuit is a problem.

You can connect pin 85 to live, and turn the relay on by grounding pin 86. However, there will only be a current of about 100 mA and that won't run the turn signal or the turn signal flasher.

You need about 5 relays, or 3 relays and a couple of diodes, to do what you want. That is because there is no place in your circuit that is on when the flasher switch is on. The turn signal flasher will make anything connected to the switch turn on and off each time the turn signals flash.

 

[PianoV]

Thanks for hanging with me on this! Yes, I realize the signal flasher will make anything connected to it flash. But I think I may have just had a good idea. What if I rig my system pretty much like I currently have it, but instead of wiring the flasher in before the turn signal switch, get a second flasher and wire one of them between the left turn signal switch and the left turn signal light and the other flasher in the same way on the right side. That way I can run a second wire from the left turn signal switch to pin 85 and another second wire from the right turn signal switch to pin 85 - but both of them before the flasher (the left turn signal switch, when activated, will power the left flasher and pin 85 and the right turn signal switch likewise will power the left flasher and again pin 85).

That should work - shouldn't it? I think it will. Seems to me the changeover relay will then be effectively powered directly by the 12VDC and get all the current it needs - and the turn signal flasher will also get all the current it needs - all I need to do is keep the flashers downstream from the changeover relay. Yes?

Thanks again!!!