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Need Help with simple 9V circuit design.

Discussion in 'General Electronics Discussion' started by RobH2, Aug 28, 2020.

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  1. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    I'm not an electronics expert but have built 100s of devices by following schematics. I don't know electronic theory beyond the very basics.

    I have the need to change an existing system. I have a motor driven by a 9V battery. It has a switch and a pot so I can change the speed. I want to remove the existing switch and pot and add a "Touch" switch component I found. I just need to be able to turn it "on" or "off" right now. The "Touch" switch is rated at 2.5-5.5V.

    Are there a couple of simple components I can add so I can use this switch without burning it up? I can't find a higher voltage version.
     

    Attached Files:

  2. Nanren888

    Nanren888

    475
    136
    Nov 8, 2015
    grr. can't cut and paste the link form the image.

    http://www.icstation.com/10pcs-ttp2...lockingno-locking-module-arduino-p-12675.html

    No. Proably not as you have drawn it.
    This switch needs power to it always, so not used in series as you have drawn. It controls it's output to provide a voltage at TTL levels, so likely has no great ability to provide the current you need to run your motor.
    Also, as you mention it's not rated for 9 volts.
    You could arrange a circuit to use iit, by rediucing the voltage down to 5v to use the switch, then adding something to use the switch output to swicth the 9v to the motor.
    .
    Might be a lot easier to look for one that works on 9 volts and has a relay or larger current output.
    .
     
    Last edited: Aug 28, 2020
  3. bertus

    bertus Moderator

    1,759
    677
    Nov 8, 2019
    Hello,

    On the page that @Nanren888 gave, in the specifications is given that the output is TTL logic.

    Touchswitch.png

    You will need more parts to drive the motor.
    Also the PCB can not run on 9 Volts.

    Bertus
     
  4. Alec_t

    Alec_t

    2,950
    803
    Jul 7, 2015
    Unless the motor is very low power and you aren't concerned that the rpm will drop as load increases, using a potentiometer in series with the motor is a poor way to control the speed.
     
  5. PETERDECO

    PETERDECO

    140
    36
    Dec 19, 2019
    If you don't care about a speed control, something like this should work. LP2950 voltage regulators are available in many voltages. Just don't choose one over 5V. I found many low dropout regulators get finicky without a capacitor. Hence the 10uF. If this is a little hobby motor, the diode can be a 1N4148. On a heavy duty motor, I use FR602 or FR603. Apparently the trace from the diode to b+ didn't print.Your speed control leaves much to be desired. The right way to adjust speed and retain motor torque is with a pulse width modulator.
     

    Attached Files:

    hevans1944 likes this.
  6. Audioguru

    Audioguru

    3,251
    702
    Sep 24, 2016
    A motor and a switch are rated for voltage and for current. You did not say the maximum current for the motor and the spec's for the switch does not say its maximum allowed current.
     
  7. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    Thank you all for these suggestions. Let me be more specific. I'm trying to modify this for a friend who has some kind of use for it. He asked if I could remove the pot and insert the touch switch. It's that cut and dry and that simple.

    This is a circuit that was given to me. It's a motor that has a 9V battery, a switch and a pot. The Pot is 1K. At full power the motor runs at about 5RPM. I'm not concerned with whether this is a proper and good way to control motor speed. It's what I have.

    I just want to add a small touch switch and can't find one of a higher voltage. I don't care about torque, it just needs to turn and have a small amount of torque. Imagine lightly pinching the shaft with your fingers. The motor has no specs on it so I don't know anything about it.

    I was hoping that this touch switch could just open up a switch in the 9v circuit. The touch switch could have the 9v reduced to 5V and then it could then be used to merely open up a switch that's in the normal 9v path.

    @PETERDECO, your suggestion may just work. That's the kind of advice I'm looking for. I'm not looking for a critique of the existing circuit, I'm just trying to modify what I have... be-it-as-it-may. :)

    Thanks to all of you. I appreciate the fast responses. I know it can be frustrating when people like me come here and they have little electronics knowledge. But there are millions of us who tinker on things like this without much technical expertise. It's nice to have access to people like you for some hand holding. Much appreciated.
     
