I am embarking on a new project to design a circuit for an automatic vehicle signal actuator.

Outline of the project is this:

The problem: I have a car that has indicators operating on a standard 12v on-off-on toggle switch. As you can imagine this switch gets left on occasionally. Both misleading and potentially dangerous.

The solution: Replace the toggle switch with a 2 way momentary switch and create a circuit that supplies 12v to the existing indicator circuit for 5 seconds. In addition, the timer is continually reset when a 12v feed from the brake circuit is applied. As soon as the brake feed stops, the 5 seconds starts again.

Effectively, this allows a lane changing signal as well as a solution for waiting at junctions or to cross a stream of traffic with the brake applied.

I'm sure there are many ways to skin this particular cat:

A 555/556/558 and relay solution? MOSFETS?

Heat dissipation could be a factor as the solution will have to be in a watertight project box.

I am playing around with a 556 on a breadboard but finding a solution that will be robust and won't overheat comes down to experience (which is something I lack). Any help would be greatly appreciated.

Kind regards

  • \$\begingroup\$ The solution: Replace the toggle switch Check if this is even allowed by the vehicle code where you live. \$\endgroup\$ – Janka Dec 24 '18 at 18:56
  • \$\begingroup\$ Do you forget the handbrake? \$\endgroup\$ – Solar Mike Dec 24 '18 at 18:56
  • \$\begingroup\$ The hand brake is not used very often with this vehicle. Changing the switch is OK as it is a kit. \$\endgroup\$ – Spencer Dec 24 '18 at 19:00
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    \$\begingroup\$ Yes Tim. I also have no windscreen, hydraulic hand brake, 250 BHP and weigh 600Kg 0-60 in 3.2s, I have to remove the steering wheel to get in and out and it is road legal. This is really a thread about electronics, not how my car is built. 👍😄 \$\endgroup\$ – Spencer Dec 24 '18 at 19:16
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    \$\begingroup\$ Just checking. Sometimes people do miss the obvious. Unless they've changed the law, if you want it to be street legal in Oregon you need a windshield wiper -- but not the windshield. Local street rodders keep a clamp-on windshild wiper under the seat in case they get pulled over (!). \$\endgroup\$ – TimWescott Dec 24 '18 at 19:23

A partial answer, in outline:

  • Yes, a 555 will work. I'm not thrilled with using a 555 for a 5-second interval because caps that large leak, but such is life. Be careful shopping for caps -- you need something in the automotive temperature range, with low enough leakage that the time constant won't be significantly affected.
  • I would get a 5V linear regulator, but one that's designed for automotive. Just follow the data sheet.
  • Assuming that each of the three inputs (left, right, brake) is +12V active, I would use a transistor buffer on each input, as shown. The idea is that you need something that'll be reliable down to around 6V (for that bad day when your charging system fails you), but will survive a load dump (Google for it) and protect the rest of your circuit. Double-check that a 1N4148 is sufficient for a negative-going load dump, and then decide if you need something that'll resist more voltage.
  • Put a bog-standard relay driver on the output of the 555; don't forget the flyback protection diode, and use a transistor that'll handle the load dump. Most 555's will drive a pretty healthy current, so you can probably use an NPN, but use a logic-level MOSFET at your discretion.


simulate this circuit – Schematic created using CircuitLab

Some reasoning:

  • A 2.2k\$\Omega\$ pullup is a bit firm, but isn't a bad value. It should demand around 2.3mA from the transistor collector in saturation.
  • For not-super-beta small-signal transistors you want about a 10:1 collector to base current ratio in saturation. So, about 230\$\mu\$A into the base. 33k\$\Omega\$ flows about 310\$\mu\$A when the base is at 0.7V; 70\$\mu\$A of that is bled off to ground by the 10k\$\Omega\$ resistor. The drive is a bit wimpy at 6V supply, so you can consider adjusting the resistance downward if you feel the need.
  • The diode protects the transistor B-E junction from reverse voltage transients. Putting it on the low-voltage side of the input resistor should protect it well.
  • Even if the circuit sees a 100V transient in either direction (which is quite unlikely), that's only 3mA in either the diode or the transistor base, so you should be OK there.
  • The 10k\$\Omega\$ resistor is there to make sure the transistor turns off -- in case you're wondering.
  • \$\begingroup\$ Cheers Tim, I'm reading round your solution and learning as I go. Can I get back to you in a couple of days with some questions? \$\endgroup\$ – Spencer Dec 24 '18 at 20:06
  • \$\begingroup\$ Sure. Just reply here, I'll see it. \$\endgroup\$ – TimWescott Dec 24 '18 at 20:17
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    \$\begingroup\$ Tim, how do you visualize the brake switch itself only re-triggering either the left, or the right (or neither, if neither were enabled to begin with), as appropriate? Assuming you imagine using a separate 555 for LEFT and for RIGHT, directing the brake switch to the appropriate 555 seems an interesting unaddressed aspect. But I may not be understanding your vision here, too. Just curious. \$\endgroup\$ – jonk Dec 24 '18 at 20:33
  • \$\begingroup\$ Oh, dangit, you're right. More logic is required. I'd recommend the same input circuit to get into the board, though. \$\endgroup\$ – TimWescott Dec 24 '18 at 21:06

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