Hi,
I’m currently making a digital music box for my daughter, rather than an analog mechanism. It’s using a particular song that was sung as a lullaby for her when she was young. I purchased an Arduino, DFPlayer Mini and enclosed 3W 8 Ohm speaker. I realised after that the DFPlayer can be used as a standalone, without the need of the Arduino. My plan is to have the lid of the music box push down a NC (normally closed) switch, which would release upon opening the lid to complete the circuit, with a sliding switch to toggle on/off elsewhere.
My question is regarding power supply. Since it is a music box, I would like the box to be portable. I would like to have the ability to recharge the batteries in situ. My research has led me to Li-PO or NiMH batteries.
Is NiMH the safest option for a childs toy?
How do I safely (over/undercharging, short circuit protection) and adequately recharge the batteries in situ, rather than removing to connect to an external charger?
Finally, how do I determine current draw from components to choose an adequate battery capacity?
Thank you for any help,
Ryan
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Hi @Ryan253685 - welcome to the forums 
Sounds like a really wholesome project.
Yes.
From memory, charging NiMH cells is not too hard with a simple current-limiting circuit. After some light research on our website I can’t see any NiMH-specific charging products that you could integrate, but there are circuits galore across the web. Here’s some inspiration 1, 2, 3.
Note I’m not endorsing any of these, but it looks like there are many resources to DIY a charger.
Use a panel-mounting USB socket as your power input and you’re well on your way.
You can measure the current consumption ahead of time using a multimeter in current mode. This will be a pretty low-power project so you could also just give it a go and trust the cells will have ample capacity. Since the duty cycle is low (the project only runs when you open the lid) I expect you’ll get really good mileage out of a single charge eg. several hours of playback time.
Happy Making 
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Hi Michael, thanks very much for the resources.
From memory, charging NiMH cells is not too hard with a simple current-limiting circuit. After some light research on our website I can’t see any NiMH-specific charging products that you could integrate, but there are circuits galore across the web. Here’s some inspiration 1, 2, 3.
Note I’m not endorsing any of these, but it looks like there are many resources to DIY a charger.
I was hoping it would be easier to buy a product, but I’m up for a challenge (very much a beginner). I’ve looked at the first source primarily. The circuit is used to charge a 9V NiMH battery. How do I adapt the circuit to charge a 3.7V battery (3.7V NiMH from Core Electronics)?
“The charging protocol depends on the size and type of the battery being charged. Some battery types have a high tolerance for overcharging and can be recharged by connection to a constant voltage source or a constant current source, depending on the battery type. If safe charging, fast charging, and/or maximum battery life are important, that’s when things get complicated.”
What are the considerations and properties of the NiMH batteries that you sell?
How does this circuit wire into the powering circuit for the box? Is this in series/parallel to the load circuit or does it need a switch involved?
Ryan
