Auto Switch Polarity of a 1.5v AA at a Given Time

[redtagblogger]

Hi everyone!

I'm fairly new to electronics and would like help on how to design a circuit that will switch the polarity of a 1.5v AA battery automatically for a given time, say every 3 minutes or 5 minutes. I will use the circuit for electrolysis with a two metal rods as probes for positive and negative dip into a glass of distilled water.

Any idea how I can accomplish this. A schematic diagram would really be a big help to me and is highly appreciated. Thanks!

[pebe]

Hi there, and welcome. Here is a suitable circuit. Note that it's the bottom circuit (the forum seems to sort them alphabetically)

The LMC555 is wired as an oscillator that switches the relay alternately on, then off. The ‘ON’ time = the ‘OFF’ time and equals 0.7 x C1 x R1, so with the components shown the times will be about 3sec each. Choose a low leakage electrolytic for C1, – preferably a bead tantalum.

You can if you like use the more common LM555, but it has a bipolar output (as against the FET output of the CMOS version) so the On and Off times of the relay will not be quite symmetrical. The action of the relay is to reverse the battery polarity.

Your PM said you would like a maximum current of 1mA. If so, the second circuit uses a lowered 12V supply in place of the battery. Set the 100K pot, R2, to give 1.5V at the output of TR1. There is then about 1mA flowing through TR1. As soon as you start drawing current from the output, less flows in TR1 until it cuts off. R1 then limits the output current to about 1mA.

If you have any questions, please don't hesitate to ask.

[redtagblogger]

thank you so much pebe! i really appreciate it.

what type of relay switch should i use? dpdt?

for current limitter, can i use a current limitter diode and place it at the output polarities of the battery?

thanks!

[pebe]

Yes, use a DPDT type (a low power one is OK).

Regarding current limiting, anything you use with a 1.5V battery must use some sort of series sensor. That means that for any current drawn you will get less than 1.5V output. The circuit I gave was for 1mA supply (the current through R1) with the maximum voltage clamped to 1.5V.

Ideally, it needed a 1.5V zener but I could not find any so Tr1 and R2 do the same job.