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Newbie needs circuit design help for automotive timer project.

Thread starter #1
Hi Guys,
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 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
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#2
It sounds like your ideas are potentially dangerous, and probably illegal?.

A more sensible solution would be to have an audio indication that sounds after x seconds while the indicators are on, and repeats every y seconds if they continue to be on. This would require no real modifications to the car, and could be accomplished inside the passenger compartment, requiring no special precautions and only low power consumption.

However, by far the easiest solution would be a microcontroller, and count the number of flashes.

I'm a bit puzzled what you're doing to make a 556 overheat?, please post your schematic.
 
Thread starter #3
Hi, It's not illegal as the vehicle is a kit anyway. Audible warnings don't work as the vehicle requires ear defence and helmet.
Hence the idea behind the self cancelling indicator.
 

AnalogKid

Well-Known Member
#7
You keep assuming that everyone knows what you know about what you are asking about.

What does the indicator indicate?
What is a bespoke vehicle?
Where are you located?

ak
 

picbits

Well-Known Member
#8
Put a counter on the indicator that starts a beeper sounding after x number of flashes - when it has been left on long enough it will alert you then you can make the conscious decision to turn it off or now. Have a reset on the counter that triggers when no flashes have been detected for a couple of seconds.
 

gophert

Active Member
#9
It sounds like your ideas are potentially dangerous, and probably illegal?.
Which items, specifically? I see nothing wrong with his methodology. Many self-cancelling turn signal strategies exist and his is closely aligned with patents from the 70s and 80s - especially for motorcycles. Cars are easier because steering works differently but the use of braking to override timing is a completely reasonable logic to use. So, still interested in the dangerous and illegal.
 

alec_t

Well-Known Member
Most Helpful Member
#10
It's a standard 12v indicator system
There are various 'standards'.
Is your flasher unit a 2, 3, or 4 terminal type? How is it presently wired? A wiring diagram would be helpful.
 
Thread starter #11
AK, Apologies, I see your point. The vehicle is road legal kit 'car'. The electrics run off a 12v lightweight racing car battery and the indicator system is similar to most cars, in as much as it has 'standard' turn signal lights (as it must to be road legal in the UK). The wiring loom is a bespoke item. As with all vehicles the system is 12v but in actuality it tun slightly above that.
As you can tell, I am no electronics expert, I am a 'problem solver'. I have deigned and built arduino based PCB's for various one-off projects in the past. I have enough knowledge and curiosity to learn as I go but not enough to come up with a solution from scratch, frustrating to say the least. Hence me needing some tuition and advice from people with a lot more knowledge than me. I am really keen to find a solution and really understand it and how it really works.

Merry Xmas
 

alec_t

Well-Known Member
Most Helpful Member
#12
Do the indicators have a self-cancelling function when the steering is centred?
Are the indicators incandescent bulbs or LEDs?
If LEDs, are they PWM'ed?
 
Thread starter #13
Do the indicators have a self-cancelling function when the steering is centred?
Are the indicators incandescent bulbs or LEDs?
If LEDs, are they PWM'ed?
Hi, they don't self cancel with the steering otherwise I wouldn't need a self cancelling circuit. They are actuated via a toggle switch on the dash. They are LED's but to be honest all I want to do is hijack the existing toggle switch feed and introduce a circuit that times that feed.
 

dr pepper

Well-Known Member
Most Helpful Member
#14
I've seen this done without a timer, a reed switch was fitted to the back of the speedo (the old rotating magnet type), and the indicators remained on until a certain number of pulses from the speedo were received, then cancelled, in other words the indicators stayed on until a certain distance was covered.
You could do this with a more modern vehicle using the Vss.
A couple of trannys, a 4040 and a relay ought to do this.
 

cowboybob

Well-Known Member
Most Helpful Member
#16
... 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). ...
Seems as though you have a handle on your needs, save for the final, higher current link to the vehicles light system.

Looks to me like a simple "small" relay (like this) between your electronics and the vehicle's existing lighting system would work. A 556 can easily handle the current required by the relay suggested. I've used such an arrangement many times.

(Modifying NM's circuit, replacing R3, R4 and the "buzzer", with the relay)
 
Last edited:

gophert

Active Member
#17
It all seems simple until you realize that you don't want the 555 timer starting as soon as you power up, you need the left turn signal to cancel when the user changes his mind and applies the right signal, and a cancel button is needed in case the user changes his mind and doesn't want to turn at all.
 
#18
It all seems simple until you realize that you don't want the 555 timer starting as soon as you power up, you need the left turn signal to cancel when the user changes his mind and applies the right signal, and a cancel button is needed in case the user changes his mind and doesn't want to turn at all.
Wouldn't the switch at the power supply reset and stop the timer if the user changes his mind?
 
Thread starter #19
The biggest issue I have is that I can easily modify any of the simple 555 circuits currently available online, I can route the output to a relay. I can modify the caps to alter the timing.
What I can't work out is the whole 'reset timer continually if brake is applied'.
I can find multiple examples where a low or a high is applied to reset a 555 count once but I don't have enough knowledge to know how to to weave the brake functionality into the mix to achieve the correct result.
Running the 555 on a vehicle voltage could mean 14v which is pushing the 15v maximum for a 555 to the max. I think running inline DC/DC converters to run the 555 on a lower (stable) voltage might result in a 'little heat' to say the least. It is possible to ditch the 555 idea and use MOSFETS but that can create heat issues. This is where I need help to know exactly what to expect.
If I had a 5v I could just program my way out of the problem with an Arduino, C++ logic and some interupts but we are back to the heat of a 14v to 5v conversion.
 
#20
The conversion is not a problem. A zener diode with a resistor or a resistor divider on the power supply can easily solve that.

The voltage can rise, thats why I have noted it in the circuit before. But you have to check if the place from which you take the power supply has a regulator for "12VDC" or not. If it does not, it will be necessary to add a limit.

Please check the circuit I gave and see if it suits your needs. There is a switch on the power supply to account for turning off the timer when needed.
 

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