Pyrotechnic Timing Device

[alec_t]

I've revised the circuit to provide noise rejection and safer power-up, as per the attached.
R3/C1 slug the circuit response to reject spurious trigger pulses less than 0.1 sec duration. R5/C3 and R8/C6 provide power-up reset to the 555s. Q1 has been added to prevent spurious motor turn-on during the 1/2 sec delay.

Edit: It might be an idea to add a push-button 'emergency stop' switch between the RESET input of 555 U3 and ground.

 

[KeepItSimpleStupid]

It would be based on this part: **broken link removed** the LM334. It's actually good for 1 to 40 V and up to 10 mA. A resistor, a diode (adds 0.6 V) for reverse polarity protection and the device.

A bridge rectifier (eliminate the diode) would allow either polarity and add 1.2 V

This becomes the FRONT END for a timer. An optocoupler (either FET or transistor) would be the output of this trigger.

 

[4pyros]

OK as far as interfacing with the fireing system.
The one system we have uses a bridge rectifier into a 5 volt reg (7805) then to the opto.
I have some 5.1 volt zeners and 4N35 optos that I would like to use.
Can anyone tell me what resisters I would need before and after the 5.1 volt zener to get the 4N35 to work off a 9 to 24 volt source?
Andy

 

[mbarazeen]

for 4N35 input forward voltage= 1.18
current= 100mA
your source would be 5.1V
for min input voltage the resistor before zener= (9-5.1)/ 100 x10^-3 = 39 ohms
verify zener power .............
at max input zener current= (24-5.1)/39 = 485mA nearly 0.5A
so power dissipation= 5.1x0.5 = 2.55 W for actual power multiply by duty cycle (factor) and make sure you have right zener. (since its a pulse you will get very low value)
for after zener the resistor would be = (5.1-1.18)/0.1 = 39.2 ohms

hope i didn't make any mistake on above calculation
Edit: you have to calculate the power of the before resistor = max power = (24-5.1)^2 / 39 x duty factor

 

[4pyros]

current= 100mA

I think that should be 20ma max for the 4N35 not 100ma. The data sheet lists 60ma max.
20ma. plus 10 persent should be 22ma.
So it should be like this?
Vin=9V Vout=5.1 Imax=22ma so the resister should be 177 ohms, is that right?
but at 36 volts the resister needs to be 1405 ohms?
So should I use the high on or the low one or somthing in between for the load resister to cover in input rang of 9 to 36 volts?
The zeners I have are 5 watt.
Andy

 

[KeepItSimpleStupid]

Datasheet is here: https://www.electro-tech-online.com/custompdfs/2011/11/83717.pdf

Absolute max is 60 mA. Typical vales are 20 mA. So make 20 mA and Vf =1.2 V the design parameters.

The design process should go something like:

Pick a current bigger than 20 mA; Say 25 mA
Limit the current to this value by R = (Vin-5.1)/25e-3
Size the resistor in series with the diode as R = (5.1-1.2)/20 mA

Check the power disipation of the resistors and the diode: e.g. P = VI

Choose resistors that are close to calulated values.

 

[4pyros]

Ok I have found several on line zener resistor calculaters that use a input voltage range.
Thay all come up with a 130 ohm high power load resister to cover 9 to 36 volts.
The load resister for a 36 volt input disapates 7.34 watts, at 24 volts its only 2.75 watts.
I am thinking with the short duration of the pulse from the firing system that I will not need such a high power resister.
I am off to see what I have laying around.

 

[KeepItSimpleStupid]

Yep, your duration is small.

The LM334 is < $1.00. Add a 0.05 resistor and your at a $1.00. 10 mA max, though.

 

[alec_t]

I am thinking with the short duration of the pulse from the firing system that I will not need such a high power resister.

It would be safer to assume that, in the event of a fault, the firing pulse could be much longer than the usual 1/2 sec or so. If you don't go the LM334 route that KISS suggested then it would be advisable to rate your series resistor (input side of the zener) at the full ~7W.

 

[4pyros]

It would be safer to assume that, in the event of a fault, the firing pulse could be much longer than the usual 1/2 sec or so. If you don't go the LM334 route that KISS suggested then it would be advisable to rate your series resistor (input side of the zener) at the full ~7W.

ya I know but................

That would be way over kill and the firing systems are designed to fail open.
I am going with a 5 watt load resister thats fine for the more common 24 volt systems.
If anything I would use a 78xx series regulator.

Anyway would you update the 1st drawing to show the zener diode and load resister?
You can use 5 watt parts if you like.
Thanks Andy

 

[alec_t]

Here's the updated schematic. It only needs 1.2mA to saturate the opto-transistor in this circuit and according to the datasheet for the 4N35 the CTRsat for an opto-diode current of 4mA is ~50%, so 4mA through the diode is plenty for saturating the transistor here. The values shown for R1 and R10 should give an opto-diode current of ~5mA and 6mA for firing voltages of 9V and 36V respectively. The max power in R1 is ~1.8W at 36V and average power will, of course, be much lower; so 1W rating should be more than enough.

