As most families with children, we happen to inherit toys and alike. We got an electronic keyboard (musical instrument) called "Miles Electronic Keyboard 3738". To my surprise it was built around year 2000, not earlier as the electronics inside would suggest. It works either on 220V AC or 9 V DC provided by
six 1.5 V D cells. My older daughter is currently playing with it around home and I don't like her constantly looking for an AC outlet. But no way I buy six heavy D cells that are probably made of AAA on steroids.
Time for a battery upgrade!
First of all, the PSU board is very simple, with a bridge rectifier and a
78L09 9.8V zener: the required current must be low. In fact the current drawn peaks to 40 mA when emitting sounds, and 16 mA in stand-by. Why D cells?! Anyway, I decided to fit a rechargable battery in there, so I chose a recycled 18650 3.7 V Li-ion (originally 2000 mAh) followed by a boost step-up cicuit (I had bought a stock of them). The current requirement does not call for two cells in series.
So I rearranged wiring in a way that the on-off switch acts between the battery positive pole and the boost converter. The AC PSU was already always on when plugged in, only audio electronics were switched off. This fact gives me the chance to repurpose the PSU as a constant voltage Li-ion 1-cell charger. It is now voltage conditioned and fed through a diode (protection against reverse discharge) right to the battery.
The first circuit uses two LEDs (red and white in series) to step down from 9 V to 4.2 V max, but the current flow is too small to recharge in a decent amount of time. It's about 20 mA, as much as the stand-by device current consumption. We've got plenty of non-use time, but I don't want to have the keyboard around, keeping an AC outlet busy just for charging.
Second thought was to fit a Zener diode as voltage conditioner (Resistor - Zener to ground - Diode to battery). Still not optimal.
Third thought: replace the 78L09 with a 78(L)05 and drop voltage to 4.2 V with a couple of diodes in series. Make sure that the recharge current lies within the voltage regulator specs and let them play! Nope! There is no 78L09 in there, just a Zener. So, nevermind, I will leave it at 9V, keep the LEDs and the slow charge.
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| The messy area inside the keyboard. |
The battery and the voltage boost are the red heatshrinked block, that has been superglued to the keyboard top cover.
