Switch circuit

Discussion in 'General Electronics Chat' started by Guinness1759, Dec 16, 2010.

  1. Guinness1759

    Thread Starter Member

    Dec 10, 2010
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    I have an electronic device that I need to power with a DC/DC converter. The problem is the electronic device is an embedded system so I can only power it by applying power (no EN pin or anything like that is available.)

    So basically the DC/DC converter will charge a capacitor and when that capacitor reaches a certain voltage I would like the embedded device to operate for about 30 seconds.

    So the only solution I can come up with is to use a n type MOSFET and a comparator. The comparator will read the voltage on the capacitor and apply the bias voltage to the gate of the MOSFET when the voltage reaches a certain level. That will allow charge to flow to the embedded device.

    So I have a few questions. What MOSFET do you recommend for this job? The current flowing through it will not exceed about 20-30mA. How can I get the comparator to continue applying the bias voltage for 30 seconds after it has started? Since the voltage on the capacitor will be dropping, the output of the comparator will naturally go low. I need it to remain on no matter what to ensure the device has time to operate (30 secodns).

    Or maybe there is a better way of doing this? I usually try to avoid these kind of designs since the MOSFET burns quite a bit of power.
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What is the input voltage, output voltage, and required current?

    The question is rather vague to me.
     
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  3. Guinness1759

    Thread Starter Member

    Dec 10, 2010
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    The output of DC/DC is 3V. the input for the sensor is 3V. The current consumption of the sensor is 10-20mA.

    I'm considering using a PIC12F1822 for this, although this would require that I learn c again. Maybe it would save me time in the long run.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What is the input voltage to the DC-DC converter?

    This could be done with a 7555 Timer set up as a one shot to drive a transistor (BJT or MOSFET) to power the circuit for a given time once power is applied.

    I am unsure what the use of a capacitor was, your original idea to make the circuit?

    Do you have a schematic of what the setup looks like currently, not what your current idea is, just the parts to connect.
     
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  5. Guinness1759

    Thread Starter Member

    Dec 10, 2010
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    I've decided to go with a PIC microcontroller since their nano power devices only consume around 30uA while running. It'll save me a lot of power.

    I'm thinking of a sensor to use with this system. I've used an LM19 temperature sensor with microcontrollers before. But I would like to do something different this time. Do you have any ideas for a very low power sensor that I can operate using the PIC microcontroller?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What temperature range and resolution do you need?

    Microchip as well as Maxim have a wide selection, including some with SPI outputs for easy communication with a microcontroller.
     
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  7. Guinness1759

    Thread Starter Member

    Dec 10, 2010
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    Well the PIC I'll be getting only has 10 bit ADC's on board so that's the best resolution that I'll have. I'll probably be running this at 3V. Temperature range will be about -20 F to 110 F.

    Are there any other very low power sensors besides temperate, for example humidity or others?
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Yes, depending on your definition of "low power", 1-2mA I believe I've seen.

    Search digikey or mouser for info and datasheets on both temp/humidity sensors to decide which you like.

    The temp sensors with the SPI output are for better temperature resolution than using the A/D on the uC, it sends the data out on two or four serial wires (SPI).
     
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