Hi

I am designing a multi stage optically triggered coil gun for a school project

I have an optical trigger which drops the voltage fron 12v to 0v

this triggeres the comparator and then an emmitter follower

my problem is this

I want to limit the maximum pulse width incase the projectile gets stuce infront of the sensor

I have an and logic in the circuit which could be used to limit the pulse

I need a one shot timer with the pulse length of about 0.5s that cannot be retriggered until it is reset

I don't mind what the reset is
it could be
a) a time period passing
b)a button
c)Vtrigger rising abouve a certain value

so far the only circuit I've found has a large parts require,ent

two 555's some resistors and caps

the first 555 cannot be retriggered until the pulse of the second 55 has run out.

this circuit

http://home.cogeco.ca/~rpaisley4/LM555.html#27

anyone got a solution with a lower part count?

 

If i understand well, You need a not retriggerable monostable.
Maybe this work fine.

Attached Images

LM555.GIF (5.0 KB, 24 views)

 

after the trigger voltage has raised above 1/3vcc and the timer has timed out could the circuit then be retriggered?

 

Yes, the 555 circuit can be retriggered. this is my solution, I forgot to show the reset, but you just have to short the capacitor to reset.

Attached Images

LATCH.gif (10.7 KB, 14 views)

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shouldn't the cap discharge through the two 100k resistors after a period of time?

 

No, sorry if my diagram was not clear.

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I meant Sebi's diagram

shouldn't the cap dscharge through the two ressistors connecter to the +v rail and reset the circuit?

 

When the trigger switch is closed, the output goes high and the capacitor starts to charge thru the 1 meg. When the voltage on the cap gets to 2/3 the supply voltage, the cap is discharged and the output goes low. It stays that way untill the trigger switch is closed again. The diode prevents the input from being re-triggered, but as I recall, you can't retrigger the 555 anyway, until the pulse has ended.

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so as the projectile passes the phototransistor the switch would open again ressetting the circuit?

 

You will have to show the circuit to make sense of that question.

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the projectile innterrupts the light between the led and phototransistor allowing the voltage to the second transistor to go high triggereing the circuit

edit: the phototranstistor must be in that configuration because it must also trigger anothe circuit

Attached Images

trigger.JPG (28.9 KB, 14 views)

 

The 555 is triggered when the 2nd transistor goes low, so it won't work.

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Oh and doing this is prety useles unles you have a MOSFET or IGBT to switch the capacitator.A SCR will stay thurnd on until very litle curent is flowing trough and this means a discharged capacitator and so there is no power source and it makes no difrance.

Only use i see is if the charging is always on wich is not very smart to do.And its stupid to do whith a SCR coilgun becuse then the SCRs wont thurn off and so short the capacitators.

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the switching will be done using mosfets

the light path between the led and phtotransistor is normally clear, therefor the phototransistor conducts when the path is clear, this in turn lowers the base of the second transistor to 0v keeping the second transistor off.

When the projectile blocks the light from the led, the phototransistor no longer conducts. This raises the voltage at the base of the second transistor to be 5v
this causes the second transistor to conduct.

This triggers the 555 it's output goes high and the pulse ends even if the projectile gets stuck blocking the phototransistor from the led, keeping the second transistor conducting.

Once the second transistor stops conducting and the pulse has ended the circuit involving the 555 timer is reset.

Have I got that right?

 

"Once the second transistor stops conducting and the pulse has ended the circuit involving the 555 timer is reset."

No, because the 2nd transistor is AC coupled to the 555. Pin 2 of the 555 goes high by itself and the 2nd transistor turning off does nothing.

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