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Objective: I am trying to build a transmitter that transmit an on signal when a button is push. The transmitter will need to be small enough to wear or hold in a pocket. I prefer not to use any microprocessor or encoder for the transmitter. For the receiver, it have to be able to receive the signal from the transmitter and play an alarm.

433Mhz RF transmitter and receiver: https://www.amazon.com/UCEC-XY-MK-5V-Transmitter-Receiver-Raspberry/dp/B017AYH5G0/ref=sr_1_1/163-3850928-5773553?ie=UTF8&qid=1470685976&sr=8-1&keywords=rf+transmitter+module

Question: I am planning to use a 433Mhz RF transmitter and receiver. Since I am only sending an on signal, I wonder if I can just send a voltage as a signal or data through the data pin on the transmitter chip. Would that be possible for the transmitter to transmit such data and for the receive to interpret it as a 1 or an on signal?

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  • \$\begingroup\$ Impossible to achieve exactly that, without encoding you can not expect to work properly. Sending just one bit over RF there would be a lot of false receiving signals. \$\endgroup\$ – Haris778 Aug 11 '16 at 12:53
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Would that be possible for the transmitter to transmit such data and for the receive to interpret it as a 1 or an on signal?

If you want a somewhat reliable system you cannot just send a high or a low voltage and expect that to be adequately decoded with any measure of reliability.

Think about the receiver - it's very sensitive (by design) so, in the absence of a proper signal, it's trying to demodulate spectral noise and producing basically garbage on the output. It will have an AGC (automatic gain control) that allows it to receive and demodulate anything that ranges from a proper signal to noise.

If it receives a non-random-noise signal it backs-off the AGC to properly receive and demodulate the signal. However, that signal could be from someone else down the street opening their car doors.

If you are looking at cheap radio transmitters and receivers you have to: -

  • Send some form of data preamble that allows the receiver to adjust its AGC or detector to properly deal with the amplitude of the received signal.
  • Send an address so that the MCU looking at the received data has some idea that it is from the transmitter it wants to focus on.
  • Send data and this could be high or low, one bit to any number of bits
  • Send a checksum so that after all is received you can get even more confidence that the signal is valid
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It would be technically possible to build a transmitter with a button which merely enabled the RF circuitry and thus transmitted an unmodulated carrier as long as it was held down (perhaps with some RC timer limit).

The problem is that without a pattern to the transmission and gaps, it is very hard to reliably recognize, especially in the presence of noise and other authorized users of the same frequency. In effect, to simplify the transmitter you end up having to make a receiver sophisticated enough to try to "guess" if a sudden increase in received power at a given frequency is due to your transmitter or something else. And however you do it that algorithm will have false positives, or negatives, or both.

It is far simpler to use a fixed function encoder or a tiny micro controller to on-off-key (OOK) the transmitter in a recognizable and somewhat unique code pattern. That is how most inexpensive products in that range work. If you are concerned about space, you merely need to pick your parts carefully - look at the size of an 8-pin SOIC MCU for example - there are smaller devices, but that has few enough large enough connections to be approachable for an initial project.

If size is a concern you may also want to look at a higher frequency - for example 2.4 GHz could mean smaller antennas, and you can get a radio and MCU combined in one package. But you wouldn't get the true size benefit until you moved away from modules and made something custom with the chip and tiny supporting capacitors, inductors, and resistors on your own board.

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  • \$\begingroup\$ I will try to use microprocessors to do this. It seem like that's a good way to approach this with my current skill level. Thanks you. \$\endgroup\$ – Meng Kry Aug 13 '16 at 14:16
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Two very simple circuit ideas that don't require a micro controller and haven't been discussed:

  1. Transmit a narrow band CW tone, build a narrow band filter at the receiver, then set a power threshold that triggers a 555 timer to make the alarm sound. Could add RC circuit as Andy mentioned to improve reliability. PROS: simple transmitter and receiver, silent while not transmitting (ideally) CONS: requires a sharp filter to reject interference, unreliable in several ways.

  2. Transmit the alarm sound and demodulate it directly. PROS: still fairly simple and removes need for a sharp filter. CONS: required demodulation (AM sill simple), noisy unless a squelch circuit is added increasing complexity.

Neither circuit uses 'bits' per se, and would act like a wireless switch. It could be a fun project as well. But I agree with the other answers: it's probably better overall to go digital. A garage remote circuit would be perfect, pre-built, and cheap.

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