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  • Author Author: balearicdynamics
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Meditech: Powering the camera probe

balearicdynamics

Introduction

As mentioned in previous posts the device RPIslave2 is equipped with the PI camera to capture still frames imaging diagnostics. This probe should be easily moved outside the Meditech unit; it remain connected to the remaining system via WiFi and should be battery-powered.

The powering characteristics should be the following:

 

  • When hosted in the Meditech unit the device is powered-up running in stand-by mode: some features will work but it is not possible to manage the camera.
  • While it is powered in the Meditech unit its battery is under charge with a simple circuit that should avoid battery damage when it reach full-charge.
  • As the device is extracted from the Meditech the charger is replaced without interruption by the device battery.
  • The actual battery - with the actual tests - should be about 1200-1500 mA
  • A 5V power regulator should be used to grant at least a stable 1000-1200 mA power at 5V
  • Maybe that a small cooler fan should be used when the device is working

 

Power test circuitimage

The power test circuit is actually using a 7.5 LiIon rechargeable battery, 1500 mA. As shown in the image there are two parts (will be a single smaller circuit). The regulated 5Vcc power is granted by a LM7805 while the battery is also connected to a simple yet small recharging device.

At the actual date the recharging circuit assolve its function to stop powering the battery as it is very near to full charge due to the final resistor that should be calculated based on the power of the battery. I have considered to erogate a lower voltage from the charging unit respect the battery and - in this case of the 1500 mA battery - limiting to about 1000 mA the power. The recharging circuit is done for now with a couple of LM350. I consider anyway to make the recharging unit with a OpAmp (LM358 maybe fine) and a feedback with three leds: under charge, charged, low battery

 

Issues

The system has beed tested for many hours and responded well. I think that some alternative solutions should be adopted for the power regulator finding a way to replace the LM7805 with some alternative: this component grant a prefect current level but dissipates too much current, specially when the RPIslave is running with all the features (camera, WiFi unit, image streaming etc.)

Attachments:
image5V PwrSupply.pdf
  • in reply to mcb1

    At the actual date there are two things I have not found yet clear documentation and I don't know very well:

    1. What is the minimum power needed to keep alive a Raspi + camera + WiFi + (maybe needed) small CPU cooler to keep the temperature around 25-30 Deg.
    2. In theory your suggestion is fine but this stuff should be boxed, with a dedicated circuit that need to be replicated. It is not difficult to update the circuit with an small Amp-Op like LM358 I suppose the solution is similar to what you are suggesting. I have experienced that these chip a custom circuits that should be get as-the-are if should be used and are not simple to find as commercial products.

    Remain the fact that the LM358 adopt the temperature dissipation method, so it is not the most efficient for cases like this and this element maybe - not great - acceptable for a first prototype but not for something to be replicated in more units.

    It is sure that at the end of the operation the entire Meditech electronics should be revisited and re-engineered

     

    Enrico

  • Nice ideas.

    I have reused a simple Car phone charger as a simple means to run a RPi, and camera.

    You can often pick them up on special for a few dollars and have a realtively efficient DC-DC chip inside, which might help with the heat problem.

     

    Mark