What is the max input voltage for a TTL?

[piratepaul]

I am trying to make a timer circuit to switch an LED on for 2 secs when I flick a switch ( ignition switch) then it stays off until I switch off then switch back on again.
I have done the 555 timer bit but now I need a single trigger pulse. I am gong to try a NAND gate where the 2 inputs are tied together via a NOT gate, if need be a string of not gates... This should give me a single pulse.
I am wondering if when I start the car there will be a voltage drop big enough to trip the pulse and therefore the LED will come on for a couple of secs when I don't want it to. The higher the inputs are the less likely the unwanted pulse.

What is the max I can input?

Tar&stuff.

[piratepaul]

Why does this baly tablet change what I write.

What is the max input voltage for a TTL? The title should be... Bolox bolox bolox.

[piratepaul]

Just to be clear. It is an airbag LED stuck to the inside of the clocks plastic thingi, sticky tape over the proper LED on the board.

A + wire from the ignition switch goes to a pot from which I can tap at least 5V, the tap from the pot goes to a NAND input and a string of NOT gates, the output from the NOT gates goes to the other NAND input. When the ignition is off, the direct wire to the NAND input is 0 so NAND output is 1... When I switch the ignition on there is a propagation delay in the NOT circuit, so both NAND inputs are high for the period of the delay... NAND output ( 555 trigger) is 0 for a fraction of a second. After the delay the NOT side of the NAND input goes low so output from the NAND is high.
Obviously the low pulse triggers the 555.

Any better ideas.

Tar&stuff.

[piratepaul]

The problem is when I turn the engine over there will be a voltage drop, this might be enough to trigger the 555.
How can I stop the second trigger pulse if it happens ( I don't know if it will until I try it on the car ). MOT aaaaaaaaaggggggggh.

Tar&stuff.

[piratepaul]

Just talking to myself don't mind me.

Well it didn't work... I think the pulse ( about 100 ns) is too short for the 555. I've had enough for today. I will try again in the morn using a relay to get a longer pulse... Yawwwwwwn.

Tar&stuff.

[Richard]

The maximum supply voltage for standard TTL is 7v, and the max input voltage is 5.5v. If you are going to use TTL in a car, it is essential to use a 3 terminal voltage regulator chip, such as the 7805 series.

A simple (and very effective - I have used it on numerous occasions) way of obtaining a switch-on reset pulse is by a resistor of, say, 4k7 value from the supply to the chip in series with a say, 10 microfarad capacitor, one side of the cap connected to ground. Connect the junction of the R & C to the reset pin.
On switch on, the cap holds the reset pin at ground for a few milliseconds - thus giving the effect of a negative going pulse. When the cap is charged, the reset pin is held high (which is usually the normal state). Obviously, if you want a positive going reset pulse, exchange the positions of the R & C, or use a simple TTL inverter. Hope this helps.

[piratepaul]

Thanks Richard... Food for thought. I have used an AND gate on a TTL one input is pulled up and the other down, when I switch the ignition, the one pulled low goes high and the one pulled high goes low... As the one which goes low is switched with a relay, there is a moment when both are high ( swithcing time of the relay ) at this point the AND output is high for a moment ( theoretically ( a bit more soldering and I will find out )) the high output switches a pull up resistor on the 555 trigger to low via a transistor...
As I say theoretically... I will find out tomorrow when I switch it on and see.

Tar&stuff.

[piratepaul]

If relay switching time is still not long enough I can use a series of relays...

[piratepaul]

Regulator wise, I have a car voltage converter, set at ... It was 4.5 v may be I will try 6 v. The only thing that is not regulated is one AND input which is tapped from a pot, one side earth the other to ignition switch. The switch side of the relay also goes to the ignition. The voltage converter is perminantly on. Fingers crossed.

[piratepaul]

Now as I think about it I could use one relay on one input and a string of relays on the other ...that way the TTL is regulated. Although the 12v side is not regulated other than in the alternator the 5v side would be.
Any way, its nearly done I will finish it in the morn ... If not ...plan C.

Tar&stuff.