How to power a Logomatic v2?

I have a Logomatic v2 that I want to run using an arduino uno running on a Lipoly 3.7v 2700mAh. I would like to power the Logomatic from one of the digital pins that is set to HIGH 5v, and therefore be able to specify when I want to turn the logomatic on & off (to save battery).

However, I am having issues - when I power the arduino with the battery, Why doesn't the logomatic log anything when I connect the logomatic's power pin to a digital pin (that is set at HIGH and at 5 volts)? The puzzling thing is that the logomatic does power up (the power LED in the board lights up but isn't super bright) but there is no activity at all in the "status" LEDs, and hence, no data is recorded.

Instead, when I power the arduino with the same battery and connect the logomatic power cable directly to the 3.3v or 5v pin in the arduino uno's "power" pins the logomatic works perfectly: the logomatic's power LED is on & bright, it logs all the serial output info, and there is some activity in the "status" LEDs (hence, because data is being recorded). My main problem here is that I cannot turn the logomatic on or off since the power is going directly to the arduino "power pin 3.3v" or "5v".

Any suggestions on how I can have the logomatic run & record the data, and be able to control when it turns on and off?


How much current does a Logomatic try to draw? I suspect its too much for an output pin.

EDIT: Up to 80mA, according to the datasheet. You seriously risk damaging the pin with that sort of current. Absolute maximum is 40mA, and recommended safe limit is 25mA.

Thanks dxw00d for the tip! I'm new at playing around with the arduino, so I'm learning as I'm going...

So, if I cannot power the logger directly from the digital pin, then, what approach can I use? Would it be possible to use some kind of relay for this?

Some sort of MOSFET driver would appear to be appropriate. You might be struggling to turn it on properly at 3.7V, I would check the datasheet. You might need a driver transistor as well. I'm not really the electronics guru - someone else might be able to suggest a good circuit.

thanks Nick

What about using a relay? I have a G2VN-234PH-US ( ... sheet.html) but it has so many pins that I am confused on how to connect it.

Specifically, how can I wire this relay so that it would take the logger circuit and turn it on & off based on the command from the arduino digital pin?

The above datasheet has the wiring details of the relay (with numbered pins) and here is a "general" wiring diagram ( Specifically... which cables go to which pin in the relay?


What does a relay give you (apart from isolation)? Judging by that thread you need a transistor anyway to turn on the relay.

Can you post a link to the Logomatic so I can see what we are talking about?