I have a pair of wires with ~22vdc across them, and I want to use the presence or absence of this voltage to trigger a GPIO line on my Raspberry Pi in order to tell my program to do something. I know nothing about electronics. How can I do this?
I am working on my never-ending Seeburg 1000 project, with the goal of putting a Raspberry Pi inside to give some cool modern upgrades to the device.
Inside the machine there is an electromechanical timer that is designed to trip on once every 30 minutes, and trip off an arbitrary number of minutes after that, controlled by a knob. This allows you to set an “on time” of ~5, 10, 15, 20, 25, or 30 minutes every half hour. In short, it sets the duty cycle of the machine.
In normal operation, this will control how many songs play every half hour, and it has a clever circuit that listens to the music and doesn’t actually shut off power until it hears a gap in the record.
I want to use this timer to control my Raspberry Pi, so that even though the Pi is playing music (and injecting it into the Seeburg’s amplifier), the electromechanical timer will still start and stop the music. I don’t want to modify anything–I am using the existing jacks of the device for everything.
There is a RCA jack on the back (labeled J103C on the schematic below) that is for externally controlling the timer switch in case you have an external audio feed you want to use and don’t want the record turning. If the jack is shorted with a jumper, the timer functions normally. If you unplug the jumper, the machine turns on, the timer turns, but the motor relay never closes and the record doesn’t turn–this is the normal “external audio” mode.
I would like to hack this jack. Though it is really intended as an input control, I believe I can detect the timer’s current on/off state from the jack. It appears to have ~22 vdc present when “on” and 0 vdc when “off”.
Here is a highlighted schematic showing what I’m looking at. I have drawn lines showing where I think the voltage is coming from.
Here’s the parts list, if the resistor values are relevant.
I will leave the jumper off, since I don’t want the record to turn, but I want to detect the ~22v that appears when the timer is in the “on” cycle. I will use this to enable a GPIO pin that will be sensed by my code and start or stop the music, just as if I were actually playing records on the machine.