Hello everyone, I'm trying to build a circuit to control my car's headlamps. I live in Brazil, where we are required by law to turn the car's headlamps everytime we are on a highway, regardless of the time. This is not needed when driving within the city, but I have to take highways several times a week so it is easier and safer to turn the headlamps on always.

A simple switch would do it, but I thought about doing a small electronic project and started to search the internet. A description of what I like:

-Circuit as simple as possible, in parallel to the original car's headlamps controls.
-Enabled or disabled by a switch (lever type or alike). If disabled nothing will happen.
-If the circuit is enabled, headlamps shoud turn on when starting the engine, with a few seconds delay (just to save some battery power lights should use the alternator's energy, when the engine is on. For that I'm guessing the delay could be anything between 5 and 20 seconds)
-If the circuit is enabled and the engine running, a push button will turn them off or on, switching from the previous state each time the button is pressed. This is because sometimes I just go to the store and there is no need to use the headlamps. In this case I just press the button and the ligths will turn off, but everything would be ready for the next time I start the engine. And while in the road if I change my mind and need to enter a highway I can press the button again to turn them on.
-When turning the engine off the headlamps will turn off if they were on, or stay off if they were already off. Circuit should be ready to start over again with the start of the engine

I found the diagrams for latch circuits using 2 transistors, having the "push button control" but lacking the initial delay before turning on. Is there a simple way to create this circuit?

Thank you for your help!

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Many ways to do this. 2 or 3 discrete transistors, one 555, two 555's, one CD4093, etc. What components do you prefer/have experience with/want to play with?

Also, you will need access to your car's headlight control wiring. Do you have a way to wire into the existing wire harness?

ak

 

Not too much experience, I would like to use a simple capacitor/resistance/transistor for the delay timer. However this simple circuit would keep turning on the latch when the condenser is charged and discharged...?

I have access to the headlamp switch, and I can find the two wires that activate the relays. Would be nice if the circuit can be in parallel with the headlamp switch so I would not need to tamper too much with the car wiring.

Thanks!

 

For an R-C timer to have consistent performance and timing repeatibility, it needs crisp switching action around a defined trip point or transition level. As a capacitor charges slowly over 10-20 seconds, a transistor being driven by that will not "snap" on, but wander from non-conducting to saturated. This can make the headlight relay contacts "chatter" as the voltage across the relay coil increases slowly, not a good thing. The solution is either a couple more transistors and a positive feedback resistor, or any of several IC options. AND, you need two timers in series, one for the turn on delay and one for the output pulse. This is not a big deal, but one or two transistors (and associated resistors, capacitors, etc.) won't do it.

ak

 

For an R-C timer to have consistent performance and timing repeatibility, it needs crisp switching action around a defined trip point or transition level. As a capacitor charges slowly over 10-20 seconds, a transistor being driven by that will not "snap" on, but wander from non-conducting to saturated. This can make the headlight relay contacts "chatter" as the voltage across the relay coil increases slowly, not a good thing. The solution is either a couple more transistors and a positive feedback resistor, or any of several IC options. AND, you need two timers in series, one for the turn on delay and one for the output pulse. This is not a big deal, but one or two transistors (and associated resistors, capacitors, etc.) won't do it.

ak

Ok, I'll search for the timers and see what I can do, thank you!

 

I can whip up a circuit. In fact, this request has come up recently from others. I'm just trying to get a feel for the level of tech you are comfortable with. Where are you located? If you want to do a little research, look up schmitt trigger circuits.

ak

 

I can whip up a circuit. In fact, this request has come up recently from others. I'm just trying to get a feel for the level of tech you are comfortable with. Where are you located? If you want to do a little research, look up schmitt trigger circuits.

ak

I´m in Sao Paulo, Brazil. I'll search for the schmitt trigger circuit, I remember reading something about it at the beginning.... Thanks again!

 

I can whip up a circuit. In fact, this request has come up recently from others. I'm just trying to get a feel for the level of tech you are comfortable with. Where are you located? If you want to do a little research, look up schmitt trigger circuits.

ak

One more question, could I put 2 latched circuits (both simple 2-transistors circuits), latched circuit 1 will feed a simple timer (capacitor/resistance/transistor) and will be energized at the start of the engine. When the capacitor is charged (or semi charged) it would "latch" circuit 2 which would feed the headlamps relay. The push button would change both latched circuits to off and back on again.... will it work?

Thanks again!