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I'm working on a device that will dock to charge very similarly to a cordless phone. The dock outputs 5V DC, which is already very low risk, but I would prefer to add a control pin that turns on the 5V DC output only when the device is connected. I contemplated having the wireless device's MCU output a signal on the control pin that would tell the charger's MCU to enable charging. The problem with that idea is that there will likely be times where people don't recharge the device for long periods of time and the battery charge might fall below the threshold to power the MCU.

The goal is that the 5V DC power will not be falsely enabled by someone dropping a paperclip or other conductive object across the charging and control pins.

What are some good options to investigate for the charging base to detect the presence of the wireless device without the wireless device having its own power?

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While a control signal might sound nice at first, I think you'd be better off with adding current limiting to the docking station as your fail-safe. Here's why:

  • Limits the charge current to the wireless device. While your wireless device probably already has some sort of battery charging circuitry, it doesn't hurt to also limit it on the docking station.
  • This inherently buys you the protection you seek. If someone drops something that would normally short out your power supply, your docking station is now actively limiting current to a point where no damage occurs to the dock. Sure, it's technically shorted if you have a paper clip across the terminals, but if you properly limited the output, your power supply will happily be supplying current and all you'll end up with is a warm paperclip instead of a fried docking station.

A step up from this could be that you have an output LED to indicate a charging fault. Based on the assumption you have current rate limiting for charging on the device side, you could program your host device to turn off the power if the current limit goes over the normal rate of consumption you'd see from the wireless device. The LED could then be turned on to indicate a fault, in this case, a short circuit across the 5V and ground terminals.

A step up even further could be some sort of momentary switch that got actuated when the device was sitting firmly in the docking station. That, in addition to the current limiting, would be a pretty sure-fire way to ensure that you weren't supplying power when you shouldn't be, but also that if you were supplying current, it would be safely limited to prevent the docking station from overheating or frying itself.

Ultimately, pulling out another signal as a separate pin offers more of a chance to short something and would still require adequate design to make sure that shorting the control pin to ground, or another pin, didn't fudge anything. As you said, it's all pretty low voltage but in my eyes making the device handle the fault condition gracefully is better than relying on another condition that is just as error-prone.

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You make some excellent points. Thanks for the suggestions. –  Jonathan Adams Dec 7 '12 at 16:44
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