I have some questions and wanted to get feedback on this circuit I have put together. Basically to see if there is something I missed or obvious that I should change or should have designed differently.

I have the valve running at around 20Hz and with a 100k POT I can adjust the duty cycle from about 6-88% which is outside of the effective range that the valve actual functions (~14-78%). I have soldered this together on a breadboard and have it running in my S10 and have not ran into any issues yet. So although they say "if its not broke dont fix it" I would like to learn from my design mistakes and try to build a better device. Down the road I would like to change this from an open loop to a closed loop setup and swap out the 555 for an Arduino and add a MAP sensor to read a value in and use a PID loop to actually target a boost pressure and not just a duty cycle.

So these are the questions/concerns I have.

Voltage Regulator: I used a 7805 for the 5v regulator due to its popularity. Is this an acceptable choice in an automotive environment?

MAC Valve Control: The MOSFET I chose is the FQP30N60L and chose that just because I have a lot of them from some of my Arduino projects and the fact that it works at logic level. I know it is much larger than I need since the MAC valve I am controlling only pulls 5.4 watts and the MOSFET is rated for 30 amps. Is there an issue with running an over sized MOSFET in my application?

Power Filtering: The C1 and C2 values on each side of the voltage regulator I got from the datasheet for the 7805. I am not sure if there is a reason for me to change these values or not.

The last question 2 questions I have are about the resistors from the 555 output (pin3) to the gate of the MOSFET. Do I need a pull down resistor in this scenario? I have only used these in the past on Arduino projects where I wanted to control a floating input pin. I had a coworker who deals with electronics suggest it to me. Is it necessary because the original circuit I tested in my S10 did not have this and it seemed to work fine.

Also is the 1K resistor that is in series needed to just limit the current from the 555 timers output to the gate? Is it necessary because the original circuit I tested in my S10 did not have this and it seemed to work fine.

I have attached a picture of the schematic.

Thank you everyone for your time and input!

enter image description here

  • 1
    \$\begingroup\$ What kind of automotive environment? Depending on that, the 7805 can be a bad choise, it quite old device so it uses a lot of current (5-8mA) so it will drain battery faster than regulators designed for automotive. Also if you have nothing else than a capacitor on 7805 supply input, it has no protection for positive or negative voltage spikes. No fuse either. The solenoid terminals +/- are marked wrong and driving a FET gate via 1k resistance makes it switch slowly, if too slowly it heats up and breaks. Make it between 1 and 10 ohms, or up to 100 ohms, but do measure how fast switching is. \$\endgroup\$
    – Justme
    Mar 2, 2020 at 6:40
  • \$\begingroup\$ Thank you for the reply! I have fixed the description on the solenoid terminals. It is installed in the cabin of my S10 which I drive on an almost daily basis during the summer months and also race it at the local drag strip on the weekends. I have an inline 3A fuse powering if off of a hob switch set at 3psi so that it is not running all the time. \$\endgroup\$ Mar 2, 2020 at 12:31
  • \$\begingroup\$ What would you recommend as a replacement for the 7805 and what other components should I be looking at for protecting against positive or negative voltage spikes? \$\endgroup\$ Mar 2, 2020 at 12:38
  • \$\begingroup\$ We discourage recommendations for specific products on this site. Such recommendations become out-of-date very quickly and don't have lasting value. \$\endgroup\$ Mar 2, 2020 at 22:46

1 Answer 1


It looks pretty good to me.

At 4.7 ohms R4 is too low resistance, use at-least 33 ohms here something like 1K would be a good size

R3 basically has the same effect on circuit operation that R4 does so it could be combined, however a resistor in series with D3 would create an upper limit on duty cycle so maybe you should put a resistor there instead.

To get close to a 14-78% duty cycle with a 100K pot use R4=18K,R3=0, and 27K in series with D3

The over-capable MOSFET will not be a problem, the larger they are the less they heat up.

R5 just wastes electricity, the 555 already has a strong internal pull down when the out pin is low.

  • \$\begingroup\$ 4.7 should have been 4.7k. I have updated the picture so that it shows the correct values. I am okay with the current duty cycle I can product since it lets me effectively shut the valve off if I go low enough with the POT. I can get rid of R5 if its not needed. Is the internal pull down R9 on this datasheet? ti.com/lit/ds/symlink/lm555.pdf \$\endgroup\$ Mar 2, 2020 at 12:35
  • \$\begingroup\$ the output is pulled down by Q26. R5 doesn't waste much electricity. \$\endgroup\$ Mar 2, 2020 at 20:07

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