I am a newbie of Electronics and learning it as a hobby. Over a course of time I realized that Electronic Blogs are not the best place to learn Electronics, unless you have a strong basics and can correct silly mistakes in circuit diagrams posted online. Often I find it difficult to get a circuit working because they have minor typos or errors.

Now, I am stuck with such a situation. I am referring two separate Electronics Blogs which are appearing in Google Search who have posted couple of contradicting circuit diagrams of a NE555 based DC Motor Speed controller. I don't know if either or both of them are correct. The circuits use pin 3 and pin 7 of the IC respectively to drive the MOSFET/Transistor.

Circuit Diagram 1:


enter image description here


Circuit Diagram 2:


enter image description here

My questions are:

  1. Are both of them correct? If yes, it's really couple of exciting circuits to understand how both of them work when the pin 3 and pin 7 are used just the opposite way in these two circuit diagrams. Does the use of MOSFET vs Transistor makes the difference?
  2. If one of them is correct - which one is that?

This question might not be new to Stackexchange because these contradicting diagrams are equally published all over the internet. Unfortunately I couldn't find it on SE. Please link the question, if you already have it.

According to my understanding circuit diagram 2 should be working. Because it is similar to astable-multivibrator config of NE555 and pin 3 seems to be generating square wave (i.e. PWM) signals to drive the MOSFET/Transistor. Please let me know if I am wrong and why.

Many many thanks in Advance!!

  • \$\begingroup\$ This is a very interesting question. In the 555 diagram we can see that the 7 (discharge) pin is connected to the collector of one transistor which base is connected to the same rail that feeds the inverter (buffer) of the regular output pin 3. This way maybe someone could use output from pin 7 (but why?). I would place my bet in a mistaken changing of pins 3 and 7 in the first schematic. \$\endgroup\$
    – mguima
    Oct 28, 2017 at 14:31
  • \$\begingroup\$ Yes, my understanding is same too! But I am surprised to see that both these diagrams are almost equally posted across the blogs, none of them are really a cornerstone - so I really doubt if any one of them is incorrect! As a beginner I would not expect the bloggers to make such a big mistake repeatedly! :) \$\endgroup\$
    – sribasu
    Oct 28, 2017 at 14:37

2 Answers 2


They are both acceptable solutions.

Notice in a 555 timer, the discharge pin is simply an open collector version of the output signal.

enter image description here

In circuit diagram 1 the 555 timer is configured as a square wave generator. The charge/discharge path for the timing capacitor is from the 555s output which will be near top rail or bottom rail. Since, with the pot at 50%, the resistance to the cap is the same in both states, it charges and discharges at the same rate. Hence the "square wave" configuration designation.

enter image description here

The discharge pin is therefore surplus in this configuration, though it still switches as normal. Instead, it is used to pull down the MOSFET gate during the discharge cycle.

In circuit 2, the more traditional wiring, the charge rate is defined by R2 + whatever the pot setting is, while the discharge is purely through the pot. With this design, mid-range on the pot is not 50% mark to space. Further, during the discharge cycle, this circuit wastes considerable current ( 12mA ) through R2 for no purpose.

enter image description here

As such, circuit 1 is arguably the better one.

Notice though in these two particular examples, the design of the MOSFET gate driver gobbles up 12mA instead, so these two circuits are pretty close efficiency wise. A better gate driver circuit would fix that.

Personally, I would have done it this way. I might even add a nice LED in parallel with the motor so I could tell how hard the controller is driving.

enter image description here I'd need to check if both end points of the pot worked as expected.

  • \$\begingroup\$ Great! Easily understood. But still one question regarding calling the 1st one "less efficient". While pin 7 is pulling down (discharge cycle) doesn't it waste energy through 1K resistor connected to it? Or am I getting it wrong? \$\endgroup\$
    – sribasu
    Oct 28, 2017 at 18:14
  • \$\begingroup\$ @sribasu I did not say it was more efficient, though it could have been. I said the second one wastes 12mA it does not need to. In these two particular examples, at 50% they are about the same efficiency since the mosfet one gobbles 12mA on the gate drive side instead. \$\endgroup\$
    – Trevor_G
    Oct 28, 2017 at 18:18
  • 1
    \$\begingroup\$ @sribasu but that is a fault of the gate driver circuit, not really the 555 circuit, if you know what I mean. \$\endgroup\$
    – Trevor_G
    Oct 28, 2017 at 18:20
  • \$\begingroup\$ Agreed. This is fault of the Gate driver in general, nothing to do with the PWM generation. \$\endgroup\$
    – sribasu
    Oct 28, 2017 at 18:46

On the 555, pins 3 and 7 are very nearly equivalent to each other, with the only difference being that pin 7 is open-collector, while pin 3 has a totem-pole output. In both of your circuits, a 1K resistor is connected to pin 7 to provide the pullup function, effectively eliminating that difference.

  • \$\begingroup\$ I learned this - pins 3 and 7 are very nearly equivalent! Thanks! \$\endgroup\$
    – sribasu
    Oct 28, 2017 at 18:10

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