OOK (On-Off-Keying) is the modern name for the basic principle of an early Marconi Morse Code Transmitter, and it is basis of RF remote controls as well as the foundation of all modern digital transmission modulations (including WiFi and Bluetooth).
Since OOK senders and receivers are dirt cheap, it’s the perfect place to start getting your hands dirty.
Overview
OOK (On-Off-Keying) is essentially describing a transmitter that can be turned on and turned off - that’s it.
There is no sophosticated overhead such as modulation (AM, FM, etc.), and OOK is the closest resemblence of an ancient morse code transmitter, similar to the one that was used in 1912 to try and save the RMS Titanic.
OOK transmitters don’t care about what kind of information you want to transmit. It is your responsibility to encode the information in a series of high (sender on) and low (sender off) pulses.
That’s why in theory, you could use OOK senders to transmit audio or even video. However, OOK is typically usable for signals in the range of 1-5kHz, so the data transmission speed is very low. Transmitting a picture or video would be entirely possible, however it would take forever to transmit.
HAM Radio Amatuers created Slow Scan Television (SSTV) in 1957 to transmit pictures at a rate of seconds to minutes to transmit a single picture.
OOK Transmitter
OOK senders are available as ready-to-use breakout board, transmitting on one of the license-free ISM bands (such as 433.95MHz). The FS1000A is a popular example.
Pick any OOK sender board at your discretion as long as
- it is transmitting on a frequency legal to operate in your region
- does not exceed the maximum transmission power
- and is indeed a OOK sender with nothing else implemented.
There are more sophisticated RF sender boards that add additional features:
- EV1527 Remote Controls: EV1527-compliant senders are ready to send 24-bit remote control IDs. They are perfect when you want to create your own simple remote controls, however these senders cannot send anything else anymore - just 24bit bursts using the EV1527 standard.
- Microcontroller-based: Sophisticated FM transceivers use microcontrollers or specialized chips to handle digital data packets and modulations. They are perfect when you intend to transmit complex information. However, they too cannot send pure and unmodulated radio waves anymore.
These boards are great - often much better suited than simple OOK boards - but for the moment (and the objective of this article), OOK is best as it lets us focus on the fundamentals, and the easiest start is to get yourself a dedicated OOK sender and a dedicated OOK receiver board.
Remote controls are very simple one-way transmissions which is why you use dedicated senders (transmitters) and receivers. Modern microcontroller-based boards (i.e. CC1101) target much more sophisticated transmission protocols including two-way communications which is why these boards are transceivers (transmitter and receiver) and can be programmed to act as sender or receiver. Many support the basic OOK mode, too, so you could use two such sophisticated boards and “downgrade” them to become OOK sender and OOK receiver. Programming the boards to act that way would be considerable overhead but proves that OOK is in fact the foundation of any digital transmission technique used today.
Pins
OOK senders have just three pins:
-
Power Supply:
GND and VCC supply the board with power. Senders typically support a wide voltage range (5-12V for the FS1000A).The higher the voltage, the more powerful is the radio signal. When you supply the full 12V, the FS1000A most likely generates a radio signal significantly over the legal limit of 10mW. Start with 5V for testing low-range scenarios without causing too much interference for others or worse, violate regulations.
-
Data:
A DATA pin can be high or low. High turns the sender on, and low turns it off. On the FS1000A board, DATA was printed in reverse by accident: ATAD.
Board Design
OOK senders are simple: made of a LC circuit and a crystal. The coil on the board is not the antenna. You typically need to add an external antenna yourself. With the FS1000A, solder it to the through-hole solder pad marked ANT.
Often, these modules come with coil antennas (that you may want to wrap in some shrink tube for better optics and insulation).
Or you use a simple wire of appropriate length as discussed above.
OOK Receiver
To successfully test a radio transmission, you need a OOK receiver to pick up the radio waves sent by the OOK transceiver.
Superheterodyne is better
Often, the FS1000A sender comes bundled with a receiver module called MX-RM-5V.
