by pebe » Wed Aug 13, 2014 11:23 am
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.
- Attachments
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- 1mA CC.GIF (2.18 KiB) Viewed 6986 times
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- 555 flip flop.GIF (3.73 KiB) Viewed 6988 times