Need help with 433mhz tx-rx module project - Page 1
Forum section: electronics / projects

author: victor
author_rank: Regular Contributor
timestamp: July 07, 2013, 07:52:32 pm
content: Hi there,First of all I would like to say, that I'm a beginner in electronics, I watch the eevblog on youtube every single week, for quite a while.I built a pretty basic schematic myself, and not surprisingly it works but not as expected since I built it myself. lolI'm using a pair of RX-TX 433mhz modules, for a IR remote extender. It works, it transmit data and the receiver just repeat the data transmitted. The problem, whenever the transmitter is not sending any data, the receiver part keeps picking noise.This part works just fineRX module keeps receiving noise.I tried adding 10k and 4.7k pull-down resistor to the data-out pin, without any success. mesuring with a multimeter there's a voltage floating around 1.8~2.8volts.But the datasheet saysData High | +200uA | Typical VccData Low | -10uA | Max 0.3vI need help.

author: David_AVD
author_rank: Super Contributor
timestamp: July 07, 2013, 10:44:44 pm
content: The random noise on the output (when not receiving a signal) is normal for an OOK receiver. Your decoding will have to deal with it.

author: amyk
author_rank: Super Contributor
timestamp: July 08, 2013, 05:10:14 am
content: The ISM bands are shared, and when the FCC says "this device must accept any interference received, including interference that may cause undesired operation", they mean it.

author: kizzap
author_rank: Unknown
timestamp: July 08, 2013, 06:29:12 am
content: What exactly is that 555 timer supposed to be doing? The Trigger, Threshold and discharge look like they do nothing at all.... shouldn't there be a voltage source there somewhere?secondly, why control the 555 by the power rail? surely you should be using the data out with the trigger input?Surely the resistor between reset and VCC is unnecessary too?I'm going to assume that you are using the 555 as a basic on/off toggle for the data LEDs, in which case what I would be doing is finding the voltage at which the data feed outputs, and set the threshold of the trigger at a point below that voltage, and that hopefully should get rid of some of the noise.as it stands though, that 555 timer looks like it is doing nothing at all.-kizzap

author: fcb
author_rank: Super Contributor
timestamp: July 08, 2013, 01:04:27 pm
content: A couple of things.I would probably have a 10K or smaller pull-down resistor on the data-in on the TX, to make sure you get a nice low voltage when off.The IR receiver part demodulates the 38-42KHz AM signal, so the data you are sending is likely just a short burst - you need to make sure that this demodulated data is going through your link correctly and not inverted or has distorted pulses.The 555 on the transmit end is to modulate the data with the new carrier frequency - I'm not sure I like the switching Vcc with data onto the 555, but it would probably work if it can settle fast enough. I'd probably leave the 555 running and gate the output or perhaps jam the RC network.Like kizzap says the resistor on the reset pin of the 555 is not necessary - but won't hurt.Your receiver data-hi must also be within 0.3V of Vcc or that transistor will start conducting, probably but a Vcc based potential-divider on the output of the receiver, say 2k2/10k.But the thing it sounds like is that you have no muting and just spitting out garbage when not receiving - are you using AM or FM modules? - do you understand AM thresholds or FM RSSI?

author: komet
author_rank: Unknown
timestamp: July 08, 2013, 01:18:51 pm
content: Those eBay 433MHz modules are huge pieces of shit and the behaviour you get is "expected".You might consider continually transmitting some dummy data as a workaround, but I've found that the correct way to use these modules is to toss them out of the window and get some zigbee or bluetooth bridges instead.

author: victor
author_rank: Regular Contributor
timestamp: July 08, 2013, 09:19:51 pm
content: Quote from: kizzap on July 08, 2013, 06:29:12 amI forgot to add a 4k7 resistor between threshold/trigger and 5v. Surprisingly it was working without it.I connected the 555 directly to 5v, and I'm driving it by the reset pin.Playing around I found that adding a 100k pull down to the undocumented linear out, seems to hold the data pin low when idle.I need to test further more, unfortunately I don't have a oscilloscope, but I managed to use my mac line in to calibrate the 555 to oscillate to 37Khz (the carrier frequency).

author: Skimask
author_rank: Super Contributor
timestamp: July 09, 2013, 04:21:29 am
content: Ain't nothing wrong with these cheapo 433mhz TX and RX modules that a little bit of EDUCATION won't fix!In very short... You can't use them like a regular old serial-in / serial-out type TX and RX. You have to send a preamble to 'train' the receiver and each 'packet' of data can't be all that long before the receiver 'forgets its training'. You have to encode the data to be sent and decode the data received.I've used these type of ASK/OOK modules for a number of years on a handful of projects.They work just fine.....If you know how to use them...

author: David_AVD
author_rank: Super Contributor
timestamp: July 09, 2013, 06:17:33 am
content: Yes, in the absence of a strong signal, the receiver starts hunting and picks up every little bit of RF rubbish. This is perfectly normal behaviour.Most remote controls that use OOK (On-Off-Keying) transmission send either a preamble or the same data packet several times in a row as the first one will be corrupted.

author: tiofilo75
author_rank: Contributor
timestamp: July 10, 2013, 05:27:10 am
content: I used these same modules to remotely switch some led lights i made for my aquarium. I used a 555 timer tied to the transmitter and a pic tied to the receiver that was programmed to count the number of pulses over a specified time interval.