one of my lecturers sent me this. no idea what he is saying. so what do you think?
"the 555's output is normally low, and goes high during the pulse. So you need to remove the PNP transistor, and feed the 555's output to the MOSFET's gate. Usually a small series resistor e.g. 22 ohms is used, to reduce the peak current (the MOSFET's gate has significant capacitance). You can keep the 10k resistor from the gate to the 0V rail.
Some designs do have a low value resistor between the 555 pin3 and the FET Gate, for basic designs I have never found it necessary for low current, low speed circuits.
You can also delete R3 and C1, and connect the pushbutton from pin 2 to 0V. This means that you have to release the pushbutton before the pulse time expires, but I expect you would be doing that anyway.
If you are prepared to 'time' the action of the switch yourself , you can do it that way, I prefer the resistor and cap coupling so that a press of any duration will not mis-operate the 555.
Also you need to use a MOSFET that will saturate (turn fully ON) with only 6V of gate-to-source voltage. In fact, it will probably only see 4.5~5.0V because of losses in the 555. Not all MOSFETs will do this. Ones that do are called "logic level gate" MOSFETs, although this term is not exactly defined and even those MOSFETs will "saturate more" if given more gate voltage.
The IRF540 is not a logic level operated MOSFET [ ie: hard ON with a 5V or 6V Gate voltage] it requires a higher Gate voltage than 6V to switch hard on.
Here are a few suitable MOSFETs that are available from Digikey:
NTD4906N:
https://www.digikey.com/product-detai...GOS-ND/2194521 USD 0.57; ON-resistance 0.008 ohms with 4.5V gate voltage; through-hole "IPAK" package.
PSMN022-30PL:
**broken link removed** USD 0.73; ON-resistance 0.034 ohms with 4.5V gate voltage; standard through-hole TO-220 package.
FDP8880:
https://www.digikey.com/product-detai...80FS-ND/976840 USD 0.92; ON-resistance 0.0145 ohms with 4.5V gate voltage; standard through-hole TO-220 package.
Lower ON-resistance values mean less voltage dropped in the device. For a small motor like this one, the difference is not important. But the TO-220 devices have the standard 0.1-inch pin spacing and are easier to use with breadboards and stripboard."
The choice of a logic level gated MOSFET with a low RDS On would be the best way to design this circuit.