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.
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.