First off, I’m pretty new to both the Pi and Beer Brewing. I am building my own take on BrewPi (found out about BrewPi much later into my adventures). Anyway, I am running solely off of an RPi B+. I have FOTEK SSR-25DA. When I connect one of the 3.3 GPIO pins to the SSR, it lights with a red light. The AC side of the SSR is connected to the compressor’s “hot” line. It doesn’t turn on though. The DC side of the SSR is rated at 3v - 32v and my DMM indicates that the GPIO is driving 3.2V to the DC side of the SSR. Alas, the compressor does not turn on when the GPIO is raised to 3.3v. However, if I connect the SSR to the 5V pin on the Pi, then the SSR activates and the compressor turns on (yay!).
Since the minimum voltage is exceeded when I use the GPIO pins, I am at a loss as to why it won’t work (short of a defect or variance in minimums). Is there a way that I can “raise the voltage” coming out of the Pi (my newness to electronics shines here) without using some sort of external PS?
I should elaborate. I’m building my concept of BrewPi. The code is running via a Google App that I wrote. The data logging lives out in Google Sheets. And, based on the data, the Pi gets control signals from the Google code via a REST API. I got way into this project (and the code) before I even discovered that other people are doing way more than what I’m doing. Anyway, my biggest issue right now is if I need to and/or can increase the output voltage from the GPIO. Can I simply add a resistor in line?
I found this on a beaglebone forum but it’s relevant
The trigger voltage listed on the data sheet is listed as 3 – 32VDC. In theory, the beaglebone’s 3.3V GPIO pins alone should be enough to trigger the active state. In testing, out of five of these relays, only one was able to switch on with direct GPIO voltage. The indicator LEDs would light up, but the active state was not triggered. Therefore, it’s a good idea to increase the voltage if you’re looking for reliability. This can easily be done with about a dollar of hardware. I elected to use a Darlington array, the ULN2003. You can get a pack of them for just a few bucks.
Brewed my beer about three weeks ago. It’s a hefeweizen and bottle-conditioning now. I was able to control everything via my own Google Script code and an API to the Pi (WebIOPi) and all the data is logged in Google Sheets. Early taste tests indicate basic success, but I have a lot to learn.
My biggest question, at this point is whether I should be controlling the refrigerator compressor based on beer temp or based on air temp in the 'fridge. For my VERY FIRST brew, I controlled via the beer temp. My actual maximum temperature never exceeded more than 0.18F above threshold/desired fermenting temperature, and never exceeded 0.95F below the threshold/desired fermenting temperature. Is that acceptable? Why would someone choose to control off of air temperature rather than wort temperature. Thanks for everyone’s advice!
You should always control the temp off of the temp of the wort. The only reason I have ever had to control my freezer off of air temp was while making yeast starters. This way you can just set the freezer to stay a certain temp and you don’t have to worry about the ambient temp.