27 MHz Transmitter/receiver Pair Made With 555 Timers

Get your feet wet with radio frequency transmitters and receivers by working your way through this pair of tutorials. [Chris] built the hardware around a couple of 555 timers so you don’t need to worry about any microcontroller programming. He started by building the transmitter and finished by constructing a receiver.

Apparently the 27 MHz band is okay to work with in most countries as long as your hardware stays below a certain power threshold. The carrier frequency is generated by the transmitter with the help of a 27.145 MHz crystal. The signal is picked up by the receiver which uses a hand-wrapped inductor made using an AL=25 Toroid Core. We’d say these are the parts that will be the hardest to find without putting in an order from a distributor. But the rest of the build just uses a couple 555 timer chips and passive components, all of which will be easy to find. The video after the break shows the project used to receive a Morse-code-style message entered with a push button. It would be fun to interface this with your microcontroller of choice and implement your own one-way error correction scheme.

33 thoughts on “27 MHz Transmitter/receiver Pair Made With 555 Timers

        1. You can hit the Wikipedia article below for the frequencies for the US CB. There where at least three stages in allocating frequencies. Right now there are 40 legal channel with a maximum power output of 5 watts. Channel 1 is at 26.965 MHz and channel 40 is at 27.405 MHz. Most channels are 0.100 MHz apart. Something else that set CB apart for other communication is that you can’t transmit data, only spoken audio and there is small restriction on what can be spoken about. Also you are suppose limit your transmission lengths to 30 second unless in an emergency situation.

          27 mhz hobbiest band is from 26.995 MHz (generally called Channel 1) to 27.255 MHz (generally called Channel 6). Anything broadcasting with an unlicensed band can transmit anything they want as long as it does not violate national security. There is no limit on transmission length. Also you cannot exceed 100 milliwatts of power.

          There are additional restrictions on both of these are the basic ones. CB also has SSB which is borderline on legality and seems to get challenged every 3-4 years. Basically it lets you transmit on a very narrow band with twice the power.


      1. Anybody who might have an idea of how easy/hard it would be to increase the range? I haven’t got much notion of these concepts so I’m sorry if I’m asking a stupid question here :)

  1. a 555 generating a 27Mhz signal!! a was quite skeptic so I checked the transmitter schematic. Ok! the 555 is used to produce a 350hz modulation signal. The real RF transmitter is made with a single 2N2222 NPN transistor. So nothing special about it.

  2. 49.860 mhz was used by by full function toy cars and 27.145 mhz used by single function cars.

    i think 49.860 is safer since since it was not used by anything else.

    i think 27 was safe for single function cars is because all it did was generate a blank signal it is like you keying the microphone of a nearby cb to the same channel and you hear nothing a more distant radio may just hear a click of the transmitter coming on and off and a slight static.

    full function cars made several tones depending on the car and if you put tones over a cb it may upset truckers, some ham radio users and especially the fcc because the tones could completely drown out the communication.

    if you are building a simple wireless doorbell then a single function 27 mhz would probably be ok (though you may want to include more than just a simple carrier otherwise any cb, single function rc car from the 80’s or garage door could ring the doorbell).

    i did just such thing one time to a vhs vcr wired in a toy car into the feedback coil on the tape drive motor marked fg and by shorting the feedback coil you revved the motor to full speed making the tape rewind and fast forward faster as well as sounding like holding the play and forward/reverse buttons on audio tape deck.

    the problem was local cb operators would cause it to trigger at the wrong time.

    and main drive motors could spin very fast and you could rewind a tape much faster.

    1. At least in the USA, 27.145 is a dedicated RC vehicle channel now. Many 27mhz transmitter receiver pairs can use this crystal for operating multiple channels for hobby quality stuff. I’ve even had 27mhz full function toy vehicles.

      It shouldn’t be any concern to CB operators because standard CB radio frequencies do not use 27.145 (a cheap transmitter may still cause interference on other frequencies though).

      1. yeah, like what could the truckers do? Turn on the Jake Brake while passing your house at 2 AM? Or key up their 1Kw linears and mess up your house electronics? But don’t get them really mad, they carry guns.

    1. No.

      Put them in a box together, give them a shared power supply add a t/r switch for the antenna and mute the receiver when the transmitter is running and sure, then it’s a transciever.

      I can’t imagine why you would do this though. It isn’t like you are going to talk to someone using this. Maybe if you built two you could use it for control w/ feedback?

      As it is the receiver is meant to be mounted in the device that is to be controlled while the transmitter is the heart of a remote control. They remain separate. It is a transmitter/receiver pair, not a transciever.

    2. No. A “transceiver” is a specific animal, a single assembly that uses parts of the circuit common to both transmitter and receiver and is bi-directional, this arrangement does/is not.

      As above, the mention of the 555 is misleading. The RF active device is the 2N2222, the 350Hz subcarrier modulation/demodulation uses the 555’s.

      The author seem a bit confused about “modulation”. Anything that changes a carrier is modulation, keying it on and off, and particularly impressing a 350Hz signal on to it is modulation of the 27MHz carrier.

      Only the anti-social will amplify this basic cruddy signal without cleaning it up first (or use a better transmitter circuit).

  3. Okay, I have an idea of a prank; remote controlled doorbell ringer. I need a receiver to be able to fit in the hole one would drill for a doorbell so no one would notice it’s there. It would also need to be powered by the doorbell. This looks like I could make it small enough to fit in the hole.

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