Fridge Timer UpdateOK so picked a relay, plenty to choose from with a 12V coil. Picked a big ass one to be sure it doesn't catch fire, and also because it's got a transparent case. I don't trust relays... I want to see inside to see condition of the contacts and if they operate properly. Also it's just cool to see the contacts
Measured coil resistance, 220R. Under 12V supply minus teh transistor saturation voltage, about 50+ mA.
Picked a 2N2222A transistor, 800mA so plenty enough. hFE 75 so a single mA in the base should be enough. Of course I doubled that hence the 47K as the pull-up in the diode NOR gate, which doubles as the base resistor for the transistor.
I added a diode in series with the base of the transistors to make sure it's not on the vege of conducting when the diodes of the NOR gate are pulled low. That would ruin my day.
I like my transistors so I added a free-wheeling diode across the relay coil. Found a 2A 600V one, should do it
Then added a green LED across the coil to provide feedback, tell me when the fridge compressor is running.
Then I fired all that up and... worked first time !!!
Look at that !!!
So now I need solder it permanently on a proper proto board, and I need to make a 12V power supply for it. Well I could just use a spare car battery I have here in the living room just a meter away from the fridge. That would the easiest and fastest way to go about it I guess !
However would be cool to power it from the mains instead... would be neater, more practical than having an external power supply...
I will see what I can find laying around. Worse case.. a cheap consumer grade supply, as I have seen so many times.... just a Zener diode with a beefy 3W or so 100K+ series resistor and call it a day !
I had that in an Espresso coffee machine.... and that was powering directly a couple CMOS chips... and it was working just fine !
I do have the relay drawing 60mA for 25% of the time though... so at least I would need a big reservoir cap to store that burst of current.
Stay tuned !