Power holdup capacitor

[Chris Smith]

The 555 is the oldest and most reliable circuit around.

Once it cost a $1000 to build, now its just pennies per unit.

It can time out a circuit from once every 400 hours down to 100.000cps.

Its bullet proof and they even have a mil spec for one version which means heat, shock, and environment is not a problem.

I have used it to retro fit delay wipers in older cars for years, and its never broken.

If necessary, use a socket design to simplify any damage and replacement.

[haklesup]

I think dyarker hit the nail on the haed.

The output from a 555 could alone drive a 15mA load without a transistor or relay to isolate it. While output current varies with the supply voltage, at 15V the bipolar 555 can source 200mA (only 15mA sink for low output). THis chip takes 10mA on its own to operate though.

The 7555 which is CMOS does the same function but can source/sink +/-100mA at the output while using only 60uA to operate.

Both chips should be robust WRT failure in an automotive circuit so long as you bypass the chip (say 10uF) and filter the source 470uF or so to clean up the supply voltage of severe spikes. A car voltage does vary a little while driving and certainly when on or off so the precision of the timer will be poor but hitting 15 min +/- 2 min should be an easy target.

Type "555 calculator" into google for some nice sites to help determine the RC values and the basic circuit diagram.

I would power the 555 (with your circuit as a load on the output) from the battery directly and connect the accessory switch to the trigger input with a series resistor to protect it from overstress (EOS).

Come to think of it you would need either a transistor on the output or an inverter on the input. This is because the output is set high when the input is pulled low and stays like that until the input is returned to high and the time cycle runs out. This is if you want it on with the car running and on after turning off the key and the timer runs out.

Unless you want the curcuit off when the car is running and the timer running only when you turn the car off, then you need nothing but the 555 and a capacitor in series with the trig input. Pullt the side of the cap far from the trig input either up or down with a resistor depending on the default action you want. THis converts the trig input to edge sensitive rather than level sensitive.

[Smoke_Maker]

Thank you everybody for your input, dyarker, haklesup, Chris and all the others. I'm going to go with the collective experience of this group and use the 7555 CMOS, I did some Google work and found a circuit very close to what I need, I will get some chips Wednesday for testing.

Next question, what is a good way to have this list look at a schematic to make suggestions? My web site with a link in the post or maybe there is a free site to post to? I have Eagle and PCB express.

Thanks again, I feel comfortable at this forum.