First off, apologies if I should have posted this elsewhere (such as under general electronics, etc.)

I have two relays (JBTek, DockerPi relay hat) I have been experimenting with to control input switching on a Kramer VS-211UHD switcher using momentary contact closure. I bought the JBTek relay first. Initially, I mistakenly wired the JBTek to the Kramer using the NC (normally closed) pin of each relay channel to the corresponding input pin on the Kramer contact closure block... "Here goes nothing!" ... "Huh, that's odd. It's switching backwards." Momentarily activating relay 1 would select input 2 and vice versa, almost like you're holding down both input buttons and then releasing one for an instant in order to switch to the other input. I checked my wiring and pretty quickly realized I had screwed up. Rewired it using NO (normally open) to each input pin and commons joined to GND and then it works like it should. Momentarily activating a relay connects the switcher's corresponding pin to GND and selects the corresponding input.

This was all good but I wanted something that was easier to mount inside a touchscreen case with the Raspberry Pi, so I looked into and eventually bought the DockerPi 4-channel relay hat. When I wired up this relay using the NO terminal of each relay to the corresponding pin on the Kramer switcher, it works just like the JBTek did when it was wired using NC wiring. Activating relay 1 selects input 2 and vice versa. I triple-checked my wiring... Scratching my head here. I wired it up NC instead (relay 1 NC to Kramer pin 1, relay 2 NC to Kramer pin 2, commons twisted together and connected to Kramer pin G) and it seems to be working correctly now.

My concern is: The Kramer manual specifically warns: "DO NOT connect more than one input pin to the G pin at the same time." When I wire it up to the DockerPi relay using NC wiring, am I not doing just that? When I hook a multimeter to the contact closure pins on the Kramer, I measure about 5.5v across either pin 1 or 2 and G. With the switcher connected to the relay board, I measure the same 5.5v between each relay NC pin and common when the relay is deactivated. So the circuit is closed on each relay under normal operation (Bad, according to the Kramer manual). It is only broken when a relay is activated, at which point the voltage measured drops to 0. Why does this relay seem to be wired opposite of the other one (DockerPi's NC behaves like JBTek's NO), and could I be damaging the Kramer switcher by connecting it like this? I'm using this to build an AV control system that would wind up unsupervised in classrooms. I have left the relay connected to the switcher for up to a few hours but I'm nervous about leaving it permanently connected when I don't understand exactly how (or why) it's working like that.

  • your post is totally unrelated to the Raspberry Pi
    – jsotola
    Nov 19, 2020 at 2:16
  • Are you using Rpi python to program I2C Kramer?
    – tlfong01
    Nov 19, 2020 at 2:26
  • Sorry if the post is unrelated (mostly), though it does involve a RPi accessory (the DockerPi hat). I am using python and the smbus library to activate the relays. The JBTek relay was driven using the RPi.GPIO library.
    – tannmatter
    Nov 19, 2020 at 16:53

1 Answer 1


Nevermind, I'm pretty sure the DockerPi board is just labeled backwards from the other one. I realized why I was measuring voltage where I didn't think there should be any. The voltage wasn't passing through the relay when deactivated, just through my leads (because I was touching them to the relay's screwposts - which were connected back to the switcher's terminals, derp). I sent an inquiry to the relay's manufacturer about the labeling being backward.

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