I just finished soldering the circuit I got from the net on a PCB for a smoke detector circuit. However, the output LED is always turned on even if there’s no smoke by default.
I’ve done continuity checks just to see if there are any electrical shorts between some solder joints and the ground or power planes, but there’s none. I’ve also tried exposing the sensor to smoke produced by heating the flux with soldering iron but I it doesn’t help to change the default condition P.S. I still haven’t tried testing it with actual fast flaming fire for which it is effective against for I am afraid I might damage the sensor because it gets too hot quickly.
But whenever I touch the sensor’s frame [the aluminum can as sources say], the output LED is turned off afterwards and reading the IC’s specs [A5368], the green LED blinks approximately after every 40 seconds, which indicates a normal running condition. I tested it with the flux smoke thing but still nothing’s happened.
Tere are the other two. The one with the Pro Mini is the PCB where power comes from. I dunno if it helps, but I included it just in case.
I think I just saw what the problem is. Whenever I cover the output pins with my hands [encircled in the picture] and expose it to smoke or steam, it is the only time when the circuit produces an output. According to the datasheet,
The output lead wire of a chamber and an input terminal of a circuit must be connected in air to keep the connected point away from the surface of a PC board(usually a teflon pin is used to support the connected point). Because of the resistance level of paper phenol PC boards, current leakage will occur on to the PC board. This will prevent to read correct outputs.
10.4 The connected point of the lead wire and the input terminal of an FET or an IC must be protected from humidity using a silicon resin or the like so that current leakage from the packages will be minimized.
10.5 Because of the necessity of reading a tiny current put out from the chamber, this sensor must be electrically shielded to minimize noises from the outside. As the most sensitive part to noises is the connected part of the lead wire and the input the terminal of a circuit, this part must definitely be shielded.
So given the layout of the PCB, I will also be attaching an XBee module beside the sensor which might generate issues in the long run. Should I attach protection cans just like this or spray the board with coating. Am I being right?