turning on an MCU with a momentary switch
courtesy of Andrew Kilpatrick
21:45 <@elmood> if the circuit doesn't need to be turned off by the switch, you can omit the two diodes
21:45 <@elmood> the theory of operation is this:
21:45 <@elmood> when the MCU is off, the switch powers up the system power rail
21:46 <@elmood> the MCU turns on and immediately raises the MCU_POWER_CTRL line to hold the power on
21:46 <@elmood> it can release it by itself to turn off
21:46 <@elmood> if you want to detect the switch when the MCU is on, you can monitor MCU_POWER_SW pin
21:46 <@elmood> the current draw should be pretty much zero when off
21:52 <greg81> waw thanks a lot andrew
21:53 <greg81> where is Vin of the MCU
21:53 <@elmood> SYSTEM_POWER
21:53 <@elmood> that's the switched rail
21:53 <@elmood> it would be almost the supply voltage
21:53 <@elmood> minus the drop from the fet
21:53 <greg81> Ok and +5V is the power rail
21:54 <@elmood> yeah
21:54 <@elmood> or whatever voltage you have
21:54 <@elmood> the values aren't critical
21:55 <greg81> is the P-FET some kind of OR GATE in a way, like one of the two top pins on the top has to see 5V in order to conduct? 21:55 <@elmood> no
21:55 <@elmood> it's a transistor
21:56 <@elmood> but one that is better for this use
21:56 <@elmood> image it like a voltage-controlled variable resistor
21:56 <@elmood> kinda
21:56 <greg81> i see
21:56 <@elmood> when off it's basically an open circuit
21:57 <@elmood> R1 holds the gate up so it's off
21:57 <@elmood> Q2 can turn it on
21:57 <@elmood> but when Q2 is off it's also out of the circuiot
21:58 <@elmood> so the draw is basically nothing
21:58 <greg81> what kind of P-FET can be Q1?
21:58 <greg81> do you have a ref in mind?
21:58 <@elmood> not really
21:58 <@elmood> something small
21:58 <@elmood> how much power does your circuit need?
21:59 <@elmood> look for a SOT23 P-channel MOSFET that has enough current for your curciot
21:59 <@elmood> leave some extra safety margin
21:59 <@elmood> the important spec of a MOSFET is the Rds on
21:59 <@elmood> which is the “resistance” when it's on
22:00 <@elmood> small fets are measured in ohms
22:00 <greg81> http://www.dipmicro.com/store/SI2301
22:00 <greg81> ?
22:00 <@elmood> that's quite expensive
22:00 <@elmood> i would get a smaller one, but that's the right idea
22:00 <greg81> 52cts?
22:01 <greg81> this one then, half the price http://www.dipmicro.com/store/SI2302
22:01 <@elmood> there you go
22:01 <@elmood> but this is N channel
22:01 <@elmood> you need P channel
22:02 <@elmood> wtf is this store anyway?
22:02 <@elmood> https://www.digikey.com/product-detail/en/infineon-technologies/BSS84PH6433XTMA1/BSS84PH6433XTMA1CT-ND/5410005
22:02 <@elmood> here's one
22:02 <@elmood> 8 ohms at 170mA
22:03 <@elmood> not great
22:03 <@elmood> but maybe it doesn't matter to your circuit :)
22:03 <greg81> no it doesn't matter
22:03 <greg81> haha this store is based in niagara falls
22:03 <@elmood> ah
22:03 <@elmood> https://www.digikey.com/product-detail/en/rohm-semiconductor/RSC002P03T316/RSC002P03T316CT-ND/5042571
22:03 <greg81> shipping is 3$
22:04 <@elmood> this one is better
