I have this 5V voltage regulator circuit that is supposed to power these 2 circuits:

  1. NodeMCU (will automatically turn on if plugged in)
  2. Turn on/turn off the LCD and sensor when the pushbutton (BTN1) is pressed. (The circuit on the right blue box.)

The problem is, when I read Q3's collector using a multimeter, it shows 5V, but when the LCD and sensors are plugged in, the voltage drops. Am I missing something?

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  • \$\begingroup\$ A power transistor like TIP32 typically hasn't got much gain. With 100K in the base there is too little base current to make it conduct significantly. \$\endgroup\$ Dec 27, 2020 at 13:30
  • \$\begingroup\$ Wow thanks! Didn't notice that. \$\endgroup\$
    – steven
    Dec 27, 2020 at 15:48

1 Answer 1


TIP32 has very low hFe, 10-50 according to datasheet, so it needs high base current.

You need to lower the value of R10 accordingly, but honestly the best solution would be to replace TIP32 with a P channel MOSFET which will have a lower voltage drop when turned on and will not require drive current. Make sure to pick a MOSFET that has a RdsON value suitable for your application according to desired maximum voltage drop at the current you'll use, and the datasheet should specify this RdsON at a Vgs of 4.5V. If it is only specified for a Vgs of 10V then it will probably not turn on with your 5V gate drive.

Note you could alos remove the 555 and do the switching with the micro.


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