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Topic: RF module false triggering relays (home automation) (Read 397 times) previous topic - next topic

Dnyaan

I'm struck at a point in my home automation project.
I'm using 4 channel 433MHz receiver and transmitter remote for controlling 4 channel relay board.

4 outputs of rf receiver are directly given to 4 inputs of 4 channel relay board. Both are given common dc 5v power supply.

The problem is, when I start the dc power supply to system, all relays are triggered once automatically without pressing any key on rf remote.


What could be going wrong? Any idea?
Anyone please suggest.

( Exactly same problem is arising with pir sensor also)

steve_mcdonald

Hi Dynaan,
In a way this is not a problem - it is normal behaviour.  But it's probably not what you want.  When your RF receiver initialises, it sets its ports high for a moment and probably then resets them to the idle low state.  You haven't given much info, but it is likely this process causes the initial on state of the relays. I expect  they reset and then behave normally. 

From the data available, the receiver is wired to output a high on the d0 - d3 outputs to indicate a relay should operate.  During the initialisation, it is probably doing that momentarily across all data outputs.  There are a few things you could do. A simple way is to "slug" all the relays so they do not respond to a very short momentary signal.  A small capacitor in parallel with the coil can do that, but I have no idea of your circuit or switching requirements so I can't say how big the cap would need to be. The idea here is the cap charges for a moment before the relay responds. The time constant would be chosen to ignore the startup sequence but still respond adequately in real signal cases.

Another way is to delay power switching to the relays while the receiver completes boot up. That's relatively easy to implement but I'd need to understand much more about your circuit. 

In short: this is normal, if unwanted.  It can be addressed by circuit design.  We need a lot more detail on your circuit if you need detailed help. 

Dnyaan

Hi Dynaan,
In a way this is not a problem - it is normal behaviour.  But it's probably not what you want.  When your RF receiver initialises, it sets its ports high for a moment and probably then resets them to the idle low state.  You haven't given much info, but it is likely this process causes the initial on state of the relays. I expect  they reset and then behave normally. 

From the data available, the receiver is wired to output a high on the d0 - d3 outputs to indicate a relay should operate.  During the initialisation, it is probably doing that momentarily across all data outputs.  There are a few things you could do. A simple way is to "slug" all the relays so they do not respond to a very short momentary signal.  A small capacitor in parallel with the coil can do that, but I have no idea of your circuit or switching requirements so I can't say how big the cap would need to be. The idea here is the cap charges for a moment before the relay responds. The time constant would be chosen to ignore the startup sequence but still respond adequately in real signal cases.

Another way is to delay power switching to the relays while the receiver completes boot up. That's relatively easy to implement but I'd need to understand much more about your circuit. 

In short: this is normal, if unwanted.  It can be addressed by circuit design.  We need a lot more detail on your circuit if you need detailed help. 
Thank you very much steve. I will try to add the capacitor. I'll keep you posted on this.

Dnyaan

So I've tried to work with capacitor 1000uf, it just slows down the relay switching speed (that too all the times i.e. Even on real triggers also) , nothing else happens.

I have attached the setup representation with single relay. Both PIR and RF representative setups are attached.
Please check.

steve_mcdonald

Hi, Your 1000uF is going to be much too big for this job.  I know these relay sets - later today i will post a calculation for the capacitor. 

Does the PIR also send a false relay operation at startup?  That's a little less usual. Unless it does something odd during setup.   I don't know the PIR unit (I build mine from scratch) so I can help with the relays and see if that solves the issue with the PIR as well. 

I'll come back to you when I sort out a cap size for you, but I'd be looking at 10 - 100 uF at the most I think.   

steve_mcdonald

Hi,
The relay has a pick up voltage of 3v8 according to its spec sheet.  Your supply is 5v. 3v8 is 0.76 of 5v.  One time constant for capacitor charge charges a capacitor to 0.63 of the supply voltage, so you would need between 1 and 2 time constants to reach 3v8 and operate the relay. 

The time constant is found by resistance times capacitance (Farads).  Let's say you can tolerate a 100 mS delay in normal operation and that is enough of a delay to prevent the initial firing of the relay.  (Both are guesses).

For a time constant of 80mS you could use a 100uF capacitor in series with  820R.  A little over 1 time constant will then charge the capacitor to 0.76 x 5 v = 3v8 and the relay would operate. 

Try a 100uF in series with 820R and wire that across the relay coil.  The relay should ignore the short transient at booting time but should respond within 100mS when a real signal  arrives.

I have not tried this out in practice for you.  I have done the same many times before with similar circuits.  Therefore "E&OE" as the lawyers say.  Good luck and let me know how it goes.

If you have access to an oscilloscope, you can easily see the length of the boot time transient and re-calculate appropriately.  Then you can shorten or lengthen the delay by decreasing or increasing resistance (or altering the capacitor).

Dnyaan

I have tried replacing the relay with multimeter and i was able to see the high pulse for about 2-3 seconds (3.3 volts) at startup.

So i guess its the sensor board which is abnormal here, and not the relay. Because its the sensor who is giving high pulse at startup irrespective of relay.

Also, before using 1000uf capacitor, I had already tried 100uf and it didn't make any change at all.

And yes, PIR is also behaving exactly similar to rf receiver board at startup.

I have also tried with different cellphone smps power supply which is more stable.


steve_mcdonald

Hi,
What is the 'sensor board' you refer to?  Your pix showed a power supply, relay and Rx or PIR only. 

Using the capacitor alone will not make a difference.  It is the series resistance that slows the charging of the capacitor.  Without the series resistor any size capacitor will charge in next to no time and will make little or no difference. 

If you'd like to pursue this further, can you post a complete circuit diagram and identify all the parts involved? 
Thanks.

Dnyaan

Hi,
What is the 'sensor board' you refer to?  Your pix showed a power supply, relay and Rx or PIR only. 

Using the capacitor alone will not make a difference.  It is the series resistance that slows the charging of the capacitor.  Without the series resistor any size capacitor will charge in next to no time and will make little or no difference. 

If you'd like to pursue this further, can you post a complete circuit diagram and identify all the parts involved? 
Thanks.
Hi Steve,

PIR sensor "module" or Rf receiver "module" I meant to say.

I will get back to you with detailed circuit diagram.


Thank you.

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