LCD Power

K

knight

Member
I just need someone to do a quick sanity check of a thought I came up with tired last night.

Picaxe 18M2 or a 20X2. Both will run between 1.9V and 5.5V and so will run off a 240mAH 3.7 Volt Li-pol battery. However even running at 3.3V or lower, their digital outputs will still run at 5V and supply up to 20mA.

Correct so far?

Okay if the above is correct I have a LCD that requires 5.5V and draws 3mA for its Logic. I think some of you will be able to guess where this is going.

Could I use one of the digital outputs of the Picaxe to power the LCD? Would that work correctly and display correctly? I'm aware that the backlight would draw far too much current for that to work, but in situations where the backlight is not required, i'm hoping this might work.

Thanks all.


Datasheet for the LCD in question:
http://www.bonafide.com.hk/catalog/MC1602-18.pdf
 
J

john2051

New Member
As far as I know, the outputs will be at whatever the supply voltage is, or fractionally lower.

regards john
 
W

westaust55

Moderator
Yes, John H is correct.

PICAXE and in general for most logic chips, the outputs operate at the same level/voltage as the chip supply voltage for all intents and purposes.
In reality there is a small voltage drop (around 0.3V) from volts across the output drive transistors.

For inputs, the input signals shold not exceed the supply voltage. The absolute max (never to be exceeded) limit for the PIC chips is 0.3V greater than the supply voltage.

If yyou do have chips that require or use a different voltage to the PICAXE chips then you need some form of level shifting. This can be as simple as a couple of resistors where a drop in signal voltage levels is needed to level shifting circuit using voltage tolerant IC (for 5V into a 3V circuit eg 74LVC245A) or using say a 2N7000 FET (transistor) which can also provide a bi-directional level shifting interface.
 
K

knight

Member
Thanks all, this again, ovbiously one of those times where the brilliant idea you had half asleep isn't such a brilliant idea...

Anyway, I've got multiple of these little Lipo cells, so I'll just hook a couple together and then regulate it down to 5v. Now off in search of a decent 5V LDO regulator....
 
V

Vroom

Member
marks said:
this is what you need lol
a stepup module designed to work from cells.
http://www.ebay.com.au/itm/Step-up-500mA-Module-DC-DC-Voltage-Converter-OUT3-6-5-2-/230624240100?pt=BI_Electrical_Equipment_Tools&hash=item35b246a1e4
With most things 5v is optimum
but specs for your lcd do give a minimum of 4.5v which i,d try first.
i always think its best if you use the same voltage level throughout if you can.
Click to expand...
Sounds great, 1.2V to 5.2V, my own design picaxe with use 3.7V Li-ion, but make own PCB, have schematic DC-DC Converter?
 
D

Dippy

Moderator
Can you expand on the question Vroom ; I realy don't understand.

If, and this is a complete guess, you are asking for a schematic of a boost (inverter) supply then go to places like Farnell and search on "boost" > "power management" > "DC-DC Converters and Charge Pumps".

You will get a shed load. Find a nice easy one and read the Data Sheet. Most will give clear SCHEMATICS. Then it will be down to YOUR skill with PCB layout to optimise the performance; trust me, layout can make a huge difference.
 
V

Vroom

Member
Right, what I was want to have a look the circuit diagram. But I've got found in websites of Australia, i will order, thanks.
 
You must log in or register to reply here.
Top