I'm thinking of starting a project for a low budget (add emphasis) RC modules to control some very simple drones, similar to cars. I've done a little research on frequencies, and tend to favor 49Mhz bands.

There will need to be 4 separate units, operating at the same time. They will be extremely local, so they would be very low power. That will take care of legal requirements nicely.

I'm not too worried about the digital encoding/decoding, that is actually very easy. Making a stable transmitter and a receiver for bottom dollar is a bit more intimidating, but not especially so.

Before I invest some serious skull sweat and solder I was wondering if folk knew what else was out there.

BTW, a frequency chart for the USA.

http://rcvehicles.about.com/od/frequency/a/frequencychoice.htm

 

What is the mode of encoding the data; Pulse, AM tones, FM tones??

 

Had not completely decided, but my thought was simple on/off encoding for the radio pulses. No servos, just brute simple. I'll draw up a rough schematic for the RF section of the transmitter and show it in a bit. I figure you could do simple tank style driving in around 4 bits, non proportional controls. Toggle switches on a box.

We have had a new combat robotics group started locally. I was thinking of making 3-4 really cheap pushy bots for the kids who come to the meeting to play with. No weapons on this batch, all identical except for color (where the 3D printer comes in).

 

OK, here is a preliminary sketch on my transmitter...

 

Hello,

For OOK, you can also use a dual gate mosfet in AGC set-up.
This will give an attenuation range of about 50 dB.
Here is a page with investigations on the circuits:
http://nt7s.com/2008/07/dual-gate-mosfet-investigations-gain-and-agc/

Bertus

 

I'm not sure exactly why, but most of my kids inexpensive RC toys use IR, and they actually work really well. I'm not sure if this is because it's cheaper/easier to make, or if it just saves a ton on FCC approval stuff. Anyway, I thought it was interesting and they must be doing it for a reason.

Also unless you're just doing it for fun, commercial RF receivers are dirt cheap these days, for example:

http://www.hobbyking.com/hobbyking/...MX_Compatible_6Ch_2_4GHz_Receiver_w_CPPM.html

Good luck!

 

Hey Wendy

I know you said you've decided on 49MHz but I thought I'd chime in as I'm trying to learn about Multiplexing so I'm experimenting with some TX/RX units I got off ebay (making the TX + RX seemed way over my head ). The ones I got were 433MHz (one of our ISM bands but frequency can vary) with variable transmitter power depending on voltage (20 mW@5V apparently and TTL logic). Listings state that the bandwidth is 2MHz and its modulation is ASK/OOK but I had to check through a few of the listing to get the complete picture of the specs. I just took a punt on these ones as I'm just messing about with them to get my footing, I'm not sure how useful they would be to your application but thought I'd link them anyway. Hope it goes well

 

If you are going to operate bots at a gathering or a match I suggest that you consider IR. At gatherings concerning radio control they are pretty adamant about you not operating a transmitter when something else is going on. At the local RC modelers air strip you have to turn in your transmitter when you arrive on site. You get it back when it is your time to fly.

In your schematic, the modulation transistor will draw max current through the inductor when on. Maybe move it to the base side.

 

Like I said, cost is everything. All the parts shown are in my junk box. Good point about the modulation transistor though, I'll rethink that a bit.

This will not be on during a real match, it is a toy, like I said. The big boys will have much fancier, safer gear. I even have one in my closet somewhere.

 

Version 2

 

Wendy,

You mentioned wanting multiple units operating at the same time. All the cheap toy RC systems I see on 27MHz and 49MHz have these characteristics:

1. They use simple AM modulation and proportional control -- 50% duty cycle means wheels straight/drive off.

2. The stack multiple channels by switching to higher modulation frequencies (usually AF frequencies) and use AF bandpass filters. So low frequency pulse duty cycle controls drive fwd/rev, high frequency pulse duty cycle controls steering. Or with a treaded vehicle, one AF frequency controls fwd/rev/speed of one side and the other AF frequency controls the other side.

3. The cheapo receivers are so unselective they usually support only two units, on different bands -- 27MHz vs. 49MHz.

So how selective do you plan to make your receivers, and how stable and tuned are you going to make that transmitter?

I think you'll have a lot more luck with 433MHz digital systems or like others have said, IR.

 

................. The cheapo receivers are so unselective they usually support only two units, on different bands -- 27MHz vs. 49MHz.....

@Wendy

I agree with the inspector

They do make 27MHz RC receivers that are selective and are tuned by a crystal, but they are not cheap (enough).

Edit: I searched for 27MHz RC receivers and it appears that they are not used any more. It is all 2.4 GHz. My stuff is so old.

 

I'm following this thread with great interest, and I'm looking foreward to some values on the schematic.

Great job, Wendy.

 

I still a 2.4ghz transmitter laying around and might have 27mhz one too but not sure ..

 

Hello,

I see that you are trying to attenuate the output power by switching a transistor on the output.
You could also try to switch the power of the output stage to modulate the signal.

Bertus

 

Not a bad thought. I also thought of using a diode to switch the signal to ground. RF diode switches are pretty easy.

 

On to the digital. At the moment I'm thinking 4015 for the serial decoder and 4014 for the encoder. I'll post more later.