DIY Kit - Installing Rechargeable Battery
Please note this guide is for v1 Gecho (made in 2018 and earlier), the v2 already has a built in charging circuit.
Recharging is the way to go!
If your Gecho board also requires some soldering, please follow this guide first.
The charging module
If your DIY kit comes with rechargeable battery powering scheme, you should find one of these modules included. It is a standard circuit utilizing TP4056, with DW-01P and 8205A dual mosfet for over-current protection.
You need one more wire
Althought the charging module has its own USB micro connector, you may want to wire it in such a way that Gecho is charged via the on-board mini USB connector instead. In order to do this, you need to take the 5V voltage from somewhere. The good point is the L2 element, a ferrite bead (which filters unwanted high frequencies on the USB power line). It does not really matter which end of L2 you solder the wire to. Ideally it should be the end farther from the edge of the board, but if the other end is easier to access, then it is fine too.
How to wire it all together
Wiring is pretty straightforward. The module's ground pins (-) and (out -) are internally connected, no need to solder the wire to both of them.
For SRAM backup voltage (which is essential for user settings and generated content to be preserved) you have more options. The MCU's VBAT pin is wired to 3rd pad on the board and it requires to have 1.65 to 3.6V constantly connected, even while unit is powered off. The current consumption is very low, STM32F405 datasheet says it's typically below 1µA. You can use CR2302 battery, two AAA batteries (old and used will do) or add 3 diodes in series to lower the voltage to safe levels. Normal silicon diodes with typical voltage drop of 0.6-0.7V should be used. The voltage of rechargeable battery will cycle from 4.2V (fully charged) to 3.5-3.6V (critically low). With three diodes, this voltage gets down by cca 2V, staying in safe margins for VBAT pin.
Mon, 05/21/2018 - 23:15
Ext 5v power supply, no rechargeable battery
If I have a +5v and GND supply from an external source but I don't want to add a Li-ion/Li-po rechargeable battery how best to connect that supply to the Gecho?
Do I do all of the following:
(a) connect my external +5v to the + and my external GND to the - on the left of the 03962A module [seems correct?], and;
(b) connect the green, red and grey lines as above from the 03962A to the Gecho as shown, but omit the LI-ION circuit, and;
(c) either use the diodes as shown in the diagram (but then no power to maintain the SRAM memory) or better, insert a CR2302 instead of the diodes to keep the SRAM memory persistent when the 5v is not supplied?
or is there a smarter way of using external 5v to power Gecho without using the 03962A, and maintain the SRAM memory via a CR2302 circuit?
Tue, 05/22/2018 - 00:02
Re: Ext 5v power supply, no rechargeable battery
You don't need the 03962A module at all, just connect external supply at GND pad and BAT+ (where the grey and green line goes). (c) is best option then - well, it depends how much you care about settings and content stored permanently. If you just want to maintain them during a session, and to avoid that low-battery warning (red LEDs blinking) at every reset, a large capacitor will do (the best is to have it charged via a diode from 3.3V pad), otherwise you can use CR2302 for longer persistence. Good luck - let me know if that worked :)