and after a couple of days, weeks or months (switching once or twice per day) they all stopped working with the same symptom: the light does not switch off anymore. Some switched 230 Volts (yes, there is an opto-coupler on the module for isolation!), some switched 24 Volts DC. So I started investigating... I found the reason: the relay is a 5 Volt relay, where, according to the specification datasheet from Songle Relay, the minimal switch close voltage (pull-in voltage) is 75% of the 5 VoltDC the relay is rated at, which is 3.75 V. I was amazed to find that the relay is actually driven with 3.3 Volt (the voltage which is derived from the low-drop out LM 1117 linear voltage regulator). This 3.3 voltage will not close the relay contacts fully, and as such there will be a considerable remaining resistance, or even an electric arc between the contacts - which, depending on the load, will destroy the contacts over time (or immediately), in fact, it is assumed that they actually weld together! This was further confirmed by the fact that when it first happens that the light does not switch off, hitting the relay might solve it and the contacts disengage. It is also interesting to note that Songle also offers a 3 Volt relay. My belief is that the original design used this 3 Volt relay, and for some reason (cost?) this was replaced by the 5 Volt variant. My recommendation is: don't buy this part, it will give you a lot of trouble!
How did you measure that voltage and what was VCC reading at that point.What was the module sitting on? The reason I ask there are several fault problems that could cause this.
voltage between the two relay-coil contacts. And, I scrutinized the whole design: the DC voltage is rated 5-12 volt for the module. There is a LDO (Low Drop Out) regulator LM1117-3.3V from the + of the DC input to the ESP8266 Vcc AND the relay. A Mosfet (A09T) switches the relay coil to GND. There is an opto-coupler to isolate the ESP8266 GPIO from the Mosfet etc. Given the LM1117-3.3V this opto-coupler is an overkill. The reason for the LM1117-3.3v is to provide the power for the ESP AND for the relay, as the DC could be as high as 12V (as spec'd). Consequently, the relay should have been a 3.3, max. 4V coil. Not 5V! The spec for the relay says: reliable close at 75% of the spec'd voltage. 75% of 5V is 3.75V. Alas, this is not a in-spec design and therefore flawed.
