one of those basic questions, google is just confusing me.
i have 2 lcd’s, one is a character type where i have access to both + and - connections for the backlight, so that’s simple, feed + 5 volts from the usb supply, and switch the negative backlight connection through an N channel mosfet like a 2n7002 (which i have some of)
switched with a pin on the arduino.
The other display is a 7 segment LCD, and i only have access to the +5v connection for the backlight, i.e. the other side of the backlight is connected internally to negative / ground,
Am i right in thinking i must use a P channel mosfet to switch the +5v supply to this blacklight?
or can i use an N channel, putting the backlight + connection to the backlight where the gnd would go on the mosfet (Source) and connect drain to +5 volts from the usb,
If i must use a P channel mosfet, … sorry for this… but any recommendations?
BSS84?
i only need to switch about 30mA,
Does the backlight switch on when you connect 5V to thet pin? Then it’s indeed a high side switch.
Do measure (or check with the datasheet) the current you really need. If <20 mA you can use an Arduino pin to do this directly. Same for the backlight of the LCD.
Otherwise a BSS84 will do fine, as will a traditional PNP transistor such as the BC557. The BSS84 comes only in SOT23 package, the BC557 in TO-92 - of course there exist equivalents in SOT-23 package as well if you prefer that. Mind that your backlight will be active low now.
yep, the display in question is a HT1621, it has a solder bridge on the back to switch from backlight being on all the time, i.e. connected to the main +5v in pin, to route it to a seperate pin, which you apply 5 volts to,
Threre’s probably a way to cut a trace and solder on a negative switching arangement, but i’m trying to make something others can copy if they want, so plug and play is best,
Which thinking about that, i’d best design 2 pcb’s, one with smt components on it that i am fine with soldering, and one using through hole, as i just know people will not even consider smt stuff, even tho i find it easier at times.