555 Monostable Circuit Puzzle

This sounds like a case for "Captain Debounce"

Threshhold,Discharge to GND, Trigger floating, ~1k resistance from Reset to GND and Rectfied (plus RC) input signal to Reset. RC can give responce to and from idle condition. Would use ~ 2sec S.M.

But hitting the Reset will set the output low, not high...right? And what keeps the output low when there is no input pulse train? And how do you implement the timing constant? This is probably 555 class 101, but I've never progressed beyond making a light dimmer with a 555 astable.

Kilowatt0, thank you kindly. The application is for a turn signal indicator used in some old European cars known as "trafficators," which consists of an arm that rotates up from a stowed position with a light on the arm. The light was steady however, not flashing. A friend is rebuilding an Austin, using a GM steering column and an aftermarket wiring kit, which will generate a flashing signal for the light; I will use the missing pulse circuit to derive a steady signal to hold the arm up while the light is flashing.

ok, SSCpal, give the man a GA.

I'd have to agree. Easy as pie, quicker to accomplish, more like the original, and more reliable.

I'm stuck with the GM column and aftermarket wiring kit. If there is an easy way to use those to accomplish the job, I'm all ears.

An indicator switch is basically a single pole ON-OFF-ON switch. You have not specified the vintage of your steering column, but if it is intended for a vehicle with flashing signals the centre contact would have been wired to a flasher unit, the other side of which would have been connected to a fused supply from the battery. If the column was for a trafficator-vintage car, the centre contact would go directly to the fused supply. Make sure this contact goes to a fused supply (or is combined with other switches receiving a fused supply). The outer contacts of the switch each then go not only to the solenoid which holds the trafficator up but also to power your aftermarket flasher, of which you need one for each side.

Thanks, phph001, I think I see. The column is from a late model GM pickup truck, so the first case you describe should apply. I can hack the fuse panel (just wire around the flasher), then use the "turn signals", which will now be steady high, to work the arms, and use a flasher at each trafficator to flash the light. Does that sound workable?

First good answer reading down from the top.

GA.

Put in a capacitor and a resitor. Wire them up so the capacitor is shorted to ground through the resitor and is charged with your pulses.

Please post a circuit diagram.

beat me to it. A simple charged cap, discharging thru a resistor to ground with the charge used to "hold". resistor size determines how long it stays charged.

Update coming as soon as I refresh my memory on how to insert an image.

I built the missing pulse detector and tried it with an "electromechanical" flasher, one of 3 or 4 types available, including the old thermal and the new electronic units intended for LED turn lights. The flasher doesn't put out a true square wave and I had to add a little circuitry to get a pulse that reliably works with the 555. Then I tried this: <broken images>

...which was suggested by several good folks in this thread, and it works fine! The turn signal keeps charging the big cap, which discharges thru the relay. Once the turn signals cease, the relay drops out in a few seconds. The diode keeps the turn lights from draining the cap, and the resistor limits the initial inrush current to protect the flasher. It works without the resistor, but I'm not sure how long the flasher would last. I'm gonna try it on the bench and see.

Sorry if the image doesn't show. I'll work on it.