As you lerned before, receivers can use different designs, and the MX-RM-5V is of type superregenerative: cheap, unreliable, and often severely affected by noise.
If you can, use a superheterodyne receiver like the AK-RXB59R or the many similar models available.
Senders and receivers may have different voltage requirements: while most senders support a wide voltage range up to 12V, receivers typically need 5V - not more, but also not less. They often cannot be operated with 3.3V.Look up the data sheet for the receiver you are using.
Antenna
The receiver should be equipped with an antenna, just like the sender. Without an antenna, the receiver most likely won’t be able to receive any signals. Fortunately, most OOK receiver boards come with a simple coil antenna presoldered.
Pins
The receiver also has three pins, just like the sender, and they work quite similar: V (5V) and G (ground) supply the power, and DATA returns the received signal.
The DATA pin on receivers often does not work exactly like the DATA pin on the sender:
- DATA on Sender: high starts sending a signal, low stops sending a signal
- DATA on Receiver: strength of received signal (voltage anywhere between low and high)
So on receivers, the DATA pin isn’t necessarily a true digital signal. It may fluctuate over the entire voltage range. That’s surprisingly because OOK receivers are optimized for receiving digital signals.
Understanding OOK Data
To successfully use OOK, it is important to understand how OOK receivers work:
-
Sender:
A OOK sender works pretty much like an ancient Marconi morse code transmitter and sends a signal for as long as you wish.OOK senders may not be designed for extended continuous transmissions and may get hot or even become damaged when operating them at higher voltages for extended periods of time.
-
Receiver:
designed to work with bursts of signals in the range of 1-5 kHz. Signals of much longer duration are treat as noise and can produce AGC drift, flooding the receiver with noise and producing unexpected DATA states.
To successfully use OOK receivers, your signals must be in the range of 200–1000 µs.
Duration Of Valid Emissions
Sending bits of data at speeds of 1-5 kHz translates to the following signal durations:
| Transmission Frequency | Bit Duration (Period) |
|---|---|
| 1 kHz | 1 ms (1000 µs) |
| 2 kHz | 0.5 ms (500 µs) |
| 3 kHz | 333 µs |
| 4 kHz | 250 µs |
| 5 kHz | 200 µs |
Sending bits at these speeds is still the Morse Code principle, only the information is sent much faster than any human telegraphist could do.
Many common OOK receivers work best with 200–1000 µs per bit. Too fast (e.g., 50–100 µs) might get filtered, while too slow (>10 ms) may trigger AGC drift.
That’s because most affordable RF remote controls adhere to the EV1527 standard and use OOK for transmission:
EV1527 Pulse Widths
At a typical transmission frequency of ~3 kHz, the pulse durations are approximately:
| Bit Type | High Duration (µs) | Low Duration (µs) | Total Bit Duration (µs) |
|---|---|---|---|
| “0” bit | 350 µs | 1050 µs | 1400 µs |
| “1” bit | 1050 µs | 350 µs | 1400 µs |
| Sync Bit | 350 µs | 10,500 µs | 10,850 µs |
The total bit time is 1400 µs, corresponding to ~714 Hz transmission rate. The sync pulse (start frame) has a much longer low duration (10.5 ms) to mark the beginning of a new frame.
Each frame is transmitted multiple times to improve reception reliability.
Uncontrollable Noise (AGC Drift)
Abusing OOK to transmit morse code signals at “human speeds” is doomed:
A duh and dah signal typically lasts between 1-2 seconds. The OOK sender can send such signals just fine. But the OOK receiver cannot interpret such signals since they are roughly 1000x longer than the signals it is listening for.
The OOK receiver isn’t simply ignoring such signals. Many superheterodyne receivers continuously adjust their gain based on signal strength in an effort to pick up even weak signals.
This is called AGC (Automatic Gain Control). When you send super-long signals, that’s for the OOK receiver as good as not sending any signals, and it cranks up its gain. Even though you are sending a very strong signal, the receiver increases its sensitivity and eventually gets totally overloaded, by your signal, and when you stop sending, by any noise floor that exists almost everywhere.
