PWM Works Great now for some Mod help

Thread Starter


Joined Aug 6, 2006
I want to thank Harry for helping me with the "PWM/Jet Engine project", now on to some other issues that may be taken care of in the learning process of this project. Included with this thread is a copy of the circuit that I am using but, I have changed some of the values for the components because of my design needs.

Here we go!
In this circuit the author used a 250k pot for the output voltage control along with 1k resistors on each side of the pot for a min resistance to this part of the circuit. What I have is a 50K slider that I am using for the throttle control and I need the full range of this slider so that there is a more positive control of fuel to the engine. It seems as though I do not get any response until the slide is about half way through and then it will go to full at about 3/4. So there is a small area that I have to work with. I did check the slider values and it does register an increase of resistance the way it was designed. I have also change the electrolytic cap to 10mf that is shown in the schematic, which allowed me to run the lager DC motor driving the fuel pump. The final change was adding a lm7812 regulator so I can drive the Mosfets at 24 volts and not burning out the 555 timer.

What I am looking for is formula or circuit designs that will accommodate the slider variable resistor so that I can have a full range of output voltage to what ever I am driving at input voltages of 12 - 24 volts.




Joined Oct 11, 2004
Hi Steve.
There are a couple of things I don't understand.
1. You have a 7812 regulator fed from a 12v rail, but there is an inherent minimum voltage drop in the device from input to output so its output will be less than 12v and it won't regulate very well.
2. As the 555 is a PWM generator then it must run as an astable. Initially, pin 6 will rise because the .01uF will charge through the 50K pot and the diode. But when it gets to about 8v the discharge pin 7 will go low and cut off the diode. I cannot see any way that the .01uF cap can discharge to take the trigger pin 2 down to 4v to make it trigger. Is there a typo in the drawing perhaps?

BTW, it would help when describing the circuit if you marked up the components R1, C1 etc.


Joined Aug 8, 2005

1) As mentioned, due to the voltage drop needed for the regulator, your PS input voltage range is more like +15V to +24V (not +12V).

2) Add a 10uF, minimum, cap to regulator output for stability (keep the 0.1uf).

3) Likewise, add a 1K to 1.2K, 1/4W, resistor between regulator output and ground so that regulator is minimally loaded to ~10mA.

4) Check the symbol drawing in the FET datasheet -- the external drain/source diodes are most likely redundant, they are inherent in MosFET's and the external ones can be removed (this is why I always advocate including the diode when creating/drawing the symbol, it's easy to forget that it's there).

5) You may want to use Schottky diodes at the 555; the BAT54S will give you both diodes in one package, and in the configuration you require.


Joined Oct 11, 2004
Steve, I missed your original "PWM/Jet Engine project".
Could you indicate: 1. What was the frequency of the pulses? 2. What was the pulse on/off ratio? Perhaps a member could then suggest a suitable circuit.