Newbie needs circuit design help for automotive timer project.

I've also just read that automotive voltages can spike very high so theoretically we have potential overvoltage, spike voltage and reverse polarity issues.

NsrMagazin said:

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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Hi, Thanks for the diagram. Can you explain the switch logic as I was hoping to use a momentary switch to initiate the counter. In addition a high from the brake light switch would keep the indicators running until it goes low and then timer count would start again.

There are a few options here. Since I don't exactly know your signals, please add your information so we can reach a solution.

0. I am not certain you want to repeat the indicator turning on/off, this is the only reason for the timer, otherwise the circuit can be simplified.

1. The idea is simple, when you receive a high from the brake(normally this is done for a parktronic device) this will activate the timer and the output of the timer will give a consecutive sequence of impulses at 5 sec period, 70% duty cycle(70% on, 30% off). When the brake is off (0V at the timer's power supply) you will turn off all indication at the output of the timer(buzzer or/and other).

-What will be the indication specs for the buzzer or other(voltage, current, frequency and duty cycle, activated by falling front or rising front)?

I am quite confused. The circuit I need supples the existing wiring loom (and the existing indicator electrics) with current.
I want to replace the existing on-off-on toggle switch with a momentary switch which activates a timed 5 second feed to the existing circuit - effectively a self cancelling power feed. In addition I need the circuit to provide a continuous feed if the brake light switch goes high and restart the timed feed when it goes low.
I do not need a buzzer as the car is driven with ear defenders and a helmet on. If I could use a sound then I could introduce a buzzer to the existing circuit and do away with the need for a self cancelling timer.

Sorry for any confusion.

This covers the 5 sec self canceling part:
In that case the timer needs to be reworked. Its possible to give a 5 sec output(100% duty cycle) and then turn off, unless a second impulse is givien after the 5sec. If a second impulse is given before the 5 sec run out, then the output will not be affected(the 5 sec will run out and wait for an impulse then).

Does this suit your needs?

The second part - a continues feed if the brake also is on, will require some time to make.

NsrMagazin said:

Wouldn't the switch at the power supply reset and stop the timer if the user changes his mind?

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Nope, 555 will trigger as soon as power is supplied (if that is how you plan to reset).

It's all very well extending a 5 sec period by use of the brake pedal, but won't you need the ability to cancel an already-started 5 sec period?

Edit: And won't you also need to allow both left and right indicators to flash simultaneously as an alarm signal? If so, would a third momentary switch be required?

gophert said:

Nope, 555 will trigger as soon as power is supplied (if that is how you plan to reset).

Click to expand...

Not if its a falling front timer 555. Then it will be activated when the power supply is removed.

As far as I understand from the OP's previous post, the timer powers a load for 5 sec and then stops? If the power supply is removed the timer output will be turned off immediately.

A second part of the circuit powers the load permanently if the brake is on?

Or is it not so?

You don't need a 556 timer IC for this idea. The technology already exists with a two-pin flasher and a time delay module from the ignition cylinder circuit ( mine is an Echlin AR-613 unit). I assume that applying the brake voltage would reset the timer and restart the time delay upon pedal release.