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RC receivers and transmitters

The RC link and ASK environment

Radio Control /Remote Control and ASK (Amplitude Shift Keying) /OOK (On /Off Keying) have been around for some time, but there is limited coherent information on the subject and the associated hardware. Applications abound, including garage door openers, keyless entry, R-C toys, etc. These all utilize one-way communications. The most commonly used frequencies are 315, 330 and 434mHZ (most popular).

Numerous transmitters may actually share the same frequency and proximity, but they coexist only because of the brevity and low repetition rate of transmissions. Minimal information is transmitted –this consists of (8) bits of identification plus (4) additional bits of identification or variable control data.

The low data rate (4800 bits per second) is acceptable for identification and limited control with a typical (4) byte transmission requiring only 8mS. The protocol in this ASK ‘party line’ environment is very simple: first come, first serve, loudest or closest gets heard. Receiver address security is high, but transmission reliability is somewhat compromised by RFI (Radio Frequency Interference) and potential collisions where more than one transmitter may be transmitting simultaneously. Due to these limitations, the operator must occasionally push the button a 2nd time to open a door etc.

This discussion builds upon the previous (excellent) work of T.K. Hareendran: RF Based Wireless Remote Control System. It is limited to the transmitter and super regenerative receiver, but not the super heterodyne receiver as that piece has yet to be delivered –the results of testing this piece will be included as a subsequent comment. Future work will include an evaluation of the Holtek encoder /decoder devices that are required to complete the system.

Photo of transmitters and receivers

Photo of transmitters and receivers

What is ASK /OOK?

Amplitude shift keying is simply toggling between two RF power levels that denote serial “ones” and “zeros.” In the case of OOK, the zero is actually No carrier –for all practical purposes in this discussion, ASK and OOK are identical. OOK is functionally identical to CW (Continuous Wave) that has long been used for Morse code transmissions, but at a much higher character or data rate. ASK /OOK is much cheaper and easier to implement than the FSK (Frequency Shift Keying) modulation technique that is commonly used for data streaming applications.

RC Transmitters

The transmitter consists of a SAW resonator configured as an oscillator.
The circuits all appear to be similar, but the layout varies depending upon the manufacturer –copies abound. The recommended supply voltage is 3 to 12V with power and range a function of the voltage. One neat thing about the transmitter is that they draw No (zilch) current when not operating because the keying transistor must be On to complete the circuit. Actual CW current runs between 9mA @ 3V to 40mA @ 12V, but that is a non-issue because it runs only when the push-button is pressed so the battery will likely never need replacing. Maximum legal RF power output is 10mW –with input power being up to 480mW, the efficiency must be low (or perhaps nobody really bothers to regulate such).

Typical transmitter schematic
CDT-88 Transmitter Schematic

Typical transmitter photo

TLP434A Transmitter

Why such sketchy specifications?

I really had to dig in order to compile a coherent list of specifications. I believe that with such cheap hardware, limited profit margins, limited engineering staff and occasional functional failure, no manufacturer wants to accept liability –this is why they make few performance claims.

Typical transmitter specifications (compiled from numerous sources)

  • Dimensions: 19 x 19 x 7.6mm (excluding pins) (some are even smaller)
  • Interface: 3 or 4 pin header (0.1” pitch)
  • Frequency: 315, 330 or 433mHZ (actually 33.92mHZ –also called 434mHZ)
  • Frequency accuracy: ±150kHZ
  • Supply voltage: 3 to 12V @ 9 to 40mA (when keyed) 0mA (not keyed)
  • Maximum input power: 480mW @ 12V
  • Modulation technique: ASK
  • Maximum data rate: 8000 bits /sec
  • Antenna length: 10 to 20cM
  • Range: Up to 150M (subject to environment)
  • Pinout: Pinouts vary, but are indicated on the circuit board

RC Receivers

Receivers fall into two categories: super regenerative and super heterodyne. While super heterodyne offers reduced noise, higher sensitivity and better selectivity (adjacent channel separation), it perhaps doubles the cost and is available from fewer vendors. Super regenerative receiver circuits appear to be very similar in design and have varying layouts depending upon manufacturer. Do not depend upon vendor’s circuit board photographs for proper circuit identification as some appear to be simply representative.

Two dimensional form factors are available for the super regenerative version: short and long. It appears that the long version was dimensioned after the larger super heterodyne version so that both have physically identical dimensions and pinouts so that they are interchangeable.

