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Hi,
I want to take some of the +5V signals feeding into the 8 triacs and connect these to an external board. Can anyone please tell me if the signals are active high or active low? I.e. +5V turns on the triac or 0V turns it on?
Thank you much for your help.
The logic signals sent to the triac are +3.3V (actual voltage may be a bit lower due to the properties of 74HC595 shift register), and are activate high. You can solder a wire from the triac’s gate pin to direct the signal to external boards.
If your external board requires +5V or higher signal, you can replace the triac by an NPN transistor (like 2N3904) and then the station port pin will become an open-drain (which can be pulled high to any voltage between 5 to 40V) with active-low logic.
Thank you Ray! I truly admire your stamina to reply to nearly every email and forum post, yet still be able to push the OS project forward.
3.3V from HC595??? The schematic ospi_v13_sch.png from your website shows there’s a +5V regulator that also feeds that chip. So am I missing something? HC595 running even at 4.5V should output around 4V min.
The idea to put an NPN in place of the triac is great! I was going to leave the triac in and just take a wire from the HC595 output to feed into a ULN2803 darlington. If I am unable to replace the triac with an NPN (and the additional resistor), then the 3.3V (if is really that) should still be able to easily drive the ULN2803 (VIL min = 2.4V @ IC=200mA).
Awaiting your reply on the 3.3V question. Thank you again!
The board provides 5V to Pi, however, the shift register and ADC are powered from the 3.3V pin from Pi, because all Pi pins are only 3.3V tolerable. The other reason is that often these embedded Linux boards (especially BBB) requires external components to be powered strictly after the processor is powered, so using the 3.3V power derived from Pi (instead of 5V from OSPi) ensures this property.
Thanks Ray. Just one more quick question please:
What is the spare current (after you subtract what’s needed for the OSPi) that I can draw from the +5V regulator LM2596S for my external circuitry?
I have a hefty 24VAC 3A (75VA) transformer and am using only 6 regular sprinkler valves so there’s plenty of capacity there. From the LM2596 spec, it seems like it should be able to do 3A but wanted to confirm.
Your customer support is dearly appreciated!!
What is the spare current (after you subtract what’s needed for the OSPi) that I can draw from the +5V regulator LM2596S for my external circuitry?
LM2596S is rated for 3A output, although in practice I don’t recommend drawing more than 1A because 3A is only a theoretical limit, and the circuit has never been tested under 3A output. Just for your reference, RPi normally draws about 250 to 400mA (with WiFi dongle), and the OSPi circuit draws very little, so the overall current draw is well below 500mA.
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