sleep mode for soil watch 10

[aylashiv]

here the scenario is: the device will be monitored from remote places.here the source of current is battery. lets say: if i want to read for every 10 mins then only a second is enough to read from analog pin rest 599 seconds current consumption is wasted. how can i over come it

Thanks,
shiva kumar

[pinolec]

Hi Shiva,

The easiest solution is to use a transistor as a switch to turn on and off the sensor. I assume you use a microcontroller to measure the values. Use 1 pin to control transistor to turn on the sensor.
Let me know if you need some more assistance with this.

 
Piotr

[aylashiv]

Hi,

can you send me the schematic of how to drive the sensor using a transistor? for whenever we want to make it on and off.

[pinolec]

Hi Shiva,

In general, you would like to use a transistor to switch load (sensor).

See attached picture.

You can use nearly any logic level n-channel MOSFET.

Regards,
Piotr

[aylashiv]

Hi,

Thanks for the schematic diagram, im currentky using an NPN transistor to switch on and off the load, but I prefer MOSFET in future, just clarify me, MOSFET has good retention than a BJT?

[pinolec]

In this application, it shouldn't be a lot of difference between BJT and MOSFET. The NPN will be good as well.

Regards

[jeanpaullm]

Hi , I was wondering if the circuit that was posted above worked with 3.3V circuits

[pinolec]

It will work as long as you choose the transistor which works at 3.3V
Look for logic level MOSFETS with VGS threshold lower than 2V or best if the RDs(on) is specified for voltage 3.3V or lower.
I think I have mentioned somewhere on the forum example transistors which will work.

regards,
Piotr

[javierb]

Hi, what is the difference between connect the sensor thought transistor or direct to a digital pin and manage it with pinmode, digitalWrite and delay?

[pinolec]

This is a good question.
In general, microcontrollers are not designed to deliver currents in the range of 20mA efficiently. Some microcontrollers are specified for less current. But even if the microcontroller can deliver 20mA, usually will suffer a voltage drop. In other words at 20mA there will be some voltage drop. Please see the voltage drop for 3V and 5V supply (from ATmega328P datasheet).
 

The voltage drop at worst 4.4V is still within the range of supply voltage of SoilWatch 10 sensor.

 

Now the voltage drop is below the specified voltage for SoilWatch 10. This will simply not work as expected.

This is the main reason we should use a transistor. With transistor, there will much less voltage drop.

[javierb]

Hi Piotr!
So I have two options to solve this, right?
- If I use a 5v arduino I can connect directly to the pin without worrying about the voltage drop,
- But if i want to use a 3.3 volt arduino, i must use transistors to drive the sensor.
It is correct?
Thanks!

[pinolec]

Hi Piotr!
So I have two options to solve this, right?
- If I use a 5v arduino I can connect directly to the pin without worrying about the voltage drop,
- But if i want to use a 3.3 volt arduino, i must use transistors to drive the sensor.
It is correct?
Thanks!

This is correct.
The first option is not pretty but it will work. Just make sure you do not overload the microcontroller and stay below the maximum current per pin and for the whole microcontroller.
The second option is my favourite.

Kind regards,
Piotr