Typical super regenerative receiver specifications (compiled from numerous sources)

  • Dimensions:
    • 30.5 x 14 x 9mm (short version, excluding pins)
    • 43.4 x 11.5 x 9mm (long version)
  • Interface:
    • 4pin header (short version, 0.1” pitch)
    • (2) 4pin headers (long version)
  • Frequency: 315, 330 or 434mHZ
  • Supply voltage: 3.3 to 6V @ 6mA
  • Sensitivity: -104dbm
  • Bandwidth: 2mHZ (super regenerative)
  • Modulation technique: ASK
  • Output signal level: (Vcc -1V) /0
  • Maximum data rate: 4800 bits /sec
  • Antenna length: 10 to 20cM
  • Range:
    • Up to 150M (subject to environment)
    • Up to 10M without antenna
  • Pinout: Pinouts vary, but are indicated on the circuit board

Super regenerative receiver function

How it works is that the gain of the resonant stage continues to increase until the point of oscillation –at this point the oscillator self-quenches so that the oscillations die out for a period of time after which the cycle repeats. At the point of self-quenching, the detector output changes states as if there was an actual signal received. This results in a noisy output when no signal is being received and the serial signal synchronization device (decoder) is always working hard to synchronize on a legitimate signal and is subject to missing the first byte.

Other types of modulation schemes such as FSK provide a much cleaner output, but require a clear channel –however that is another discussion.

Super heterodyne receiver specifications (Wenshing RS-434N)

  • Dimensions: 43.2 x 12.2 x 9mm
  • Interface: (2) 4pin headers
  • Frequency: 315 or 434mHZ
  • Supply voltage: 3.5 to 5.5V @ 6.5mA
  • Sensitivity: -115dbm
  • Maximum data rate: 12kBaud
  • Temperature range: -20 to 85°C
  • Modulation technique: ASK

Antennas

Antenna lengths for both transmitter and receiver are generally specified between 10 to 20cm. For close-range interference-free operation, the receiver antenna may be omitted. To avoid interference in busy, high density environments (multiple RF links under your control), the transmitter antenna lengths may be reduced somewhat but not eliminated. I determined that the transmitter would sometimes not oscillate with no antenna connected –it needs an RF load. At least one vendor claims that the antenna needs to be connected to prevent damage due to load mismatch. Under such conditions, the resonant circuit can ring up higher than normal voltages.

Maximum range experiment

For a simple experiment to determine range, I connected a transmitter to a function generator set for a 2HZ square wave. An LED was connected to the data output of the receiver –note that the LM358 IC that is used as a detector /driver can both source or sink current, so I sourced an LED through a 1K limiting resistor –this seemed to provide good results. The receiver, being battery powered could then be hand carried to the reception limits at which the LED blinks incoherently. Without an antenna on the receiver, the range was 10M. With an antenna, the range was 100M. This is exactly what was expected.

RC range experiment schematic

RC Range Experiment

Video clip

No signal noise (super regenerative receiver)

This is the nature of the beast –if no signal is received, there is an incoherent output signal otherwise called noise. The super heterodyne receiver (as far as I know) does not act like this, but can receive real noise (RFI) due to its high sensitivity rather than introducing its own noise like the super regenerative does. In the oscillograph observe the output noise. Also, observe the minimum modulation frequency –it clearly indicates that the super regenerative receiver cannot handle DC modulation due to the introduction of noise. This is a bit of trivia, but I think that the super heterodyne version will demodulate a DC modulating signal.

Determining the max data rate

The typical maximum data rate (4500 bits /S) appears to be marginal at best. I would reduce the data rate further if there were framing error problems –that is where the serial 1’s and 0’s do not occur within the acceptable time frame window. I believe that the super heterodyne receiver offers better performance in this regard.

Obtaining a transmitter /receiver pair

Cost is about $5 for the pair containing the super regenerative receiver –very inexpensive, and you may find the shipping exceeds this figure. I had good results obtaining devices on eBay from a domestic supplier –they are made in China and, of course, you may purchase direct, but shipping time may be an issue. The first transmitter that I purchased from the Robot Shop malfunctioned and despite wonderful web page claims, they provided no support, so make sure that you purchase a backup set. I think that the SAW resonators may be fragile like crystals.

SAW confusion

Do not confuse SAW (Surface Acoustic Wave) devices with SAW (Synthetic Aperture Window) Radar. SAW radar can actually render a 3D image of the inside of your residence from a Black Helicopter (by my namesake, the late Jim Keith –no relation –LOL).

For more reading

http://jap.hu/electronic/rf/el9805.pdf
http://valontechnology.com/images/REGEN.PDF

For the future

Encoder and decoder application info

Undocumented words and idioms (for our ESL friends)

party line –idiom referring to early telephone landline systems where numerous phones were connected to one ‘party line’ due to want of circuits –each subscriber had a different ring pattern –sometimes it was necessary to tell nosy neighbors to hang up because the additional telephones loaded down the signal excessively –LOL –this was in my parents’ day…

zilch –slang indicating the quantity ZERO

13 Comments

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  • jonathanscottjameshotmail-com

    i successfully developed a circuit to transmit the state of my thankless water-heater(which is prone to go off when the water faucet is turned to low or when it overheats or when other people use water and the pressure drops too low)(you want to know when your shower water is about to go ice cold in the winter and freeze you sugarcookies)

    i used 120v usb wall chargers modified to put out 7.5v by simply changiung the zeners to drive to-92 7805’s to power the transmitter and receiver which i mounted on usb connectors that plugged directly into the wall chargers.

