Peter Jakab has written a new article on how to adapt character LCD modules for 3 volt operation.
Generic character LCD modules contain an industry standard HD44780 compatible controller, which can operate down at 3 volts. But the modules are usually specified to work only at 5 volts, unless you choose a specific one designed for 3V operation. It is possible and simple to adapt the 5V modules to work on 3 volts.
via the contact form.
That is a nice hack and good research, however, the adaptor likely costs more than simply picking a 3.3V display module. Not to mention the extra hardware – that could be a problem in tight cases.
I bought a batch of 5V modules that have pads for an on-board charge pump. Unfortunately, the charge pump chip is more expensive than the entire LCD module at hobbyist quantities.
What’s he using that’s more expensive than an entire LCD module? We don’t need much current for biasing; TC7660-like pumps can be found under USD1. Or you can make a 1-gate oscillator using a schmitt trigger inverter, that’s under USD0.40. A PIC10F is less than USD0.50 too.
If you have a square wave clock somewhere, just add 2 caps and 2 diodes to make the inverting charge pump. (Oversimplified, but you get the idea.) You can even get some dual diodes in sot-23-5 packages and save one footprint. A schmitt trigger (square wave) oscillator will need 1 R, 1 C and 1 gate.
if you need to spend $1 to populate a charge pump chip on the lcd module or need extra footprint on your pcb for the voltage doubler then why not simply buy a 2 dollar 3.3v LCD module and be done with it?.
Obviously, it’s hard to go least-cost all the time. Think hobby time here. Can’t super-optimize things like a Wallmart executive all the time. We don’t say our hobby time is a complete waste of time too.
A charge pump circuit can go a lot under USD1, I was just giving ballpark estimates. A bunch of 1N4148 in a parts collection is viewed as practically free to most of us. A charge pump running at 20kHz can easily provide a few mA. The footprint is just a thought; I bought BAT54S at nearly the same price as BAT54, and I can use BAT54S for BAT54 circuits as long as the other pin is NC.
In the end, when we have amassed a huge collection of parts, sometimes we want to put some into use, even when it’s a bit harder than ordering new parts. Some of us old timers have plenty of old parts that we wistfully feel we want to use someday. Metal can transistors, anyone? The article is definitely informative tough.
It would have been nice to see a schematic for the add-on board. Or a github reference, or even a link selling them for $3.50 [for example]
I failed to find identify the SMD markings. If it’s expensive, it may be a regulated charge pump. If not, it may be a TC7660-type part, some derivatives are sot-23. IIRC the cheapest IC alternative is to use a 74LVC1G14 as an inverting charge pump.
that seems to be a TPS60400dbv
http://www.ti.com/product/TPS60400/technicaldocuments
You will get them for ~$1 in single quantities.
Thanks. Hmmm. Here are some comparison data from Farnell/Newark/Element14 (in USD):
TPS60400DBVT Texas Inst 10+$0.983 unregulated charge pump 60mA
TCM828ECT713 Microchip 10+$0.549 unregulated charge pump 25mA
TC7660COA Microchip 26+$0.66 cheapest classic 7660-type part
A funny thing, you can get MAX232 type ICs at less than the price of TC7660, they have 2 charge pumps, but are usually specified for 5V operation. For making a schmitt trigger oscillator, a 1-gate parts is often the cheapest option:
74LVC1G14GV NXP 1+$0.087 schmitt trigger inverter
Other square wave oscillators using 555 or opamps or comparators or multivibrators can often be made from parts already existing in a parts collection, ‘free’ in terms of parts cost, ahem. A square wave can then drive an inverting charge pump, I think almost anything can switch a few milliamps.
For completeness, one more item from Farnell/Newark/Element14 (in USD):
PIC10F220T-I/OT Microchip 1+$0.46
(Often there are cheaper discounts, this is a part at regular price.)
Of course, this will be connected to the inverting charge pump.
Any other way to get an itty-bitty negative voltage rail?
Yes, the integrated circuit is a SOT23 TPS60400 to make the board as small as possible. You can find the schematic and board layout of the circuit at https://github.com/PeterJakab/lcd_bias