I need to switch a 9vdc power source to feed either circuit A or circuit B. Both A and B have a common ground. The switch can use the same 9 v source. Maybe use a trim pot to adjust how long the power is on A before it goes back to B. Maybe a range of 5 to 20 seconds. Something like this. Load on each circuit jumps around but max on A or B is .45 amp.
thanks in advance for any help

 

Maybe use a trim pot to adjust how long the power is on A before it goes back to B.

Not clear.
So you want a delay after the switch is operated, to delay going from A to B?
How about from B to A?

 

Not clear.
So you want a delay after the switch is operated, to delay going from A to B?
How about from B to A?

On power up say circuit A turns on first, after say 5-8 seconds the power switches to circuit B and it operates for 4ish seconds ( could be fixed time) before falling back to circuit A for the 5-8 sec again-repeat

 

On power up say circuit A turns on first, after say 5-8 seconds the power switches to circuit B and it operates for 4ish seconds ( could be fixed time) before falling back to circuit A for the 5-8 sec again-repeat

So it goes back and forth between A and B with those delays, continually after switched ON?

 

So it goes back and forth between A and B with those delays, continually after switched ON?

Sure. I don’t need it to vary really. Mostly circuit A and then a shorter time on B. Repeat

 

I think you can set up a 555 with 2 separately adjustable state times by using the output to charge and discharge the timing cap.

It involves using some diodes.

Then just drive a relay with the discharge pin.

Of course, there is still the issue of the first on time being longer than the rest.

I believe you can also configure the Schmitt trigger inverter gate F/F to have separately adjustable state times, but by the time you add buffering you may as well use the 555.

 

Below is the LTspice sim of a circuit using a 555 timer, one BJT, and two P-MOSFETs, that should do what you want:

For the circuit values shown, the nominal A on-time (green trace) is about 7.5s and the B on-time (yellow trace) is about 4s.

The B on-time is determined by the value of R2 and C2.
The A on-time is determined by the value of (R1+R2) and C2.

The P-MOSFET can be just about any with an on-resistance of less than 0.2Ω.

 

On power up say circuit A turns on first, after say 5-8 seconds the power switches to circuit B and it operates for 4ish seconds ( could be fixed time) before falling back to circuit A for the 5-8 sec again-repeat

Does it matter which circuit turns on first? Or can this be random? How critical is it, if at all, that each circuit be powered for the same amount of time each time it is powered? In other words, if the first time Circuit A is powered, it is powered for 7.3 seconds but each time after that it is somewhere between 5.3 seconds and 6.1 seconds, would that be tolerable?

Is the 9 V available all the time, even before either circuit is powered? In other words, can the control circuit be running ahead of needing to start actually providing power to either circuit?

Is power consumption by the control circuit an issue?

 

Does it matter which circuit turns on first? Or can this be random? How critical is it, if at all, that each circuit be powered for the same amount of time each time it is powered? In other words, if the first time Circuit A is powered, it is powered for 7.3 seconds but each time after that it is somewhere between 5.3 seconds and 6.1 seconds, would that be tolerable?

Is the 9 V available all the time, even before either circuit is powered? In other words, can the control circuit be running ahead of needing to start actually providing power to either circuit?

Is power consumption by the control circuit an issue?

Yes 9v always available and times don’t matter

 

Yes 9v always available and times don’t matter

I have a 2 amp supply and load is no more than .5 amp. So power consumption should not be a problem. Simple circuit if possible

 

Simple circuit if possible

Not likely to get much simpler than that in my post #7.

 

Yes 9v always available and times don’t matter

Then I would think that crutschow's circuit above is a strong contender, with the slight mods needed to make the times adjustable.

 

Below is the LTspice sim of a circuit using a 555 timer, one BJT, and two P-MOSFETs, that should do what you want:

For the circuit values shown, the nominal A on-time (green trace) is about 7.5s and the B on-time (yellow trace) is about 4s.

The B on-time is determined by the value of R2 and C2.
The A on-time is determined by the value of (R1+R2) and C2.

The P-MOSFET can be just about any with an on-resistance of less than 0.2Ω.


View attachment 321423

Breadboarded this up. Works and I can change times. Always begins with A. When load goes from A to B it seems clean but when the load changes back to A from B both loads A &B are on for a small bit. I need a clean isolated change. Both loads cannot be on together? Ideas?

 

What are the loads? If they are electronic and have a capacitor across the power input, that would explain why it stays on. The only way to fix that would be to have dead time between the transitions.

 

Make sure R3 is connected.

 

What are the loads? If they are electronic and have a capacitor across the power input, that would explain why it stays on. The only way to fix that would be to have dead time between the transitions.

I breadboarded it and tested it with two LEDs. Not my loads I need it for. Test first to see if it worked. No capacitor. LEDs have a common ground

 

Below is the LTspice sim of a circuit using a 555 timer, one BJT, and two P-MOSFETs, that should do what you want:

For the circuit values shown, the nominal A on-time (green trace) is about 7.5s and the B on-time (yellow trace) is about 4s.

The B on-time is determined by the value of R2 and C2.
The A on-time is determined by the value of (R1+R2) and C2.

The P-MOSFET can be just about any with an on-resistance of less than 0.2Ω.


View attachment 321423

Works

Below is the LTspice sim of a circuit using a 555 timer, one BJT, and two P-MOSFETs, that should do what you want:

For the circuit values shown, the nominal A on-time (green trace) is about 7.5s and the B on-time (yellow trace) is about 4s.

The B on-time is determined by the value of R2 and C2.
The A on-time is determined by the value of (R1+R2) and C2.

The P-MOSFET can be just about any with an on-resistance of less than 0.2Ω.


View attachment 321423

Breadboarded circuit and it works. Tested with a couple LEDs, one as load A. One as load B. A comes on first then changes to B crisply but when it changes back to A both A & B are on together for a short bit. Loads A & B cannot be on together. Has to be a clean switch. Any thoughts?
Thank You

 

that is a very simple circuit so it clearly started working ok.
my guess is that your 9V battery is dying. make sure LEDs have current limiting resistors.
monitor supply voltage or use PSU. Q1 is in linear mode - change R5 to something much smaller and add resistor across base and emitter to help it turn off...

 

Loads A & B cannot be on together. Has to be a clean switch. Any thoughts?

I tried adding some delays to the original circuit but that didn't work well so I added a Schmitt-trigger quad NAND gate package, non-overlap circuit (below):
It adds one IC package but eliminates a transistor.
The non-over lap time is determined by the R8*C3 time-constant.

The bottom two traces are expanded, and shows about 10ms of non-overlap.

 

that is a very simple circuit so it clearly started working ok.
my guess is that your 9V battery is dying. make sure LEDs have current limiting resistors.
monitor supply voltage or use PSU. Q1 is in linear mode - change R5 to something much smaller and add resistor across base and emitter to help it turn off...

9v is a bench supply. The LEDs have current limiting resistors. Can you elaborate a bit on the alterations to Q1?