Motor Run Timers

[Happy Hobit]

Hi All,

Can anyone recommend a source for a circuit to control the motor run time for a free flight electric airplane.

I'm planing on using a n-20 geared 4.8:1, a 125mm x 110mm Gunther push on prop and a 7.2v, 70ma NiMh battery pack

I've read about a circuit using a fet, an off biasing resistor and a capacitor that when charged, causes the motor to run until discharged.

The Happy Hobit :D

[Terry Lyttle]

How big is the model?

If it isbig enough, my answer is to use a lever switch hooked up to a Tomy mechanical timer, made froma windup toy. I have seen circuits using old tech (555 chips), but I have yet to use one. Usually my model/power system doesn't require timed flights...

[Happy Hobit]

Thanks for the response Terry,

How big is the model?

Well I have a Guillows Piper Super Cub 95 (24” WS) and I’m currently building a Fairchild 24 (15”WS). I’ll probably set up a Rubber powered stick first for my 6-year-old granddaughter.

I’ve built circuits to cycle a motor on and off, non model application, using a 555 driving a fet but I was looking for something simpler. I can probably breadboard a circuit, but why reinvent the wheel?

Jay

[ArtSvensson]

www.pennvalleyhobbycenter.com

Has a device (Item# MPT) for $10 US. Wt = 0.12 Oz. Handles 4.0 Amp or 15.0 Amp w/heat sink. Operates on 1-6 cells. Motor run time adjustable from 0.25 - 2.5 minutes "via a simple screwdriver adjustment.

[Happy Hobit]

Thanks for the Web site Art.

Their On-Line store nice, if they ever get it running right. I had to go to their ‘Old’ store to find the device you referred to. It looks like what they’re selling is kit with 6 components, probably a version of a 555 timer driving a fet.

I’m retired and have more time than money so I’ll probably build my own.

Thanks again for taking the time.

Jay

[KnifeEdge51]

Hobit, what's the AUW of your Cub 95? I've got one on the bench, and I'm in the process of trying to select my first free-flight motor. Can I ask you what motor you selected for it? I've got the KPOO and the LV N20 in mind. I think as soon as I get them both (en route), I'll experiment a bit. Also, what prop did you use? Thanks!

Nick

[steve lewin]

I knew I'd seen some circuits somewhere, but as they're both simple 555 timers they're probably not of very much interest.

http://www.aefa.asn.au/bulid.htm#Bladier
http://www.uoguelph.ca/~antoon/gadgets/rc-sw2.htm

Just for my own interest I did a quick breadboard of a simple capacitor/resistor discharge cct on the gate of a logic level MOSFET and it does sort of work. What sort of times are you looking for ? My main worry would be that MOSFET gates are not exactly calibrated for turn off voltage and big capacitors are notoriously imprecise and leaky so it might be tricky to get the timing accurate or even very repeatable. Fortunately the gate takes very little current indeed so you can afford to use quite high resistance in parallel with it.

Steve

[Happy Hobit]

Hi Nick,

Sorry about the delay, my cable modem has been out for the last two days

Let me first say that this was the first model I’d built in 35 years, and I used the balsa supplied with the model (I’ve since learned that this balsa is very heavy, but it does make a solid model.).

The 'Guillows Piper Super Cub 95' weighed in at 53 grams, 77 grams with LV N-20, gears, prop and battery. This will produce a wing load of .917 grams/SqIn.

I'm not sure what the capacity of the KP-00 is, but the recommendations for N-20 I’ve seen are, max AUW of 80 grams with a max wing load < 1 gram / SqIn.

My gearbox shaft is 2mm so I used a 125mm x 110mm Gunther push on prop. My current draw with this prop, a 4.8:1 gearbox and 7.2V, 70 ma NiMh battery, is .75 Amps.


Jay

[Happy Hobit]

Hi Steve,

I was interested in motor run times from 15 sec. to a couple of minutes. This is more of a rubber-band replacement than anything else. A repeatable of 25% would be fine. Can you give me some recommendations for component values and times? Most of my electronics gear is in storage and it’s going to take a shovel to get at it.

Jay

[KnifeEdge51]

Hobit,

Guillows wood IS heavy, but I can attest to a stong build. I've still got the wings to my Cessna 150 from them, and it is the only surviving part of the model. The wings are complete with no broken parts. Some torn tissue, yes, but the structure still rings true. :D

Thanks for the info on the Cub. Where did you obtain your gearbox from? Also, how in the world did you mount the motor WITH a gearbox inside that cowl? I tried it, and perhaps it's my mounting idea, but it seemed like it would be difficult to line the prop shaft up with the whole in the cowl. In any case, I'll figure something out.

Oh, where did you get your batteries from? Thanks for your help!

Nick

[Happy Hobit]

Hi Nick,

I bought the batteries and gears from http://homefly.com/ and built the gearbox my self, 10 tooth pinion and 48 tooth bull gear.

Next time I’ll go for 6 tooth pinion and 30 tooth bull then it will probably fit into the cowling.

Also check http://toddsmodels.com/ for gears.

Jay

[steve lewin]

Originally posted by Happy Hobit
I was interested in motor run times from 15 sec. to a couple of minutes.
It's like 2 different threads mixed up in here :). Anyway if you're still interested....

Try something like 100uF discharged through a 1M pot in series with a 220K resistor. That gives me around the sort of times you're looking for but I'm playing with 6V and high power logic level MOSFETs (all I've got handy but it shouldn't make any real difference).

I tried a couple of different MOSFET types and the time can vary quite a lot with the different gate voltages required to switch the device off so you may need to do some tweaking of values. You want logic level devices so the gate voltage required will be quite low. To save damage when charging the cap I'd also put a small resistor (1K or so) in line with it.

Good luck - Steve

[Happy Hobit]

Hi Steve,

Well I finally dug out some of my electronics junk.

I found an ECG66 (an IFR511 equivalent), a47uf electrolytic and a 330K resistor. Soldered them together on a piece of perf board with a diode for noise suppression and a 100 ohm charge current limiting resistor (a good idea). Wired on the N20 motor, gearbox and prop, clamped it in a vacuum base vice, wired in an old micro-switch and plugged in a 70 mah NiMh battery.

I got 10 sec. full speed run and 5 sec ramp down with a 7.2V battery, and 15 sec. full speed run and 5 sec ramp down with a 9.6V battery. :D

Now to find a 1Meg trim pot, a smaller push button and decide what plane to put it in.

Thanks for taking to time and helping me test out the Idea.

Jay