  8. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    @PETERDECO , is this a correct modification of your schematic. I added two connections that might not have printed. You mentioned one but I was another that maybe should have been there.
     

    Attached Files:

  9. Nanren888

    Nanren888

    475
    136
    Nov 8, 2015
    The LP2950 is a regulator bringing the 9v down to 5 volts for the touch switch. According to
    https://theorycircuit.com/lp2950-adjustable-micropower-voltage-regulator-circuit/
    pin 3 should go to the 9v, pin 2 to gnd and the output is pin 1.
    .
    The Fet provides that extra current capability that I suggested.
    The diode is to protect against the back emf of the motor, so the diode should go across the motor.
    The LP2950 comes in different output voltages, so get the 5v one.
    I have not looked at the touch switch, but since it is across the battery, it will somewhat drain the battery even when the motor is off, so might thik about keeping a switch as well.
    .
    I've not checked the design.
     
  10. Bluejets

    Bluejets

    4,764
    999
    Oct 5, 2014
    When most refer to 9v, they are talking smoke alarm battery which is not suitable for driving motors other than those extremely small.
     
    hevans1944 likes this.
  11. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    This is all great info and has been educational. Little by little we learn, with lots of help. I've ordered parts. I'll post my success or failure later when I've assembled things. I appreciate the assistance very much.
     
  12. PETERDECO

    PETERDECO

    140
    36
    Dec 19, 2019
    No. The input to the voltage regulator goes to B+ 9V, not ground.
     
  13. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    Thanks for clarifying. So ok, I'll take the blue lead of the LP2950 that I showed going to another ground and connect it to the 9v(+) between the battery and the motor like the attached.

    upload_2020-8-29_9-36-2.png
     
  14. PETERDECO

    PETERDECO

    140
    36
    Dec 19, 2019
    Yes. The regulators have an input (B+), output (regulated) and ground.
     
  15. bertus

    bertus Moderator

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    677
    Nov 8, 2019
    Hello,

    You can get a LP2950 in a TO-92 housing.
    It has the following connections:

    LP2950_pinout.png

    Bertus
     
  16. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    Thanks for all of this feedback and advice. I have parts in now and I'll start to test this. I'll report back.
     
  17. RobH2

    RobH2

    19
    2
    Aug 6, 2020
    Hi, I'm returning for a little advice. I've attached what I have wired (left hand schematic) and what was suggested above (right hand schematic). I have two needs:

    1. When I power the system, the touch switch actuates the circuit but very quickly the 'LP2950' begins to smoke. I turned it off quickly and let it cool. I then momentarily applied the 9V and it started to work, so I didn't fry it.

    2. Once I get it to run and not stress the LP2950, I'd like to add a pot in for speed control. The pot I show is from the original circuit I'm dismantling to add the touch switch in place of a standard mechanical sliding switch. How would I wire that pot into the circuit.

    I appreciate the help with this. Obviously I'm not an electronics expert. I know just enough to wire from schematics but can't diagnose much and don't dare start reversing connections so I don't blow my components up. I'm a product designer but not an electrical engineer.

    Components for Touch Switch_For Schematic_LP Smokes_2.jpg
     
    Last edited by a moderator: Sep 22, 2020
  18. Alec_t

    Alec_t

    2,950
    803
    Jul 7, 2015
    Are you sure you have the LP2950 wired correctly? The middle pin should go to ground according to the datasheet.
    Your choice of colours for the wiring in your schematic is unfortunate in that it differs from convention so can lead to confusion..
     
  19. bertus

    bertus Moderator

    1,759
    677
    Nov 8, 2019
    Hello,

    In your drawing you show the TO252 version of the LP2950.
    As @Alec_t said, check the wiring of the LP2950:

    LP2950_TO252_pinout.png

    Bertus
     
  20. PETERDECO

    PETERDECO

    140
    36
    Dec 19, 2019
    Yes. The schematic doesn't match the pinout of the actual device.
     
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