I've built 555's before but it's been a long time.

I'm working on a new rocket launcher to launch Este's model rockets. Going to initiate the grandkids into rocketry.

The target goal is to rebuild a circuit I built a very long time ago. It had a manual LAUNCH button. You pressed it and the rocket launched. This time I want to integrate a count-down timer. No, the timer itself doesn't need to count down it just needs to give a 1 second high pulse at the end of 30 seconds, then shut down. So it's going to be a "One-Shot" that starts cold then goes high as described.

I hate when people ask for free circuits; and here I am asking. My apologies but the old grey matter isn't working like it used to.

For the curious: here's the original control circuit with push button. Forgot to label the electric match.
View attachment 357293

And here's a timing diagram:
View attachment 357292
I want to replace the NO Momentary PB switch with an electronic circuit. Relay or MOSFET - either is fine. One safety concern is I don't want the electric match lighting off the moment I clip the two alligator clips to the match wires.

 

Probably could use pin 7 (discharge) as a ground circuit for a relay as long as pin 7 isn't held to ground at all times when not powered. Then the relay would be active constantly until I turned the circuit on. Then I'd only have 30 seconds to make the connections and get down range before the launch.

I would like to use a decade counter to activate a loud signaler when there's 3 seconds to launch.

 

Is the timer initiated by a momentary push button?
If so is there need for a reset during the count down?

 

HMMmmmm. Good questions.
Well, I don't want any accidental firing when connecting the alligators to the electric match. So I suppose I want it cold until I'm ready to launch. Then with the push of a button or toggle of a switch I have 30 seconds to get down range for recovery. After the 31st second the one-shot should go cold again. That may exclude a toggle switch.

[edit] Just coming to mind - Would probably want an abort switch. Perhaps a DPDT switch to show when active and when in the abort position via LED's. A RED LED could indicate an armed situation. When the switch is activated (thrown to abort) it disconnects the firing circuit and lights up a green LED. [end edit]

 

I have an ignition system the gives a beep every 5 seconds until the last 5 seconds when the rate changes to 1 beep per second.
Gives noticed if it's still safe to disarm the launcher

 

I think I'm going to change the second switch (on RED LED) from SPDT to DPDT. Switching that off I can use the other pole to rapidly discharge the capacitor through a low resistance device.

 

Do you prefer a relay to fire the igniter or a mosfet?

 

I think this 555 pulse delay circuit does what you want:
C1 is connected to V+ instead of ground so that the application of power is less likely to trigger the 555.

Rload is just for sim purposes and would be whatever load you want to drive.

 

Version 1:
When power is applied through Sw1 LED1 will light to indicate the unit is in the Disarm mode.
At the same time LED2 will light to indicate mosfet M1 is not activated or shorted and it's safe to connect the igniter.
When Sw2 is moved to the Start position LED2 will brighten indicating the igniter has continuity and begin the 30 second count down timer.
Additional bells and whistles can be added.

 

Currently at a rocket launch.
More designs later.

 

This winter will be a good time to rebuild a launch controller with the features I want.

I'm pondering using a couple decade counters. One for the countdown and the other to blast a horn within moments before the launch. Use your imagination. I haven't decided on anything specific yet.

Using two decade counters I can divide out up to 100 counts. Use of an AND gate connected to both 10th outputs would fire the rocket. Exact timing is not a concern. Just want time to get out into the recovery zone. Meanwhile, with the second counter at 10 the first counter can begin blasting a horn at outputs 1, 3, 5, 7 & 9. That'd give five one-second blasts right before the second counter reaches the 10th output. Also at the 10th output I can disable the counting.

This is evolving in my head along with the cobwebs and mold. This is more of an open discussion to hone in on the best possible design. I've used DDFF's before as a clock source using both set and reset as part of the timing. I have that drawing somewhere but havent' seen it in a long time. Might require luck to find it. But it's in here somewhere. Hopefully.

 

Perhaps one day I'll take the kids to a real launch. Not far from ATK Thiokol, so watching a solid rocket booster test might be exciting as well. On rare occasions I've heard tests from my front door. Window was rattling so I went out and heard a distant thunder. Later found out it was an SRB test for a shuttle booster.

 

Here's what I've managed to come up with. I'm almost certain there are better ways. I'm open to suggestions. Except micro-controllers. I'm old school and have never learned coding.

OK, after much editing and reconsidering how I want things to work I've come up with this:

DRAWING REMOVED. SEE POST 15

So, OK, let me have it! What'd I goof up on?
The thinking is that when SW3 is closed the timer goes active. However, because MR (Manual Reset) - OH GEEZ! I FORGOT THE RESET BUTTON!
I NEED A BREAK. I'll be back after dinner.

 

I see where you're going with that.
Will work on it also.

 

OK, Here's my latest and greatest: (I think this is right) (but y'all will let me know if not).
View attachment 357318
Master switch (SW1) off, all is grounded.
Master switch ON, SW3 set to "Manual" will force a reset on the 4017's. U1 (NE555) will not be running.
Switch SW2 to ON (armed) - you can manually fire the rocket via SW4pb or you can switch SW3 to "Auto" and the timer will begin and the counters will count. When U3 reaches Q2 - U2 outputs Q0 through Q9 will start blowing the horn at Q1, Q3, Q5, Q7 and Q9. When the next clock pulse hits U2-CO will carry out and clock U3 from Q2 to Q3 and will fire the rocket. The counter will continue until U3-Q4 disables the counting process approximately one second after the rocket has been launched. As long as U3-Q4 is held high U2-CI will not allow the counter to count. U1 will still be running but no action will be taking place. Switching SW3 back to "Manual" will reset the 4017's and U1 will come to rest.
U4c acts as an AND between POWER and CLOCK. Every time the clock pulses the LED will count. This should be a ready signal to determine the clock is running. This acts as an alert to the operator. I've thought about adding a beeper to the output of U4c as an audible means of knowing when the clock is running.

With SW2 armed the rocket can be launched automatically or manually. SW4pb is just for that purpose.

 

Is Sw4 to be a remote operated switch that would plug into the control unit?
Are you using Estes igniters?

 

Will the 4017's reset if they have no power?
Should I directly power the 4017's and only power the 555 from "Auto" (SW3)?

 

Is Sw4 to be a remote operated switch that would plug into the control unit?

I figure the SW4 to be box mounted on the controller, not plugged in from a remote location.

Are you using Estes igniters?

That is the intention - as long as I can find them. Why? Do they have low resistance and are easily ignited?

 

If Sw4 is on the box how are you at a safe location when pressed?
I assumed the box is only a few feet from the launch pad.

 

Estes igniters are typically less then 1 ohm and require appx 1 amp to fire.
Will need a beefier mosfet.