Radio control question - Page 1
Forum section: electronics / general

author: Fishplate42
author_rank: Contributor
timestamp: May 17, 2012, 07:41:06 pm
content: Hi all, this is where I show my how little I know about electronics...Over 30 years ago, I think I remember that Futaba Radio control units had a selection of 'crystals' so the user could change the channel it operated on thereby allowing more than one unit to be used in the same area. For all I know that might be how they still are as I have never been that interested in radio control models.Recently, we bought some cheap radio control Meccano cars with the idea of racing them at shows and letting the kids have a go. The problem is they are all set to the same frequency They have a sticker on them that says 27Mhz. I assume this is the radio frequency and there is a code transmitted over the signal? See I told you I don't know much about thisI have taken the transmitter and receiver apart and there appears to be a crystal on the transmitter PCB and something that looks like a tuning coil on the receiver. Neither have any markings. I was wondering if I change the crystal and retune the coil if I can change the code and run two systems independently?Thanks in advance for any help.Ralph

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 18, 2012, 04:00:54 am
content: This may help:http://rcsource.hobbypeople.net/faqs/freqlist.htmGenerally RC Aircraft use 72 MHz bands and RC Cars and Boats use 75 MHz bands.The channels on 27 MHz are close enough that you shouldn't have a problem with crystal swaps. It might me possible to detune both transmitter and receiver. Most 27 MHz cheaper RC are just AM modulation with timing pulses for each of the control channels. With a scope or frequency counter you should be able to adjust the transmitter. The receiver is a little more difficult, depending on what style they used.

author: Psi
author_rank: Super Contributor
timestamp: May 18, 2012, 06:35:30 am
content: The 27Mhz band was the most common for professional radio controlled gear in NZ/AUS 15 years ago (when i was using it). 72Mhz is probably the USA band.Pretty sure most of the RC gear available now is in the 2.4ghz range.

author: pullin-gs
author_rank: Regular Contributor
timestamp: May 18, 2012, 06:47:06 pm
content: Dont count on it unless the RX is a crystal superhet.The cheap 27mhz RXs' selectivity is so low that it picks up pretty much everything within the 27MHZ RC band (I think there are 6 channels??).

author: typeglob
author_rank: Contributor
timestamp: May 18, 2012, 10:11:18 pm
content: Quote from: sacherjj on May 18, 2012, 04:00:54 amGenerally people have transitioned to the 2.4 GHz band by now.Futaba's FASST system constantly hops (http://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum) to different channels in the 2.4 GHz band while JR's/Spectrum's DSM2 system selects two clean channels at startup (http://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum) that are widely spaced in the band. Both systems are highly resilient to interference and some of the newer receivers enable 2-way communication (i.e. send motor speed or battery pack voltage from model to transmitter, so it can be displayed).Vendors like Sanwa and Airtronics may have different systems, but do use the 2.4 GHz band.With these newer systems there's no longer a need for keeping track of who's using what frequency and you no longer need a license to transmit on the frequencies used (I know you needed a license to use the 35/40 MHz band in the Netherlands).This has nothing to do with the topic starter's problem, but I thought I'd post it.

author: RCMR
author_rank: Frequent Contributor
timestamp: May 21, 2012, 03:55:01 am
content: The transmitters in these toys are usually crystal controlled (a requirement for certification in most countries) but the receivers are often just super-regenerative, using a free-running self-oscillating mixer with regenerative feedback (so the same stage performs RF amplification, local oscillator and amplification of the demodulated signal (clever stuff!).Unfortunately, the selectivity of these receivers is limited by the Q of the tuned circuits (an LC setup) so you usually can't reliably get two to operate on the same band without interfering with each other.