I have searched the internet but have not quite found the right solution. I have not found anything here in the forum either and hope that you can help me. My Arduino is powered by 2 x 3.6 V batteries connected in parallel. To switch a relay, the batteries should be connected in series for a short time. see diagram. Specifically, pin D8 goes HIGH so that transistor Q2 is blocked. Then D2 is switched to HIGH to switch the batteries in series (Q3 switches through). D12 activates the relay. Can this be realized with this circuit? Are the two diodes sufficient to protect the Arduino from voltage peaks? Do you expect other problems?
Looks to me that if Q3 conducts you basically short VCC1. I'm not much of an electronics engineer to advise on better approaches.
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@zapatas that all looks rather complicated. Why not use one of the readily available relay modules like this one, for example:
To get the Batts in series one Batt + should go to the other Batt - , but both Batts - neg is fixed connected to each other.
Like @zapatas suggested a relay DPDT.
Why not run the Arduino from 6V.
Correct.
Your batteries are shown with the polarity reversed (or did you intend them that way?). When I was a boy first learning about electricity I remembered which end of a battery is positive by imagining the long bar that symbolises positive as being cut in half and made into a '+' sign.
Why are you trying to power a Uno R3 at 3V3?
Diode D2 does nothing useful.
Diode D1 would provide the required protection if the circuit worked, which it doesn't for the reason @sterretje said.
The better way to do this is to provide a power supply of the correct voltage for the relay and use a buck converter to lower that voltage for the Uno, which should be powered at 5V. Alternatively have 2 batteries, one for the Uno and one for the relay, at different voltages, or better still use a 5V relay as @mikedb suggested while I was typing this.
Thanks for the suggestions so far. The swap of the polarity was unintended. I will rethink ..........
NEVER connect batteries in parallel. They could overheat, give off toxic fumes or even catch fire.
I would use a SEPIC (buck/boost) converter, that will keep everything at voltage as the batteries discharge. You can keep the batteries in the parallel configuration. Use the correct voltage for the coil and it becomes a simple circuit.