# hard circuit for alternate ON-OFF operation

#### blue11

Joined Aug 1, 2008
5
can somebody please give me a working diagram for alternate on-off operation?!

as a specific example:

two pumps connected in series. when a high level switch is triggered, pump 1 will operate while pump 2 remains off. when a low level switch is triggered, pump 1 will turn off (both pumps now are off). when the high level switch is triggered again, it is now pump 2 that will operate while pump 1 is off. when the low level switch is triggered, pump 2 will turn off (both pumps now are off)... and cycle repeats. the two pumps should not operate simultaneously.

i easily created a circuit with plc diagram using one-shot relay. but when an interviewer asked me to draw a diagram for hard circuit (not plc), too bad i cannot think about it easily.

thanks!

#### RiJoRI

Joined Aug 15, 2007
536
I would go the route of using a J-K flip-flop, with the outputs driving something like a ULN2003, and that driving the relays.

--Rich

#### blue11

Joined Aug 1, 2008
5
I would go the route of using a J-K flip-flop, with the outputs driving something like a ULN2003, and that driving the relays.

--Rich
i'm kinda slow here...how can i do that exactly when j-k flip-flop has two input while my problem has only one? can you please show me your actual diagram?

#### beenthere

Joined Apr 20, 2004
15,819
Then use a D flip-flop with the not-Q input tied to D and the single input tied to the clock. The output state will change with each clock.

#### thingmaker3

Joined May 16, 2005
5,084
i'm kinda slow here...how can i do that exactly when j-k flip-flop has two input while my problem has only one? can you please show me your actual diagram?
The j-k flip-flop actually has three inputs. (Some of them have five!) Tie j and k to +V, and run your high level sensor to the clk i/p of the ff. Both outputs will now toggle. You can use the second ff in the package (they come two to a package, as you know) to generate a generic on/off for the pair of pumps based on the level switch inputs. Then all that is needed is a pair of AND gates and some MOSFETs or triacs and any required drivers.