I have this delay circuit. I am trying to delay the power to R1(which will be an android phone) by 1 second. SW10 is my SPST relay. Can i replace the relay with something cheaper and physically smaller without losing significant voltage across it as the phone requires 5V and 500mA. Delay circuit

I tried using npn transistor and does not work due to low HFE ratio and high voltage drop, like in the picture below: circuit with NPN transistor

Darlington pair but could not make it work due to their high voltage drop as well.

Enhancement N-fet might be a solution? but I am just not able to figure it out. If someone can draw me a circuit, I would be high grateful.

  • \$\begingroup\$ google arduino motor driver \$\endgroup\$
    – jsotola
    Mar 15, 2018 at 6:05
  • \$\begingroup\$ reduce R2 to 100 ohms, replace T1 with 2N2222 or some other tansistor that's good for more than 500mA Ic \$\endgroup\$ Mar 15, 2018 at 8:09
  • \$\begingroup\$ How much smaller do you need? There are fairly small relays available. pic-control.com/product/mini-relay/5v-mini-relay/… \$\endgroup\$
    – Passerby
    Mar 15, 2018 at 9:49
  • \$\begingroup\$ @Passerby - that relay looks promising size wise, any idea of the Part number or manufacturer? Can it handle 500mA? Also cost is a factor as i assume this would cost at least a $3-$4, I am trying to bring down Cost Of Goods. \$\endgroup\$ Mar 15, 2018 at 18:58
  • \$\begingroup\$ @Jasen - I cant reduce R2 to 100ohms as it controls the timing for my capacitor getting charged. If i make it 100ohms, it goes to 5v almost instantaneously. Also npn transistors have high voltage drop across them. I need some other magical component that will not cause significant voltage drop. That is why switch/relay works well but its big and expensive \$\endgroup\$ Mar 15, 2018 at 19:07

2 Answers 2


Use a USB load switch in place of the relay.

eg: https://www.diodes.com/assets/Datasheets/AP22802.pdf

the enable threshold voltage isn't very well defined though so you may need an adjustable resistor or to use a circuit with a better defined speed


Using mosfet

Do you think this will work? Please comment on if I am missing something. Phone need 5V and 500mA to charge.


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