I found a circuit to protect DC motors from stalling:

this is the schematic: schematic

and this is the board I made based on that schematic:

PCB board

Actual board

When the power is connected to the circuit makes all LEDs on but no power in LOAD, when i press the SW1 push button motor connected to the LOAD is working only one LED is on but as soon as i release the SW1 motor stop working. stalling the motor cause short circuit. VR1 have no effect.

Why isn't the circuit working? In the video it is working...


edit: the IC number in schematic is LM358 but mine is LM358N, not sure if there's a difference...


closed as unclear what you're asking by Marcus Müller, Dwayne Reid, Finbarr, Sparky256, Edgar Brown Jan 4 at 17:57

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  • \$\begingroup\$ sorry, with the level of information you're giving us, there's no way we can help you. I'm afraid you'll first need to acquire familarity with how the circuit is supposed to work, to be able to diagnose it, before we can help you based on the diagnosis – we're simply not close enough to even start diagnosing things. \$\endgroup\$ – Marcus Müller Jan 2 at 13:35
  • \$\begingroup\$ also, this thing is a bit complex for something that any form of current sensing, a 1€ microcontroller, and a relay could have solved digitally. \$\endgroup\$ – Marcus Müller Jan 2 at 13:36
  • \$\begingroup\$ @MarcusMüller what information you need to know in order to be able to diagnose it? as i said i'm a newbie what's that microcontroller you mentioned? \$\endgroup\$ – newbie Jan 2 at 13:44

Judging by the the relay markings of NC and NO on the PCB, you have swapped the NC and NO of the relay.

NC means Normally Closed. NO means Normally Opened. 'Normally' in the sense that this is the state the relay is in when it's not powered.

In the schematic you can see that the NC side of the relay goes to the 0R1 power resistor - which is correct. The board you've made routes the NO side of the relay to the 0R1 power resistor - which is incorrect.

--UPDATE 1--

I checked the datasheet of the relay:

Your PCB was right after all, the markings on your PCB (NO and NC) just aren't... Sorry about the confusion, I assumed the markings would be correct. Lesson to both of us: Always check the datasheet :-)

--UPDATE 2--

Ok, since I've started this, I'll try to finish it. I thought it might be a simple solution, but unfortunately...

  • I looked at the PCB and the schematic, and I think they match.
  • LM358N is fine. The N at the end just means it's in a DIP package.
  • Measure the voltage at the + of C1. Should be 5V. If not, did you use a zener diode of 5V? (It's not a regular diode, just checking.)
  • Remove IC1 from the socket. Relay&LED1 should be off and motor should turn. Only LED3 should be on. If not (relay&LED1 on, motor off): Transistor is BC1815, right? If you've used a different type, check if the pins are at the right position. Like a BC547 has the pins in a different location. Otherwise: Replace transistor.
  • If you've ever accidently reversed the polarity of the INPUT, you might want to replace IC1.
  • Check the PCB for bad connections or short circuits.
  • Then, most importantly: The design is just wrong. The datasheet of the LM358 says: The upper end of the common mode voltage range is VCC−1.7V, which in your case is 12-1.7=10.3V. With this design, the common mode voltage is 11.47V, which is not acceptable. It won't destroy the IC, but it might not behave correctly. So even if you've done everything right, this circuit might still not work (possibly depending on the batch or brand of IC1).

--UPDATE 3---

I just realised this can never work, because the + and - of IC1G1 are also reversed. That's the problem when looking at someone elses schematic - you assume things are right...

Swap R3 pin2 with R4 pin2. That's still wrong common-mode wise, but it gives you some chance of getting it to work. Honestly, I don't see how the youtube video ever worked with this schematic. You'd really be better off redesigning it... I'm sorry :-(

Hope this helps. I'll leave it at this.

  • \$\begingroup\$ just swapped NC with NO on the board, relay is not connect disconnecting anymore but the is circuit is not working either... \$\endgroup\$ – newbie Jan 2 at 15:51
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    \$\begingroup\$ That mistake alone should not burn out anything. So unless you've made more mistakes they should still be working just fine. Please explain what you tried so far and what you mean with 'is not working'. What LED's are on? Did you connect a motor? Did the motor turn? Does it switch off when you stall or short circuit the motor? What setting of VR1 did you try? Is your power supply able to handle the high current draw from the motor when you stall it? What happens when you press SW1? \$\endgroup\$ – MartinF Jan 2 at 16:21
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    \$\begingroup\$ 500mA is really not a lot, possibly not enough to trigger the overcurrent protection of your circuit if your PSU cuts power before the circuit does... But that's not your main concern right now. Because when you power things up, LED3 is a power LED, so it should for sure light up! Did you swap + and - ? You've connected DC power right? \$\endgroup\$ – MartinF Jan 2 at 16:39
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    \$\begingroup\$ Please don't call yourself an idiot, it really doesn't help. I updated my answer. Your PCB was correct after all, I guess you've made a double mistake, making it right again...? Your IC and transistor should be fine. Did you swap INPUT with LOAD from the start? Maybe it just works now? \$\endgroup\$ – MartinF Jan 2 at 17:58
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    \$\begingroup\$ Correct. Unswap NC and NO, make it like it was before. Then tell me what it does, like what LEDs are on and so on. \$\endgroup\$ – MartinF Jan 2 at 18:50

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