 

[Reloadron]

Way back in the beginning KISS mentioned the use of this timer. When triggered it uses the 4N35. Looking at the data sheet on page 3 they show the triggering circuit using the 4N35 and they also get into a detailed explanation of the opto coupler. Wouldn't something like that work or am I missing something? If using that configuration is way off base forgive me as I got here somewhat late.

Ron

 

[4pyros]

Ron; Although its a vary nice timer it would not work for what I need without modifications.
It dose not have a timing mode that I can use, and the speks for the opto are off. It should be 10 to 60ma for a 4N35 not 5 to 80ma. It will not do timing down to a 1/2 sec. and its a $25 kit.

 

[4pyros]

Here's the updated schematic. It only needs 1.2mA to saturate the opto-transistor in this circuit and according to the datasheet for the 4N35 the CTRsat for an opto-diode current of 4mA is ~50%, so 4mA through the diode is plenty for saturating the transistor here. The values shown for R1 and R10 should give an opto-diode current of ~5mA and 6mA for firing voltages of 9V and 36V respectively. The max power in R1 is ~1.8W at 36V and average power will, of course, be much lower; so 1W rating should be more than enough.

alec; Thanks for taking to time to update the schematic. Why would you limit the input of the opto to 5ma. I have seen problums in the past with using the low rating for the LED. In fact some different manufacturers 4N35's would not even work. I think it safer to go with 20ma.

 

[Reloadron]

OK, got it. I understood about the timer not having what you needed as to features I was just unsure about the 4N35 opto. I return to work today after being out of town, I'll look at my stash and pices parts and see what I may have lying around that may help you out. I may have a commercial solution lying around here.

Ron

 

[alec_t]

Why would you limit the input of the opto to 5ma. I have seen problems in the past with using the low rating for the LED.

I'm helping to save the planet by not wasting power . The LED current required depends very much on the load on the opto-transistor collector. Perhaps those problems were a result of poor design in the past. With a 10k collector load the 5mA LED current should be nearly double what is needed (according to the datasheet) to drive the transistor into saturation and trigger the 555 if using a 12V supply.
As you already have the 4N35, convince yourself this works by trying a simple experiment. Pass 5mA through its LED and measure the collector voltage when using a 10k collector resistor fed from a 6V or greater supply. The answer should be << 1V. Anything less than 1/3 of the intended timing circuit supply voltage would be enough to trigger the 555.
Edit: The only other reason I can think of for using a higher LED current would be to reduce sensitivity to noise pick-up. My suggested circuit already has noise filtering, but you could beef that up by connecting a capacitor (say 100nF) directly across the LED.

 

[4pyros]

OK, got it. I understood about the timer not having what you needed as to features I was just unsure about the 4N35 opto. I return to work today after being out of town, I'll look at my stash and pices parts and see what I may have lying around that may help you out. I may have a commercial solution lying around here.

Ron

Looking forward to seeing what you dig up.

 

[4pyros]

I'm helping to save the planet by not wasting power . The LED current required depends very much on the load on the opto-transistor collector. Perhaps those problems were a result of poor design in the past. With a 10k collector load the 5mA LED current should be nearly double what is needed (according to the datasheet) to drive the transistor into saturation and trigger the 555 if using a 12V supply.
As you already have the 4N35, convince yourself this works by trying a simple experiment. Pass 5mA through its LED and measure the collector voltage when using a 10k collector resistor fed from a 6V or greater supply. The answer should be << 1V. Anything less than 1/3 of the intended timing circuit supply voltage would be enough to trigger the 555.
Edit: The only other reason I can think of for using a higher LED current would be to reduce sensitivity to noise pick-up. My suggested circuit already has noise filtering, but you could beef that up by connecting a capacitor (say 100nF) directly across the LED.

alec; The problem we had was not so much as a result of poor design. It was an interface to a mainframe printer. The printer manufacturer changed the interface on there side and then only the interfaces we with a sertin manufacturers opto would work. We had to go back and change all the opto load resisters to make them all work. If the load on the output transister is unknown what current would you use for the LED?

 

[alec_t]

If the load on the output transister is unknown what current would you use for the LED?

Personally, I would hope never to be in that situation and I don't see how you're in that situation here. However, if hypothetically you were, then you would have to design for the maximum non-destructive current, minus a margin for safety. So if Imax is 60mA the LED current could reasonably be as much as, say, 40mA. If I had to use that much current and cope with a 9-36V input range I'd go with a constant-current feed to the LED rather than use a dropper resistor.

Edit: What sort of pyrotechnic displays involving mainframe printers do you put on ??

 

[mbarazeen]

I think that should be 20ma max for the 4N35 not 100ma. The data sheet lists 60ma max.
20ma. plus 10 persent should be 22ma.
So it should be like this?
Vin=9V Vout=5.1 Imax=22ma so the resister should be 177 ohms, is that right?
but at 36 volts the resister needs to be 1405 ohms?
So should I use the high on or the low one or somthing in between for the load resister to cover in input rang of 9 to 36 volts?
The zeners I have are 5 watt.
Andy

ok i missed that information, if 20mA is the nominal current, you can work out the resistor for min input current as you mentioned. 150 or 120 ohms can be the right load resistance in this case. the current limiting resistor after zener can be 180 ohms. the rest you have to conform the power capabilities...