Nuts and Volts - September 2013

It is a boost regulator, generating

3.3V from a 3V battery. However, I found a few errors in his design/ documentation.

1. The buck regulator is called TPS72630 in the text and in the schematic, and does not exist. He first mentions the TPS62730 correctly, so I assume it is a simple transposition.

2. The PUMH10 is misrepresented in the schematic as having FETs, rather than the NPN bipolar devices mentioned in the text and parts list. The ports of the BLE112A have enough drive to easily pull the camera inputs low, so the buffers could be eliminated (though don't forget to change the firmware).

Unfortunately, the BLE module's ports are not truly open-drain. Because of the uC port's ESD diodes, the outputs should not be pulled beyond 0.4 volts of the supply, preventing a direct connection to the camera without buffers.

3. The 3024 coin battery is specified at 3.0 volts, not 3. 6 as in the text and schematic. A fresh one may read 3. 25 with a Hi-Z meter, but will quickly drop down with a load applied.

4. The camera detect power-on circuitry doesn't fit the text description regarding the resistors connected to the input of the Schmitt inverter. I re-arranged and merged the schematics to get a clearer understanding of the connectivity. As drawn, the detect node will ALWAYS

be low — 0.27 volts. As redrawn (and shown above) as I think was meant — with the 1M pulldown connected to shutter rather than detect — the circuit is functional. Note there is a 1M + 45K load on the camera battery which causes a continuous

3. 2 µA drain. When disconnected, there is an 11M load on the coin cell as well. (Every nanoamp counts.)

5. I would tend to leave the camera plug in place for long periods, since the connector is not very sturdy and I don't want to cycle it excessively. I would disconnect the dongle using the 2. 5 mm plug. With the plug dangling, it may make contact with something metallic and fire the trigger — or at least cause camera battery discharge. I would use a female connector on the camera side and a male on a cable from the controller.