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
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.
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 photo
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)
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.
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)
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
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.
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
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