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I have an existing latching button switch that controls a low power LED (no more than 1W) from a 12V power supply. I'd like to add an extra circuit to the same power source that is controlled by the same switch, the other circuit will consume more power (up to 80W).

I'd like the on/off applications of the switch to cycle the on/off state of the two circuits as follows:

Switch mode             Circuit 1         Circuit 2
Off                     Off               Off
On                      ON                Off
Off                     Off               Off
On                      Off               ON
Off                     Off               Off
On                      ON                ON
(and repeat the cycle)

I'm not clear what I am looking for to achieve this desired effect, is there something off the shelf that can be used to achieve this and wire to my existing switch? If not, what should I be searching for to breadboard up a suitable prototype? As an aside, I'm not 100% sure the switch can handle 80w throughput, which may be one consideration.

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3 Answers 3

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I agree, a microcontroller would be the easiest, as you will need de-bouncing for your switch, and a state machine of some kind. There are other solutions, but they rapidly get more complicated. If you want to dabble in a tiny amount of programming, that is probably the solution for you. This dev kit is cheap, includes the programmer, and has access to enough pins for what you need. You would need to add a connector, and have an external circuit with a couple mosfets, a couple diodes, and a couple relays. Let us know whether this is a possibility for you, and I could draw it up in further detail.

http://www.mouser.com/ProductDetail/Texas-Instruments/EZ430-F2013/?qs=sGAEpiMZZMumoJNx8xCU5nFxmy%252b2zsQ4h%252bdVrWunAKI%3d

If you wish to do it completely without SW, this is the process that you would have to go through to design a logic circuit to do it, where the clock would be replaced by a debounced version of your switch.

http://www.allaboutcircuits.com/textbook/digital/chpt-11/finite-state-machines/

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  • \$\begingroup\$ Thanks for the the mouser link. Power consumption is a concern as the system will be on 12v batteries, but looking at the datasheet I think it can be very low power on standby. Claims as low as 5 microamps on standby (@3.6v). My low power led is 0.2 amps (@12v), so by my reckoning the microcontroller would be using over 100k times less power, so wouldn't cause much drain. \$\endgroup\$
    – user524261
    Jan 19, 2016 at 21:04
  • \$\begingroup\$ And if standby power is a concern, there are microcontrollers that are even lower standby power. That's just a pretty popular one, and easy to develop with. \$\endgroup\$
    – Peter Frey
    Jan 19, 2016 at 21:08
  • \$\begingroup\$ And thanks for reference to debouncing, all makes sense but is new to me! This would be an interesting project without SW (I do enough SW stuff!), too challenging for a beginner? \$\endgroup\$
    – user524261
    Jan 19, 2016 at 21:09
  • \$\begingroup\$ Nah. It's not complicated. If you know what all your logic gates do, what a flip flop does, and how to minimize a k-map, that's all you need. If you don't, there are plenty of articles to teach you. Up to you how much work you want to put into it. \$\endgroup\$
    – Peter Frey
    Jan 19, 2016 at 21:12
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Maybe a impulse relay (step relay), forget the MCU it will drain the battery, old school, this: http://gfinder.findernet.com//assets/Series/408/S26EN.pdf

Unfortunately it has only 4 steps: both off, 1st, off, 2nd, off. No both on. Perhaps you'll find something with 6 sequences.
Edit: 26.04 model you have: both off, both on, 1st, 2nd,... both off,...
12VDC model 20.24.9.012.4000 http://gfinder.findernet.com//assets/Series/406/S20EN.pdf

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  • \$\begingroup\$ One question, would I need my switch to be non-latching for the 26.04 model to work? ie a pushbutton type switch where each impulse goes through the four modes off/off, on/on, off/on, on/off? \$\endgroup\$
    – user524261
    Jan 19, 2016 at 22:08
  • \$\begingroup\$ @user524261 Yes, a pushbutton. The relay will consume power only when you press the button. It depends on your needs, but I think the functions can be expanded with use of more relays, also mere expensive and complex, but still very robust solution. youtube.com/watch?v=Lf-n7_vWt2o \$\endgroup\$ Jan 20, 2016 at 9:48
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The simplest method is a microcontroller, and a pair of relays. The switch would just be a simple input to the microcontroller. You can then use a simple state machine as you have posted to control the two relays.

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