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circuit modification

Discussion in 'General Electronics Discussion' started by maanas.adi, Jul 2, 2013.

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  1. maanas.adi


    Jul 2, 2013
    Hi all,

    i have been given a circuit of someone else to modify/minimize the parts needed. the circuit is a dc motor driver with switches and interlocks. dc motor moves in left or right direction with two switches. we have two limit switches mounted which gives the motor end position. also included is a stop switch which will stop the motor from further movements. what are the minimum logic ic requirements. could you please help me in designing the circuit.

  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi there
    welcome to the forums :)

    how about showing us the circuit you have been given

  3. maanas.adi


    Jul 2, 2013
    the ckt is for operating a small dc motor. when sw1 is pressed it starts moving in left direction. after reaching the extreme left position, we get indication through No/NC contacts. the motor cannot be stopped in between. when in left position then only switch 2 will work.

    i cannot use digital pulse based h-bridge or microcontroller. also another switch might be required to stop the motor in between and then change the direction.

    Attached Files:

  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi and welcome to the electronics point forums :)

    I've had a look at your schematic and I have a few comments and questions.

    1. Is this a complete schematic of the board? I ask this because there are four OR gates, which would normally be a single CD4071 IC, but three of the gates are in U1 and the fourth one is in U3, and there are four AND gates, which would normally be a single CD4081 IC, but three of the gates are in U7 and the fourth one is in U4. Both of these facts seem to indicate that other gates in those ICs are used for other functions on the board.

    2. What are your priorities with this redesign?
    a. Update the design to use more modern components
    b. Reduce the component cost
    c. Reduce the total number of components
    d. Reduce the total size of components
    e. Convert the design to SMT (surface-mount technology)
    f. anything else?

    3. Can you deal with SMT components on this board, or does the design need to remain fully through-hole?

    4. The logic requirements of this board would be best implemented in a microcontroller, or a programmable logic device of some kind, e.g. an FPGA. You said you don't want to use a microcontroller. Do you also not want to use an FPGA? What is the reason? Do you want to avoid a programming step in the production process?

    5. Using a microcontroller would hugely reduce the cost and size of the circuit - all the logic would fit into a single 14-pin device costing around USD 1.00.

    6. Have you searched for a commercial product that does what you want? For example, a garage door opener contains the logic you need, although they are normally controlled with only a single pushbutton.

    7. U2, the LM311 comparator, is only used to implement a short power-on delay and could easily be replaced with a CMOS gate, to save space and cost.

    8. The limit switches, SW3 and SW4, have both the normally open and normally closed contacts used. There's no need for this; I would prefer to use a single contact.

    9. U5, CD4043B, is an old and little-used IC. It's probably on a "to be discontinued" list somewhere. It can be replaced with a CD4001 or a CD4013.

    10. U6, CD4098, is also an old device and could be replaced with a CD4528 or CD4538, or some simpler gate-based circuitry.

    11. I don't understand the function of U6, the CD4098 dual monostable. Its output pulse is only about 340 microseconds wide. Can you explain what it does?

    12. The two relays are a significant issue in terms of cost, size and power consumption. I would like to replace them both with an H-bridge IC. You mentioned that you don't want to use a pulsed H-bridge, but is there any reason you don't want to use an H-bridge IC instead of the relays?

    13. How much current does the motor draw?

    14. Would you be able to have a heatsink for the H-bridge IC if needed?

    Please answer every question thoroughly.
  5. KJ6EAD


    Aug 13, 2011
    Kris, I'm often impressed with the quality of your contributions on the forum but this one strikes me as amazingly comprehensive.
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Thanks KJ6EAD :) I appreciate your contributions too.
  7. maanas.adi


    Jul 2, 2013
    hello kris,
    i will try to give answers to all your questions.

    1. yes this is a complete circuit. the problem might have arose while doing annotations. all the four OR gates and AND gates are used of single IC.

    2. mainly is to reduce total number of components. where i WORK we have to purchase components in a proper manner and takes too much time. hence purchasing is not an option.

    3. if possible design should be through hole.

    4. since it is not a big project so microntroller/FPGA requirements has been nullified. Further system has to work in industrial environment, thats why working at 12/15V supply.

    5. purchasing problem???????????

    6. since the system is working and already well tested hence only modification required and no purchase.

    7. U2, LM311 is acting as power-on reset. to drive the motor to a default state.

    8. the motor should not operate in either direction further even if user accidentally presses the same direction button. e.g. motor has reached left side, so even if user presses left button motor should not move. that's why NO/NC were employed BUT if there are any changes that can be done.

    9, 10 Can be included in the modified ckt.

    11. 4098 is for resetting SR flip-flop , hence pulse width is not important.

    12. the motor is installed at a far distance and ckt can not be placed near/ on it as no space available. My Question is if h-bridge used then would there be any problem of pulsating analog to travel this far distance.

    13. motor takes 3 A current at full load.

    My main aim is to minimize the components while performing the logic. the circuit has added interlock eg if Relay 1 Operates then it will remove the power of Relay 2 even though the transistor of relay 2 is off.

    i am waiting for your suggestions

  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I don't understand why a microcontroller would cause a problem with purchasing. I would suggest something like the Microchip PIC16F1503-I/P which is a 14-pin DIP device. Digikey ( has these ex stock for USD 1.08 in single quantity, and less for larger quantities.

    Each device would need to be programmed. I suggest you download the data sheet from and look near the end for the section on programming it, to see what equipment would be required and whether it would be workable for your company.

    Using a microcontroller would eliminate a huge amount of circuitry and I would highly recommend it. I can help with the code.

    Re point 7. The LM311 causes the motor to initially move to the right hand limit, correct?

    8. I understand what you're saying. But there is no need for both contacts on each limit switch to be used. Only one contact needs to be used on each limit switch.

    11. I see what's happening. When one flip-flop is set, the attached monostable generates a short pulse to reset the other one, and vice versa. Correct?

    12. The distance to the motor is not an issue. The H-bridge would be used to produce steady voltages to the motor, without any kind of pulsed operation. Using an IC will save space and cost. I have identified several ICs that look suitable on Digikey:

    Infineon TLE5205-2 5A TO220-7 USD 6.89
    Infineon TLE5206-2 5A TO220-7 USD 6.89
    STMicroelectronics L6203 4A Multiwatt-11 USD 9.53
    ON Semiconductor STK681-320 5.2A 19-pin staggered-lead SIL USD 11.49
    ON Semiconductor STK681-332-E 6.1A 19-pin staggered-lead SIL USD 12.10

    Have a look at those devices. You may want to ask your purchasing department whether they have any preference.

    If you don't want to use a microcontroller, the changes I would suggest would be:
    a. replace the relays with an H-bridge IC;
    b. eliminate the LM311, CD4043, and CD4098.

    The first change should reduce the cost and size, although a heatsink will probably be needed for the H-bridge device.

    The second change would reduce the number of different items on the bill of materials, and remove the reliance on unusual components. But it would not significantly reduce the number of components in the design, especially if you want stop/resume and/or stop/reverse features added. The best way to do that would be to use a microcontroller for all the logic functions.
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hello...? What's happening with this project?
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