Timer with long "fire" time

I'm looking for a timer with a programmable time for the firing circuit (in the 5s range). I think the Missile Works PET timers could do this, but I'm not aware of any current timers that can. Do you know of a timer that has both programmable delay and activate times?

Note that this is not to ignite a motor, but to activate solenoids.

If you can't find anything off the shelf it should be pretty easy to design a simple circuit to do that with a 555 timer. Lots of YT videos on how to do this.

I'm pretty sure the new egg timer proton could be programmed to do that

I think the Eggtimer quantum can too.

John- would love to hear more about your project!

Mike Momenee
TRA #12430 L3

The Eggtimer's Quantum and Proton both have a long dwell time mainly for use with servo's (and solenoids probably too).

jmmome said:

John- would love to hear more about your project!

Click to expand...

This is for drop-away boosters on a 6" Ariane 6 model:
jcrocket.com/ariane6.shtml

I will look into the Eggtimer. At first glance, I'm leery of baro-only in this application (not a great location for venting and I want a short staging delay) and would prefer accelerometer triggering.

The Eggtimer Proton uses baro and accelerometer. I would email Cris Erving about it.

The Eggtimer Classic, Quantum, and Proton all would support this, they offer up to a 9 second "on-time" in airstart mode. I did that just for this type of project. How much current are you looking at? You'll need to add a reverse flyback diode across the solenoid coil, and a capacitor (about 47 uF or so) across the solenoid's +V lead to GND isn't a bad idea either. See the diagram below. (I love Google images, they're way better than anything I can draw...)

Thanks for the response!

There are four solenoids, each one draws 0.45A at 12v. I have tested and they work fine from a 3-cell LiPo battery, although the coils get hot after 10s or so activated.

For my instruction (electronics neophyte here), why is the extra circuitry necessary? It looks like you're creating a solid-state relay, but presumably 2A for 5s falls within the specs for direct output from the Eggtimer.

I think the Raven altimeter can "hold the switch closed". When I ask Adrian, for the same reasons, I replied to me with the FIP interface and some explanations. But I don't know if an extra circuit is necessary...



You can also add some thresholds before releasing boosters.

Steeve

John- it looks like an amazing project!!

JohnCoker said:

Thanks for the response!

There are four solenoids, each one draws 0.45A at 12v. I have tested and they work fine from a 3-cell LiPo battery, although the coils get hot after 10s or so activated.

For my instruction (electronics neophyte here), why is the extra circuitry necessary? It looks like you're creating a solid-state relay, but presumably 2A for 5s falls within the specs for direct output from the Eggtimer.

Click to expand...

You can ignore the transistor part of that schematic, that represents the output stage of the altimeter. You definitely need the flyback diode across the coils (it's in the REVERSE direction of the energizing current flow), and the capacitor across the +V and GND is probably a good idea too.

The Classic, Quantum, and Proton will hold the output closed continuously if you want, but that's probably not something you'd want to do with a solenoid. Your 5 second "on-time" is probably more than adequate, and the combined current draw shouldn't be a problem.

cerving said:

You definitely need the flyback diode across the coils (it's in the REVERSE direction of the energizing current flow), and the capacitor across the +V and GND is probably a good idea too.

Click to expand...

Thanks for the info. Since there are four solenoids wired in parallel, should I put a diode and cap across each or just across the outputs of the timer?



By the way, this is the solenoid (I have no idea about the inductance, that was just required by this online circuit drawing program I found).

JohnCoker said:

Thanks for the info. Since there are four solenoids wired in parallel, should I put a diode and cap across each or just across the outputs of the timer?

View attachment 380632

By the way, this is the solenoid (I have no idea about the inductance, that was just required by this online circuit drawing program I found).

Click to expand...

You can probably get away with one since they're in parallel, just make sure the diode is rated for at least 2A. Something like a 1N5401 should work.

John,

Your schematic in post #15 may have issues. What does the voltage source symbol represent?

In general, you don't want to quickly switch the voltage across a capacitor. That can lead to current spikes which may damage the driving circuit or the capacitor itself. Do you have a more complete schematic that we can check?

Randy

The capacitor increases the discharge time of the coil after the solenoid is de-energized, it's an additional circuit protection for the output transistor in addition to the flyback diode. Here's an app note from Silego for one on their power switching devices... we don't use them, but it has some good advice.

https://www.silego.com/uploads/Prod...nductive Load Power Switch Considerations.pdf

You definitely want the diode. Since this is pulsed with a very low duty cycle, a 1N4000 series is overkill. (8.3ms half cycle surge rating of 30A) A 1N4148 would do in a pinch but its surge current rating is marginal.

The capacitor is useless. When the outputs turn on, current will be zero, building until limited by the coil resistance. That is just the nature of inductors. Capacitors on the other hand will start at V=0 but current equal to as much as it can get. When the outputs turn off, it forms a LC tank circuit that wants to oscillate.

Oh, and using a polarized cap there is a bad idea because it sees both positive and negative voltages.

RCBrust said:

Your schematic in post #15 may have issues. What does the voltage source symbol represent?

Click to expand...

The firing circuit of a flight computer (here specifically the Eggtimer). Note that I will be holding it on for 5s.

Only cerving knows if the Eggtimer has any output current limiting or what the current handling capability is, but if it's a simple low-side low Rdson MOSFET switch, I would not put a capacitor on it.

Note that the capacitor shown in the first schematic in post #10 is across the power supply, not the solenoid.

Randy

All Eggtimer altimeters except for the Proton use BJT's, coupled to the logic with optoisolators. The Proton uses automotive-grade driver IC's with built-in current limiting, thermal protection, and a number of other protections as well. RCBrust, I missed that the cap was across the power supply... of course you need one there!

OK, so I take it that I don't need the cap (driving four solenoids from the Eggtimer output), but the diode is still a good idea.

The Eggtimer Proton has both accelerometer launch detection and 6 channels - that can be ‘ganged’ in software. If there was a current load concern, the four could be put on individual channels and triggered together.

JohnCoker said:

...but the diode is still a good idea.

Click to expand...

The diode is a must.

Randy

Crumb fire said:

I think the Raven altimeter can "hold the switch closed". When I ask Adrian, for the same reasons, I replied to me with the FIP interface and some explanations. But I don't know if an extra circuit is necessary...

View attachment 380571

You can also add some thresholds before releasing boosters.

Steeve

Click to expand...

Thanks for responding; this is correct, though the latest interface has a little different location for the latched outputs. The latched outputs stay on until landing is detected.

Adrian A said:

Thanks for responding; this is correct, though the latest interface has a little different location for the latched outputs. The latched outputs stay on until landing is detected.

Click to expand...

Thanks for the info. That is a useful feature, although it won't work in this case (I think it would burn out the solenoids).

OK, I got the booster locks CR completed today. I built a simple breakout board for the four solenoids and you can see I took the advice to add a "flyback diode." It works nicely with a very satisfying snap when powered by a 3-cell LiPo pack.

For more info, see my Ariane 6 page.

Not sure how I missed this thread until now...love the project, subscribed! Is there a target for the first flight?

Viperfixr said:

Not sure how I missed this thread until now...love the project, subscribed! Is there a target for the first flight?

Click to expand...

Thanks. Not this year for sure (no launches on the books). Hopefully next June at Mudrock. (I have quite a backlog of rockets that need to fly at this point!)