GEM Accessory Power
Posted: Mon Jun 29, 2020 11:35 am
This is a two part question, and I haven't been successful in coming up with the right search terms to see if this has been covered elsewhere - if it has, I apologize for the redundancy.
The GEM board has a connector to supply 3.3 and 5V power, "for future expansion". Without shutting it down and dissecting it, I'm curious to know if the 5V pin is directly tied to the 5V input from the external power supply (and with common GND). I ask this, as I have built a board to optically read my water meter - not much to it, but I'm using the laser diode and optics from a cheap laser pointer to pick off the needle that sweeps by with every litre consumed, and a small tunable circuit with a photo-transistor and op amp are generating the pulse. There's also a bit of filtering before the final signal gets passed through a Schmitt trigger to give me a decent square wave (just using part of a 74LS14 for this). The whole circuit draws less than 40mA at 5V.
Would I be advised against using the power directly off the GEM via the aforementioned expansion pins?
Also, it begs the other question - inasmuch as the output from this circuit is referenced to the power supply's ground - connecting this to the pulse input on the GEM would effectively be tying the "0V" signal of the pulse input directly to the GND supplied at the power header. is *this* problematic?
Seems daft to have another 5V wall wart plugged in just to feed this little circuit.
Cheers!
The GEM board has a connector to supply 3.3 and 5V power, "for future expansion". Without shutting it down and dissecting it, I'm curious to know if the 5V pin is directly tied to the 5V input from the external power supply (and with common GND). I ask this, as I have built a board to optically read my water meter - not much to it, but I'm using the laser diode and optics from a cheap laser pointer to pick off the needle that sweeps by with every litre consumed, and a small tunable circuit with a photo-transistor and op amp are generating the pulse. There's also a bit of filtering before the final signal gets passed through a Schmitt trigger to give me a decent square wave (just using part of a 74LS14 for this). The whole circuit draws less than 40mA at 5V.
Would I be advised against using the power directly off the GEM via the aforementioned expansion pins?
Also, it begs the other question - inasmuch as the output from this circuit is referenced to the power supply's ground - connecting this to the pulse input on the GEM would effectively be tying the "0V" signal of the pulse input directly to the GND supplied at the power header. is *this* problematic?
Seems daft to have another 5V wall wart plugged in just to feed this little circuit.
Cheers!
.