F/F Timer - RC Groups
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Jun 29, 2005, 05:19 PM
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F/F Timer

Hi everyone.
I'm from Poland and I'd like to build 5inch wingspan Wilga for electric power.
I'm looking for schematics of little timer for my model.
I found picture of timer like that, but there i no word about schematic.

Help me guys. Pleaase.

Sorry for mistakes.
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Jun 30, 2005, 09:47 AM
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JMP_blackfoot's Avatar
I think it is detailed here :
http://www.aeronutz.flyer.co.uk/FFTimers.html ?
Jun 30, 2005, 11:24 AM
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peter frostick's Avatar
I have just started playing with these timers to freeflight test a model built for future Plantraco RC guidance! If you make the capacitor 22 Mfd, and the leak resistor 270K, the motor runs for about 15 seconds before starting to slow down --- (this assumes you are using an IRLM 2502 mosfet?) Doubling the resistor value will I hope! double the run-down time; so a one meg resistor value should be good for about 35 seconds.
As the mosfet chosen has such a low gate voltage requirement, a quick connection of the yellow coloured swith on the diagram is enough to initiate the motor run sequence.
This is the the simplest use of the timer circuit, and so does not need a "voltage injector" circuit to set the duration --- the injector circuit can of course be used to fine tune the flight duration if you need more precision!!
I would recommend a tiny pcb (as drawn) to give the Fet a bit of heat-sinking, as it is not designed to be a controlled resistor --- for a single Fet 300 ma should be OK as absolute maximum current!

Good luck with the project

Regards Peter
Last edited by peter frostick; Jun 30, 2005 at 11:35 AM. Reason: spelling!
Mar 08, 2006, 01:49 PM
Registered User
Peter Frostick,

Thanks for posting this little timer. I tried it out and it works great.

With IRF540 N channel mosfet, 100uF cap, and R=940K, the motor run time is about 20secs. But IRF540 is a dip type with heat sink …… so really heavy.

With IRLML2502, 47uF cap and R=1M, the motor run time is 53 seconds.
IRLML2502 is the only surface mount component on my PCB.
My timer at present is aprox 0.8 grams.
I still have to find a 1M SMT Resistor and also a 47uF SMT cap, that should make it lighter.

Last edited by Raj Quest; Mar 13, 2006 at 08:25 AM. Reason: added next post
Mar 13, 2006, 08:40 AM
Registered User
Just finished the timer posted by Peter, added a Li-poly battery monitor.
100uF cap, 1M resistor, and an LED are "dip" type while others are SMT components. Weight is 0.95g and motor run time is 80secs.

Last edited by Raj Quest; Mar 19, 2006 at 01:02 PM. Reason: added image
Mar 16, 2006, 10:57 PM
Registered User


Sounds like a very cool idea. I build Free Flight rubber power and I've never tried electric. A rubber powered Wilga would be a good project too. Maybe a peanut scale?

How will you build your Wilga? I have a small 3-view. (See the attached file). If you have better plans I'd like to see them. Do you have reference materials? Photos?

Best of luck!

Jul 31, 2006, 02:38 PM
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mr badwrench's Avatar
Hi, Ive been toying with micro Planes using a 1/64th scale RC car electronics for control. I was wondering If someone could lay out a circuit similar to your timer for me. I know what I need it to do but I drew Comicbook pages instead of paying attention in Electronice class So I dont know how to make the circuit.

I was wondering if you timer coulde be wired to run for 5-10 seconds every time it gets a signal from the RX??? this way I coule get a little more creative with my flying and controller setup...
Jul 31, 2006, 06:45 PM
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mr badwrench's Avatar
hey, I found something similar. I guess I just need to know what to look for.

some nice schematics and drawings. with this I could gently pulse the power button and the motor would run constant even if there was RF interference causeing it to lose signal you would still have smooth power untill you stoped pulsing or signal was lost completly.
Aug 17, 2006, 02:26 AM
Registered User
I just read this thread
Raj Quest do you have any photo from this pcb? i really want to try it
one more question, at what voltage the MAX flashes the LED? from MAXIM's data sheet they say 2.5-3.3V

Originally Posted by Raj Quest
Just finished the timer posted by Peter, added a Li-poly battery monitor.
100uF cap, 1M resistor, and an LED are "dip" type while others are SMT components. Weight is 0.95g and motor run time is 80secs.

Aug 30, 2006, 06:00 PM
Registered User
billystiltner's Avatar

That is some really awesome changes to that circuit.
About 15 seconds runtime is all you really need when testing out new models in the living room. Any longer and the plane would probably hit the ceiling.
Thanks for sharing your circuit and parts layout.

Aug 31, 2006, 08:30 AM
Registered Billy
I have built a couple of these as shown on the Aeronutz site and they work nicely. To keep the time consistent the voltage injected to the cap has to remain constant. In other words, as your motor battery voltage drops, the cap takes less time to discharge. The Aeronutz site also mentions using a separate injector battery to avoid this problem. The circuit also has a trim pot to make precise adjustments to the voltage/time. Nice and simple. I'm working on a simple design to try these out with as well. If it comes to fruition I will post it here.


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