I am working on a project within my company that involves a relay board used to control three different motors when an external 24V button is pressed. The existing setup works based on this principle.

Now, I have been tasked with designing a new board that maintains the current logic while introducing PWM (Pulse Width Modulation) capability for one of the motors. An important requirement is that the motors should still be controllable in their default manner even if no microcontroller software is running, if the software crashes, or if the controller malfunctions. To achieve this, I've come up with a circuit design, and I'd appreciate your insights on it.
here is the sketch:

1. Here's a breakdown of the proposed circuit:
   1. Input Processing: The external 24V momentary switch's signal is processed using a voltage divider before being routed to six drivers.
   2. Default Mode: In normal operation (when the MCU is functional), 1OE (Output Enable) is set to low (pull-down), and 2OE is pulled up..
   3. Software Control: Once the MCU software is developed, it will set 1OE to '1' and 2OE to '0', enabling the MCU to control the motors.
   4. Failsafe Mechanism: To ensure failsafe operation, an external watchdog with a latch circuit is integrated. This watchdog monitors the MCU's status. If a software crash occurs, the MCU is held in reset (RST), and the outputs are set to a high-impedance state (high-z). At this point, the pull-down/pull-up configurations take over to ensure the motors remain controllable.

The motor control side involves using three different 2N3904 BJTs.




I have a few questions:

1. Could you please review the attached sketch and let me know if you notice any potential issues with this design?
2. I am aware that connecting the inputs together is feasible, but I am uncertain about how to handle the outputs in this setup. Any advice on this matter would be greatly appreciated.

Thank you in advance for your valuable feedback and insights.

 

Welcome to AAC.

My first impulse for this circuit would be to use a synchronous counter IC, like the SN74ALS867A wired as a one-hot (ring) counter. How did you decide on the SN54HC as your solution?

 

Welcome to AAC.

My first impulse for this circuit would be to use a synchronous counter IC, like the SN74ALS867A wired as a one-hot (ring) counter. How did you decide on the SN54HC as your solution?

I did mistake in p/n
How 8 bit counter connect to all what I said?
Did you read my question?
Its just a 8 I/O buffer thats all!

 

I did mistake in p/n
How 8 bit counter connect to all what I said?
Did you read my question?
Its just a 8 I/O buffer thats all!

I may have misunderstood. I thought you wanted a state machine that would select one of three options with a pushbutton. If not, then just ignore my suggestion.

 

I may have misunderstood. I thought you wanted a state machine that would select one of three options with a pushbutton. If not, then just ignore my suggestion.

So what now you think about my circuit?

 

So what now you think about my circuit?

I don’t know because if my understanding is not correct I failed to decipher your post, so I can’t help.

Sorry about that.

 

You really need to describe what exactly what your circuit needs to do. Like ya’akov, I thought you meant the button would alternately activate the three motors. Now it looks like it activates all 3 at the same time. And it looks like you want to control the motors with a microcontroller as well. And don’t just say I have it right, because my understanding is still limited.

 

I want to design a new board , and keep the existing logic

I see no logic.
It appears that all motors are turned on by pressing the one button.

this is the driving side of motor (the BJT in my sketch are 3 different 2N3904):

You are driving the MOSFET with a gate voltage of 24V (when 2N3904 is on) and most MOSFETs can only tolerate 20V or less, so you need a voltage divider on the gate to reduce that voltage to about 10V.

 

You really need to describe what exactly what your circuit needs to do. Like ya’akov, I thought you meant the button would alternately activate the three motors. Now it looks like it activates all 3 at the same time. And it looks like you want to control the motors with a microcontroller as well. And don’t just say I have it right, because my understanding is still limited.

I think I described it well,
the circuit should work with MCU/ analog switch. if the MCU Crash the supervisor/watchdog should floating the OE1+OE2
and the button will work without need the MCU

PLEASE LOOK AT MY NEW EDIT, NOW I THINK IS MORE CLEARLY

 

I think I understand this now.

There is a momentary switch which is normally input to an MCU with in turn controls some motors. The desire is a failsafe backup so the switch will either signal the MCU, if it is running, or operate the motors directly if it is not.

I don’t really have any suggestions about this idea since it certainly isn’t something I would do. I would isolate the backup control by using a second switch which would become active if the MCU was not operational.

Maybe someone else can work out the best way to do this though. Best of luck.

 

I think I understand this now.

There is a momentary switch which is normally input to an MCU with in turn controls some motors. The desire is a failsafe backup so the switch will either signal the MCU, if it is running, or operate the motors directly if it is not.

I don’t really have any suggestions about this idea since it certainly isn’t something I would do. I would isolate the backup control by using a second switch which would be come active if the MCU was not operational.

Maybe someone else can work out the best way to do this though. Best of luck.

True, that's my intention, happy for the clarification and refinement
what you see wrong in my circuit?