Hi guys,

This is my first post on the forum, and hope someone can help. I've been through the threads and haven't found a solution yet.

I have, amongst other r/c aircraft a Dragonfly 2 channel helicopter, really just a toy. Controls presently are simple up, down, hover, spin left and right, non-proportional. Although described as a 2 channel control, chips used are the tx/rx-2b 5 function job designed for toy cars. Data sheet is here:

datasheet4u.com/html/T/X/-/TX-2B_SilanSemiconductors.pdf.html

It occured to me that if one of the functions could be made proportional, it would leave 2 spare channels to control direction (currently not available).

Early experiments are good. Replacing the original full speed on/off switch in the transmitter with a PWM circuit to control the main motor works well, the motor speed now being truly proportional, hopefully giving much better up/down/hover control. (so far without any changes to the receiver end)

However there's one snag so far with the circuit I've used. If any part of my body gets too close to the circuit (or the aerial), the motor spins up to full speed, dropping back to controlled speed if I move away again. I've tried various decoupling caps etc to no avail.

If anyone can help I'd be much obliged.

Davegriff

Attached Images

circuit.jpg (28.5 KB, 37 views)

 

Hi Davegriff,

That's a very clever solution that harks back 50 years to when we used "galloping ghost" controls for model airplanes, namely a PWM modulated control at very low frequencies. The inherent control was either full on or off.

I am not familiar with that chip set per se; however, I notice that your PWM frequency of 35 Hz is very close to some of the control word lengths. For example, "turbo" is
30 mS (33 Hz) (i.e., 4x2mS + 22X1mS). Have you tried a much lower PWM frequency, such as 10Hz or lower. That shouldn't make too much difference to the motor driving a helicopter blade. It can't respond to any frequency very well.
Good luck. John

 

thanks John for response. Yeah well I am getting on a bit myself, but young at heart.

Good idea though. I actually started at about 200 hz, went up to a few khz and back down (all same effect). Wasn't sure whether motor would run too jerkily at 10hz but will try anyhow.

Davegriff

 

more.......

Just tried circuit with PWM running about 10hz.

Seemed to work ok at first - no speeding up but not getting full revs because battery in chopper was a bit low so tried with fresh battery.

This gave zero to full revs but speeding up effect returned! Could it be I'm overloading the tx (or rx?) somehow. Tx is on 27mhz, uses 5 transistors, but apart from that I haven't given much attention to it so far.

I'll work out the tx rf circuit and post it when I'm done as that might help.

Davegriff

 

I think the very cheap single transistor radio receiver (a super-regen) overloads when the transmitter is near.

A real radio receiver has a lot more than just a single transistor.

__________________
Uncle $crooge

 

Suppose it could just about be the receiver at fault, and it doesn't show up normally because the signal's not usually pulsed.... but its surely more likely to do with added circuitry - that's why I asked the question initially. I've tried it again - full battery, 20ft between tx rx, aerial not extended - same result.

I have real tx's/rx's and could rebuild the chopper with one of these, or easier, go out and buy a £200 chopper to play with. but that wasn't the point of doing this. I just like tinkering, and pushing the limits of a £10 toy (albeit with real/imaginary receiver) appeals to me. I'd also like to learn something along the way.

Davegriff

 

I don't think the problem is in the super-regen receiver per se. That was all we had for years. I still suspect the problem is in the coding/decoding. Either the transmitter or receiver isn't handling it. When we had signle-channel, super-regen receivers and escapements, our pulse rates were manually generated and were very low, because the feedback loop was what the airplane did in flight. Albeit, helicopters are a lot more sensitive than our intermittantly guided free-flight models were, but for motor power, I think you can still go lower in frequency.

Now, why did the battery charge have an effect? Do you have effective capacitors and/or other noise suppression on your motor? It does seem odd, though, that the receiver can decode reliably the transmitter signal, but not simple on/off modulation.

As for low frequency PWN, obviously you can do it with discrete elements as you have shown. Last week, however, I bought a little signal generator on e-bay for about $120 USD that goes to fractional hertz, has adjustable duty cycle, and display of frequency and amplitude. I don't know if yout tinkering budget would include something like that, but I am very happy with it. The brand in the USA is Victor VC2002. Here's a link:
http://cgi.ebay.com/New-0-2Hz-2MHz-B...QQcmdZViewItem

John

 

Cheap toys use simple super-regen receivers.
Ordinary half-decent model airplanes, helicopters, cars and boats have used good super-heterodyne receivers for years and they work very well.

__________________
Uncle $crooge

 

Thanks for the link John - might get me one of those. Got several hobbies including computer programming and current pc coming up for renewal so should really look into replacing first as its used for business as well (but money not really an issue issue here) nice kit at good price though. So many things to do.

I'll work out tx circuit (this is for FUN and knowledge, audioguru) and post again. Got super het tx's and rx's lying around but like I said, could buy another heli but don't intend to yet.

Right now I need some sleep.

Thanks both for interest
Davegriff

 

The signal generator is not a kit, nor is it based on the old Maxim chip. It's made in China (of course) to ISO9001. The real manufacturer is not given, so I can't help you there. I have seen identical units on the Austrailan site for a similar price, if converted, and haven't checked the UK site. I got it because of the display and low-frequency range. It's max is 2 MHz, and the square wave looks pretty bad at the frequency. John

 

Sorry John, language thing - kit (or 'piece of kit') here also means any sort of equipment.

 

No offense taken. Right after I posted, I recalled the different use of the word, as in soldier's kit, mess kit, etc. I decided to leave it as was, because the price is so relatively low, one might think that it was a kit to be constructed and not a completed unit.

Any updates? With the PWN frequencies I am suggesting (e.g., 2 to 4 Hz), you could probably simulate them with your thumb or a mechanical switch, just to see if the problem still exists. John

 

Haven't tried lower frequency yet but here's the circuit + that of pwm.

I posted query first about six hours earlier, but when it didn't appear in the forum and could find no record of it I posted again. Its now appeared so is showing twice! Know how I can get rid of the other?

............................

Had gone to bed when I had an idea. The original circuit has a pull up resistor + diode linking the left/right and boost control pins (presumably to give full throttle during spins). Tried a pull up res on the output transistor of the pwm and its solved the problem! Great - now all I need to do is tidy up the tx and somehow use the two spare channels for direction (work on rx I think).

Guess that will teach me not to mess with a circuit till I know what it does.

Many thanks for your help John, will let you know how I get on. Davegriff

Attached Images

dragonfly tx.jpg (30.1 KB, 13 views)

 

Just one more thing. Am I likely to be upsetting the rf stages in any way by doing this mod?

 

I've looked at the two schematics you have posted, and nothing jumps right out at me as being wrong. But, I am more of a "try it" type person with a breadboard, rather than one who etches a PCB and solders in components based on my knowledge of design. One question though: Why do you power the PWM add-on from "B" rather than from Vdd? Could the added voltage drop across R9 during the PWM "on" cycle be enough to upset the transmitter?

The result that still bothers me is why the system seemed to work when the receiver batteries were low, but not when fully charged. John