    so i tested the 4 pin receiver and 433mhz round saw transmitter

    turned out the transmitter was nothing special raw 5v usb wall charger to the +5v pin and ground.
    i fed the data pin of the transmitter from the armature of a grounded 20k trimmer fed from “gl5516 photoresistor”(2 cent) with a 100k metal film resistor connected to the active 120v ac .

    the 20k trimmer adjusted out to about 5k when the water-heater was on with the photo-resistor scotch taped to the “flame view slots” on the outside of the thankless water-heater.

    after 2 weeks of adjusting and befuddlement,,,
    i found the 4pin receiver requires a mandatory to-92 7805 (+5v regulator).
    after that. i discovered i got the best filtering with a 10uf cap in parallel with a 1watt bead lamp led from data out to ground was perfect.
    for the antennas,, i first took a single strand piece of straightened cat5 wire about 2meeters long ,, then attached miniature insulated alligator clips to each end of the wire,, i then made a coil and attached one alligator clip to the receiver antenna pad ,, then i stretched the coil till i got a peak in the signal strength then attached the alligator clip to the most convenient thing i could find that would hold it in that position which was the bathroom mirror light shade. to form the coil,, i wrapped the wire around a 12oz aluminium can then removed the can and stretched it till it peaked and fastened the alligator clip to the wall paper.

    cost me under 5$ per pair. but took 2 months to develop .
    thanks for reading .
    your friend, jonathan scott james

    i made a 50 hour video on my youtube sight. lol also some scheematics of the walcharger scheematic on http://www.jsj.8k.com

  • amal

    i use 433MHz ASk receiver, it shows out put(+4v) with out having input on ASk transmitter. help me
    thank you

    • Jim Keith

      This is an inherent weakness in a bare bones system. Without an RF signal, the output can go high –this is a function of the extremely high gain of the receiver, whether super-regen or superhetrodyne –gain increases until it outputs noise. The solution is to continually watch for a specific bit pattern in the output –this is what the encoder & decoder chips do. A simple, less intelligent solution is to watch for a specific keyed frequency.

  • the mechanic

    what are the capaacitor values of superregenerative type schematic

    • Jim Keith

      Capacitor values are neither indicated on the schematic nor the components (being SMD). To obtain such, it is necessary to disassemble a unit (they are inexpensive) and measure each.

  • mojtabavahdat

    hi
    thank you for circuit
    i need schematic tx-4000 or fst-2 module

    or

    rf amplifier circuit for 315mhz , 1 or 2 wat

    thank you for help

    • Gabri

      Jim, is AC freq (50-60hz) effect stppeer motors? My XYL has an embroidery machine the runs on 120VAC 60hz and we are now living overseas at 220AC 50hz. A step down transformer does not operate the motors in the machine. I suspect that the reason is due to? that the step down passes the 50Hz which is to slow for the motors.

  • Tropicaliano

    Elucidative article. Answered most of my quetions. Thanks to Electroschematics for enabling such valuble means of knowledge sharing.

    • Hermas

      Jaycar Electronics in Canberra have a couple steros that still sells caps, resistors and other sundries related to? electronics and have helped me many times, a big thanks to all the lads at the Belconnen store.Another great show guys! 73 svk1fxjc

  • sulemasn

    hello
    i am suleman tariq.
    i am student of BS Electronics.
    i am making a project is 20 feet range magnetic indicator.
    i have no circuit diagram kindly please help me for circuit diagram.

  • Jim Keith

    Finally got around to testing the super-hetrodyne receiver. This unit (transmitter /receiver) pair is made by Shenzhen JMRTH Tech. Co. Ltd. Model # is RXB6-433 (433.92MHZ). I purchased this on ebay.

    Sensitivity is noticeably better than the super-regenerative receiver.

    However, I was incorrect in suggesting that it was free of No-Signal noise. In that regard, there seems to be no difference from the super-regenerative receiver.

    Max ASK On pulse = 50 to 100mS, while Max ASK off time is 2 seconds. That provides an indication of how low the modulation frequency may get before it malfunctions (of course nobody ever operates that low, but this is simply a test exercise).

    This is the preferred type due to superior sensitivity and selectivity. There is also a power-saving mode feature mentioned (the DER pin). However, I could find no info on this interesting detail.

    pdf datasheet:
    http://www.jmrth.com/en/images/proimages/RXB6_en_v3.pdf

    Next exercise is to dig into the Holtek Encoder & Decoder. I have many questions.

    • Yogi

      Congratulations on reaching the nedeed goal for the new transmitter. I will be chipping in my $pledge$ tomorrow, and appearing on Ken Till Ten to help inspire our bluegrass listeners to chip in even more. Great idea for a new goal!

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