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QT110 Trouble with batteries

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harryvermeulen

New Member
Hello,

I've put together a basic circuit using a QT110 capacitive sensor (http://www.qprox.com/assets/Downloadablefile/qt110_104-15456.pdf). The output of the QT110 is connected to a small relay.

I've had a test setup running fine for a few days, using 4 AA batteries, respectively 6 volts. Now I'm trying to switch to a smaller 6 volt camera battery (6V Battery PX28A > Maplin), and I'm running into problems.

When I connect the battery, the relays coil turns on (I can hear it click). When I touch the touch sensitive part of the QT110, the coil turns off, but it doesn't turn on again when I let go of the touch sensitive part. It switches on and off fine on the 4 AA batteries.

My first thought was the small 6 volt battery wouldn't have enough ampere to switch the relays on, but it does as soon as I plug it in to the circuit. It also switches fine when I connect it directly to the relays.

My second thought was it's a grounding issue with the capacitive sensor, but I'm not sure why it would work with the AA batteries then.

I'm basically out of clues. Any help would be greatly appreciated! The circuit is going into an exhibition in about two weeks, and would really like to use a small battery to fit it in an object.

All the best,
Harry

p.s. I'll try to post the schematics I used when I get to work today
 

audioguru

Well-Known Member
Most Helpful Member
The tiny battery cannot provide enough current so when the relay actvates then the battery voltage drops down too low for the sensor to drive the relay for a second time.

Four AA cells weigh nearly 10 times as much as the tiny battery so they provide much more current without the voltage dropping.
 

harryvermeulen

New Member
Yeah, that sounds like a plausible theory. Is there any way to tell how much current a battery can provide? Usually they only state how much mAh they last no?

Thanks for your help!
 

audioguru

Well-Known Member
Most Helpful Member
Yeah, that sounds like a plausible theory. Is there any way to tell how much current a battery can provide? Usually they only state how much mAh they last no?

Thanks for your help!
Most battery manufacturer's websites have detailed datasheets for their batteries.
Duracell don't make the old alkaline tiny 6V battery anymore and have replaced it with a much better lithium type the PX28L. It is shown with a load of 11.1mA to last about 12 hours.
I bet your relay draws much more than 11.1mA.
 

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