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Ic555 Timer Advice


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I'm trying to use an IC555 timer circuit to send a 1 second 12V pulse when the ignition is first turned on.

If I use a 555 timer chip, and leave the trigger pin permenantly connected to ground, will this start a cycle every time power is connected, or does the 555 need to see a falling edge before triggering?

Cheers in advance

Ian

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From teh datasheet...

Trigger Pulse Width Requirements and Time

Delays

Due to the nature of the trigger circuitry, the timer will trigger on the

negative going edge of the input pulse. For the device to time out

properly, it is necessary that the trigger voltage level be returned to

some voltage greater than one third of the supply before the time out

period. This can be achieved by making either the trigger pulse

sufficiently short or by AC coupling into the trigger. By AC coupling

the trigger, see Figure 6, a short negative going pulse is achieved

when the trigger signal goes to ground. AC coupling is most

frequently used in conjunction with a switch or a signal that goes to

ground which initiates the timing cycle. Should the trigger be held

low, without AC coupling, for a longer duration than the timing cycle

the output will remain in a high state for the duration of the low

trigger signal, without regard to the threshold comparator state. This

is due to the predominance of Q15 on the base of Q16, controlling

the state of the bi-stable flip-flop. When the trigger signal then

returns to a high level, the output will fall immediately. Thus, the

output signal will follow the trigger signal in this case.

So from that, if it wont work if just held to ground all teh time.

PDF is here -> http://www.semiconductors.philips.com/acro...A_SE555_C_2.pdf

HTH :)

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One thing you need to be carefull of with the 555 (or tripple 5 as well used to call them ;) ) is the crowbar effect of the chip, it can be solved by the addition of a capacitor but I have forgot the value and location :(... i'll do a quick google :)

I think the crowbar was limitted to the TTL versions of the 555, CMOS ones dont tend to exibit the same characteristics dur to the way they operate.

CMOS Datasheet here:

http://www.intersil.com/data/fn/fn2867.pdf

It's been a very long time for me, I used to make those fake alarm flashing LEDs from 555 astables when I was a kid for extra pocket money... its been a good 5 years now since I gave up being a design engineer and started consultancy..... I miss playing with toys :(

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hmm... Summer 1989... well done Jason for finding such a detailed answer mate

OMG thats spooky... ;) same here... but I went on to do A and a degree and then a few more years after that in electronics design.... shame I left it realy :)

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Put a 100k resistor from pin 2 to positive, and a 100nf cap from 2 to earth.

The resistor will keep it high, and the cap will send it low briefly for a couple of milliseconds causing it to trigger.

Ooh is the 555 still used? I used to do electronics as a hobby in my teens - 20 years ago. Wow. Sorry don't remember much about it though...

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  • 2 weeks later...

An easier way would be to use a low consumption 12v relay with a series large value elec cap in line with one of the coil connections. You would also need a resistor in parallel with the cap to discharge it, must be high enough value not to hold the relay in. They idea is that when power is applied it charges the cap via the coil and thus brings in the relay at the same time. As soon as the cap is nearing full charge the relay will drop out. The resistor will discharge the cap when you turn off the ignition, ready for next time.

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That would do the job fine - but it depends on the timing.

The cap will charge at different rates according to the Battery, plus the electrolytic variant with high value capacitors.

If it's not that critical, then it doesn't matter. The simplest methods are somtimes the best.

Don't forget to put a diode across the relay so no reverse emf spike goes back into the car system.

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  • 2 weeks later...

tim.gif

Here's both.

Not sure what components you have lying around or what relay you will get, so on the simple circuit you will have to play about with the resistor and cap until you get the timing right. keep the resistor as HIGH as possible - at worst it will take a few seconds more to discharge the cap.

The 555 can sink about 200mA so is enough for many of the low powered relays out there. These values will give you 1.1 secs.

So.... whatya doin?

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Cheers Geoffers - magnificent help!

So.... whatya doin?

Nothing too interesting.... while I'm waiting for my mota to clear ESVA, I'm playing with some stuff to go into it.

I've got a Lilliput screen that I'll probably connect to a DVD player and maybe sat nav if the quality through the composite input is good enough. I'm trying to make it come on automatically with the power instead of having to press the button on the front (I've got plenty of time to play with :whistling: ) - the timing circuit is to close a relay that shorts the contacts of the press switch.

I'll let you know how I get on.

Ian

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Ahhh - you should have said.

The circuit Necromax gives is dead right - just put a cap across the power switch. The resistor will just change the timing of the cap and still hold it high.

If its just auto power on for the liliput you want... you can have a look here its a solution to modify the lilliput so it auto powers on :)

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Yeah - I saw that a while ago, by my screen is a different model - just composite input, no VGA or touchscreen - and the PCB is a different layout. Rather that try to find the analogous connection points with the associated high risk of me knackering the screen, I figured I'd stay within my skill level and do a slightly less elegant but hopefully successful alternative.

To complicate matter further the screen appears to need 2 presses of the button to start up when power is first applied, and thereafter only one to put into standby and wake up.

Ian

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Bloody thing - I wish I'd never bought it in the first place! I've got everything together, with an extra circuit to give 2 pulses to the 'on' switch when the power comes on - it worked great for a day.

Come this morning, it's now decided it only needs one press to power up, so my circuit is turning it on, and then immediately back off again.

How can something pretty fundamental like this be so random? It's easy to take out one of the pulses at the moment because it's not installed, but once it's in the car - what happens if it goes back to the two pulse behaviour.

Rant over....

Ian

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