I needed to build a delay circuit so I'm following this guide: http://www.learningaboutelectronics.com/Articles/555-timer-delay-before-turn-on-circuit.php


Using this circuit, I get a delay of about 5 seconds before the LED turns on. The only big difference is the output of the 555 is going into the gate of a IRFZ44N MOSFET as I needed more power.

The MOSFET source is hooked to ground, and the drain is hooked to a 330 Ohm resistor -> LED -> 5V.

However, with 5V input voltage, I can only get 3V across the drain and source. How could I improve this circuit to give me more efficiency, closer to my 5V?

EDIT: I'm following the schematic from the guide. It's exhibiting the same behavior as the guide, so the timing circuit should be correct. The only difference is the MOSFET. I also added a 330 Ohm resistor between the 555 pin 3 output and MOSFET gate and now I'm getting full voltage from drain. Looks like the gate was leeching some voltage.

  • \$\begingroup\$ Here is a comment, replace the word "here" with "you", replace "is" with "should make" and replace "a comment" with "a proper schematic". \$\endgroup\$ – Harry Svensson Apr 12 '18 at 3:33
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    \$\begingroup\$ Well, this is a pretty simple circuit. Most of the schematic is there. In the meanwhile, I think I solved the issue by moving the resistor between the 555 and gate. I'm getting the full 5V. Closing this. \$\endgroup\$ – DTOAL Apr 12 '18 at 4:04
  • \$\begingroup\$ It would be nice if you add a schematic instead of a snap of the breadboard connection! \$\endgroup\$ – Dhans Apr 12 '18 at 4:05
  • \$\begingroup\$ Yes, the snap was a bad idea. \$\endgroup\$ – DTOAL Apr 12 '18 at 4:14
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    \$\begingroup\$ I don't see any MOSFET in that schematic. \$\endgroup\$ – duskwuff -inactive- Apr 12 '18 at 4:24

Vgs(th) is when it barely conducts at Vgs=2~4V so you need 2.5x to 4x this value minimum RdsOn for low Vds at high current.

Since Vout is Bipolar it is not rail to rail. So either use a sub-threshold FET or V=+12


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