As a result, the receiver DATA pin outputs a solid high when you send, and all kinds of random highs when you don’t.
Conclusions
OOK is not the resurrection of the ancient Marconi Morse Code transmitter. It is a modern form of morse code transmitter operating at much higher speeds. It cannot be used for slow human morse code transmissions.
That’s coincidentally why basic OOK senders and receivers are typically designed to be plugged into microcontroller-based solutions. They are not intended to be operated directly. Microcontrollers can easily supply digital signals at the supported rate of 1-5 kHz.
You can still create a great “human” morse code transmitter using OOK: just don’t send all the time. Instead, design a digital start and stop signal sequence with signals lasting 1ms or so. Your receiver would need to be “intelligent” and recognize your signal patterns to start and stop the audio tone, respectively.
That’s essentially how any other digital service (meter readings, remote controls, sensors) work. And it is much more efficient, too:
Sending “brute force” continuous signals may be rude at best and illegal at worst. Your signal is eating up the valuable duty cycles that are shared among the many other users (and devices) that share the same ISM band with you.
Getting Practical: Operating OOK Transmitter
You can experiment and apply the theory from above now:
- Sending Signal: set DATA to high on the sender board. The sender starts sending out a continuous radio wave
- Receiving Signal: the receivers’ DATA pin turns high when it receives the signal from the sender.
Now you can test OOK yourself and better understand why some users can’t get OOK to work, while others happily use it to transmit sensor data or control devices.
Setting Up Sender
Connect the FS1000A like this:
| Pin | Connection |
|---|---|
| VCC | 5-12V |
| GND | Ground |
| DATA | Morse button, other end of button to VCC |
Setting Up Receiver
Connect the AK-RXB59R (or any other superheterodyne reciever you picked) like this:
| Pin | Connection |
|---|---|
| V | 3.6-5V |
| G | Ground |
| DATA | connect LED or buzzer anode |
Experience Failure: Using OOK wrongly
Observe what happens when you manually send “morse code” with invalid signal lengths.
Observations
The receiver seems to return random high signals, and when you attach a LED or buzzer to DATA, the LED will flicker, and the buzzer will emit random sounds.
Once you transmit your “morse signal” (a long transmission), DATA on the receiver side will go all the way to VDD. A LED lights up in full brightness, and a buzzer emits a strong signal.
You may even see the LED turn off when you transmit, indicating that it now draws more energy than the receiver DATA pin can deliver.
In a nutshell, while you can see some response on the receiver side when you send a signal, the received signal is unusable as it is embedded in a very high noise floor.
Simply add a logical layer that can distinguish signals from omnipresent background noise. EV1527—as used in cheap remote controls—is such a layer and enables OOK to reliably transmit simple data, including Morse code.
Experience Success: Using OOK Correctly
Attach a digital signal in the range of 1-5 kHz to the sender’s DATA pin:
- Custom Signal:
Usetone()on Arduinos or create a PWM signal on ESP32, then send it to a GPIO connected to DATA on the sender. - Real Remote Control Signals:
Use the rc-switch library to emit real remote-control signals (or decode existing remote-control signals).- Your sender can act like a real remote control or even mimic an existing one.
- Your receiver can pick up signals from real remote controls and decode their IDs.
- By combining your sender and your receiver, you can create self-learning remote controls that first pick up signals from real remote controls and then play them back.
- ESPHome:
Use the components remote_receiver and remote_transmitter to control your OOK receiver and OOK sender. Both components internally use the rc-switch library. - Hardware Solution:
If you are not into microcontrollers and want to keep things simple, use a hardware solution: swap out generic OOK boards with EV1527 boards. The latter include a specialized chip that takes care of all the heavy lifting. With this solution, you can remote-control devices or transmit Morse code without microcontrollers, programming, etc.
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(content created Mar 16, 2025)