SMD Circuit & part check before ordering pcb

This is my 1st time designing a circuit and pcb for surface mount parts. I have not been able to bread board this since obviously it’s be rather hard. I have spent a great deal of time going over data sheets looking for suitable parts and believe every thing I have should be fine but I’d like a second opinion from some more knowledgeable on this subject since I’m more of a tinker then an engineer. For the sake of simplicity we will pretend I’m only running a button sketch and rather then turning on a single LED I’m turning on a couple of small light bulbs. I’m aware that my output has 1 positive and one negative so just assume the lights are wired so that one bulb turns on with a path to Vcc and the second light bulb tuns on with a path to ground though my Mosfets. Will this work?

Data sheet on Mosfets
P-channel https://datasheet.lcsc.com/szlcsc/MOSFET_ON_NTR4501NT1G_NTR4501NT1G_C85416.pdf
N-Channel https://datasheet.lcsc.com/szlcsc/MOSFET_ON_NTR4501NT1G_NTR4501NT1G_C85416.pdf

Cap 1 https://datasheet.lcsc.com/szlcsc/Aluminum-Electrolytic-Capacitors-SMD_HONOR_RVT1J470M0810_47uF-63V_C40078.pdf

Cap 2 https://datasheet.lcsc.com/szlcsc/Aluminum-Electrolytic-Capacitors-SMD_HONOR_RVT1V220M0505_22uF-35V_C23289.pdf

V-Regulator https://datasheet.lcsc.com/szlcsc/Linear-Voltage-Regulators_ON_MC7805CTG_MC7805CTG_C22456.pdf

AtTiny https://datasheet.lcsc.com/szlcsc/FLASH_ATMEL_ATTINY85-20SU_ATTINY85-20SU_C89852.pdf

Resistors are package 1206 & 1/4 watt rated

Input fuse of 3 amps, P-channel fuse is 2 amps and N-Channel fuse is 1 amp.

Will this work, if so any thing I should improve on. If it doesn’t work what part is incorrect or what would need to be added/changed to make it work.

Thanks ! ! ! ! !

I should add that based on my assumption of this chip that Pin 1(PB5) does not need a pull down resistor. Is that a correct assumption?

HI PeteS160,

Couple of comments:

  • I don't believe the PFet will work. To turn it off you need to get the gate within a volt or 2 to the 14 V source connection, the ATiny will only reach 5V, you have PB0 PB1 Reversed and PB0 will have to go high to turn the PFet off
  • A matter of schematic style. You should never make cross connections as you did near pin 1 of U2. Always offset one wire making 2 "T" connectsions
  • You should consider breadboarding your circuit using similar non SMD parts. For the ATTiny you can find a number of "breakout" boards on Ebay

I'm curious about the goal behind R2 and R3? In the current configuration they would tend to turn Q2 of and Q1 off if the ATTiny were not booting or during booting.

Good luck

John

PeteS160:
Will this work, if so any thing I should improve on. If it doesn't work what part is incorrect or what would need to be added/changed to make it work.

Asking in here as to whether the forum watchers think your PCB\Circuit will work is no substiute for breadboarding it and checking for yourself .............

JohnRob:
HI PeteS160,

Couple of comments:

  • I don’t believe the PFet will work. To turn it off you need to get the gate within a volt or 2 to the 14 V source connection, the ATiny will only reach 5V, you have PB0 PB1 Reversed and PB0 will have to go high to turn the PFet off
  • A matter of schematic style. You should never make cross connections as you did near pin 1 of U2. Always offset one wire making 2 “T” connectsions
  • You should consider breadboarding your circuit using similar non SMD parts. For the ATTiny you can find a number of “breakout” boards on Ebay

I’m curious about the goal behind R2 and R3? In the current configuration they would tend to turn Q2 of and Q1 off if the ATTiny were not booting or during booting.

Good luck

John

I see where I made the mistake. I have added a 2nd N-channel in this version(see attacgment or link) that is used to control the P-channel gate. I used a pull up resistor on the drain of added N-channel to keep this high so the P-channel will turn off.

If I could find a smc relay that was nearly as cheap as these mosfets it would make make this so much easier for me.

As for the resistors you asked about, they are there make sure the mosfets stay off when the AtTiny is booting and so the lines didn’t float. The intention was to keep both mosfets in the off state but i made an error with the P-channel.

Connect R4 between gate and source if you want the p-channel to turn off.

Wrong p-fet though. Absolute max gate/source voltage is 8volt.
Will end up in silicon-heaven on a 12volt supply.

This page might help you understand why.
Leo..

No, you do not want an SMD relay, mosfets are what you want, just use appropriate mosfets and they're just what you need. Relays are expensive, bulky, power hungry and have a limited lifetime (in terms of switch closures); MOSFETs are where it's at nowadays.

You don't need a breakout board to breadboard an ATTiny85 (though I sell some nice ones) - they come in a DIP version, so you can just use scraps of prototyping board (hey, I sell those too!).

For a circuit this simple, I would definitely breadboard it first using generic breakout boards - ideally with the same parts that you're planning to use.

Where's your layout?

How are you planning to program the parts? Most people include an ICSP connector so they can program the chip in place; this is a practice I strongly recommend.