Relay help

I’m working on adding after-market fog lights to a Toyota, normally we just connect a relay to 12v off of the high beam feed and ground and all is good, but the voltage to the high beam is only 5v, it goes into a headlight assembly (sealed) where I have to assume the 5v triggers either a relay or a circuit on a board. I tried using a standard 5-terminal automotive relay, but it won’t trigger, I then tried a Crouzet 3-32v solid state relay that I had in stock, but the relay still won’t trigger. I am guessing that the current requirements are too high for these relays, so I need something that doesn’t require a great deal of current to trigger. The load side is only 12v and connects to the trigger on another relay, so the current requirements on the load aren’t all that high.

Any suggestions?

Since 5V is far too low a voltage to reliably be used for a fixed voltage power source over more than 1 meter of wire length, I’d guess the line with five volts is either a constant current power source wire (LED drive) or a control wire.

In either case I would not tap into that wire to add a normal 12V lamp assembly since improperly changing the loading on either type of line can be very problematic. I’d simply tap into a an actual 12V power line or run a new 12V power line for the add on lights.

The 5v is a control, that’s why I need to use it. It has 5v when the high beams are on, there is no 12v source that is only on with the high beams.

I need a relay triggered by 5v to control the 12v load to the fog lights. Thus far what I’ve tried requires more current than the 5v provides.

Without seeing the wiring diagram for your lights and the wiring diagram for the fog lights, it is difficult to give advice on the hookup. Usually fog lights are used with the low beams and would be off when the high beams are on.

Exactly, and that is, typically, controlled with a relay, which is what I am looking for. The wiring diagram wouldn’t help, it’s not as detailed as it should be. We know how to do the setup, we’ve done it dozens of times, the kicker is the way Toyota has wired the lights. Instead of individual wires for each bulb one connector contains 4 wires:

  1. 12vdc as soon as the column switch turns to DRL.
  2. Ground.
  3. 5.14vdc, on for low beam, off for high beam.
  4. 5.14vdc, on for low beam, on for high beam.

I need a relay that is NO to connect to wire #3, but that triggers at a low current. EMR requires too much current to keep it closed, as does the SSR that I used. I don’t know if there is a different SSR that I can use, or if there is a Reed Relay or something else.

What is the make and model of the SSR that you tried?

CROUZET GNA5 84134910 SOLID STATE RELAY 25A 240VAC 3-32VDC. It’s the only SSR I had on hand in the 5v trigger range. Not exactly meant for this purpose but, like I said, I had it on hand, worth a shot.

Data sheet title: “GNA5 Series, Panel Mount SSRs, 24 to 280 VAC”

That series of SSR is for switching 50/60Hz AC power (TRIAC output), try a DC switching SSR.

Here’s a search that returns only, in stock, DC output, SPST NO, SSR’s that operate with a 5V control voltage.

You can filter the list for the fog lamps current draw and the packaging style you prefer.

Thanks, limited selection, least expensive is $43, have to check with the boss and the customer. I understand the fog lights only cost $65, not sure if they want to spend close to the same amount, or if the boss wants to be stuck with it if it doesn’t work.

I found two (one was a suggestion from the first page), so have a question on them:

Returned from search: SSRDC-200D12, mentioned as a possible: EL100D10-05N

I checked the wiring that will go on the trigger side and it’s below 10a. Before I do anything, do you think either of these might work?

Either of them might work.

The first catch is we don’t know how much extra current the cars controller circuitry can deliver on the high beam control line reliably over time. It seems like it tolerated your first experiment but we don’t know if the extra load would cause the cars controller to burn out over time.

The next catch is the derating of the output current capability at higher temperatures. Depending how hot it gets and how good the external heat sinking is, a 10A unit might need to be derated to 5A at standard automotive application temperatures.

Given those unknowns I think there is only a 50/50 chance of short and long term success.

Thanks. I went with the EL version, if this doesn’t work then we’re just going to have to tell the customer that it can’t be done. I hope that other mfgrs don’t follow Toyota’s methodology. If the battery was 5vdc I could understand the reasoning, but the secondary battery is a 12vdc, makes no sense to drop everything to 5vdc.

I think 5V and 3.3V are the most common signaling levels used in electronics today (e.g. ISO 11898-3 fault-tolerant CAN bus, USB, etc.). My guess is that by choosing 5V signaling they are saving money with lower cost integrated circuits and microcontroller interfacing.

Hi MikeL,

Do you have the ability to use a relay with pins rather than spade terminals or or screw terminals? If so, perhaps you could use a small SSR with a lower input current requirement to energize the coil of a standard mechanical relay.


With the LCA182, one would have to be sure you found a mechanical relay with coil current draw requirements below 100mA and you would need to connect a resistor in the neighborhood of 4.7k Ohms in series with the input to limit the current to around 1mA. The CPC1218Y has a built-in resistor, so no external resistor is required, and it can handle the coil current of pretty much any standard automotive relay, but it would draw upwards of 6mA, compared to only 1mA for the LCA182…

These would add a little complexity to the design, but would be significantly less expensive from a component perspective, and would require even less current to be pulled from the 5V signal than the EL100D10-05N, which will draw between 12-27mA.

I had considered these, but they present their own problems. Of course, they need to connect to the wiring somehow, typically done using circuit boards, which then means etching boards or using generic boards. I don’t know that they can, bare, handle the temperatures in the engine compartment, entailing longer runs of the wires through the firewall. The bottom of the board needs to be coated to prevent accidental contact with metal body parts. I have not given up on the idea, but am hoping the EL100D10-05N will work.

All good points. Yes, they would probably need to be located out of the engine compartment and would need some sort of board, though a simple plated through-hole board either enclosed in a small box or potted might prove sufficient.

Best of luck!

Thanks. Been a while since I created circuits, by through-hole board are you referring to a generic board with pre-drilled holes and circuits running perpendicular to where the IC rests? Or is this something different?

Yes, you got it. Something like this:

Here’s a link to some more similar ones. Some have interconnected strips and others are isolated per hole.

Man, that brings back memories. Like building my first Apple computer.

:grinning_face_with_smiling_eyes: Obviously, the preferred method would be to have a small circuit board made, but for a one-off, that’s not all that practical. That’s why I mentioned potting it after it’s wired up. That both helps seal out moisture and protects it from vibration.