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Single Pushbutton Run-Stop Circuit Schematic

Single Pushbutton Run-Stop Circuit

There are solutions to this problem—mechanical (push On/push Off switch), electromagnetic (latching relay) and electronic (CMOS logic), but few (if any) good discrete electronic solutions. I have scoured the web, looking for such and have not found any decent circuits. One recent job required such and I had to resort to CMOS logic — I will be posting that one in the future.

As a result, I have been racking my brain for the last few months and have finally come up with a really neat circuit. It is a little busy, having 19 components, but they are small, inexpensive and commonly available.

Related Products: Electromechanical Switches | Switch Indicators

What are the benefits of such a circuit?

Good question — one may wonder what use this could be. Besides being compatible with any normally open pushbutton, it is a great way to add multiple pushbuttons to a system—all normally open pushbuttons are simply wired in parallel — any can start or stop the device. Also note that push-on/push-off switches are quite special and have a limited offering in regard to size, mechanics and aesthetics—they also have an unpleasant feel, in my estimation.

Single Pushbutton Run-Stop Schematic

Single Pushbutton Run-Stop Circuit Schematic

How it works

When the pushbutton is initially closed, it directly turns on the gate of Q1 via D2. Q1 turns on after a brief delay determined by the charge time of C1. Q1 then biases Q2 on, and Q2 seals in the pushbutton signal and C2 charges up to 12V via R8 and D2.

When the pushbutton is closed again, the top side of C2 is grounded via the pushbutton action through D1. The lower side of C2 goes negative and dumps half of its charge into C3. The negative voltage on C3 turns on Q3 that is connected in the common collector configuration (emitter follower). The emitter of Q3 shorts the bias voltage of Q2 to common thus turning off Q1 (as soon as the pushbutton is released).

Related Products: Switch Push Button Switch Tactile 

Oscillographs

Minimum cycle time

Minimum time between pushbutton cycles is approx. 1.1sec. If this timing is violated, the circuit will not re-latch on subsequent pulses. In the event it does not turn off the first time because it occurred within this 1.1sec window, all that is necessary is to wait for an additional 1.1sec and push it again.

FYI, Typical pushbutton closure time varies from about 0.1 to 0.2sec.

Noise immunity

This type of latch circuit when comprised of bipolar transistors is very noise susceptible due to the high gain of common emitter transistors. However, Q1 is in this case a P-Channel MOSFET that has about a 3V gate threshold. In addition, the gate is swamped via a 1000pf capacitor to enhance noise immunity. Good noise immunity is essential to prevent the circuit from turning on by itself—I had no issues with noise.

Why a 27A MOSFET?

MOSFET power transistors are characterized by Rdson that in this case is 0.07Ω. If operated at 27A, the voltage drop will equal 1.9V—way high for 12V applications. If the transistor is operated at 3A instead, the voltage drop is 0.21V and that is acceptable. So to assure low voltage drop, large, low resistance power devices are applied at low current. Since both the voltage drop and current is low, the power loss is very low, so only a small heat sink is required.

Photos

For the future

Single pushbutton CMOS logic Run-Stop circuit

Preferred components for the serious experimenter

FQP27P06
FQP27P06 P-Channel MOSFET power transistor, 27A, 60V, Rdson = 0.07Ω, TO-220
DigiKey FQP27P06-ND, $1.21 each
2N3906
2N3906, PNP, 200mA, 40V, bipolar transistor, min hFE = 100 @ 10mA, TO-92
DigiKey 2N3906FS-ND, $0.20 each
Complement to the popular 2N3904 NPN transistor

13 Comments

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  • asep

    Hi Jim,
    What is the D1…D5 part type?
    thank you..

    • Jim Keith

      D1 – D5 may be virtually any silicon diode.
      Suggested parts: 1N4004 or 1N4148

  • Chris

    Ah, very cool, would you possibly be willing to help me figure out what I need and the schematic? please 🙂 if email is easier that works too.

  • Chris

    Hi Jim,

    I’ve been looking for a schematic to incorporate into my 2-cycle engine. It has electric start button and a separate shutdown button. My issue is that I want to combine the two via a switching logic like this. However, my concern is the amperage during the “startup”, in the components in the design and then, using the same shutdown portion, to “ground out” the ignition once the engine has started. And on that end, not sure if the ignition source will cause any damage to the circuitry in either action, starting or stopping. Kind of like the same feature as Seadoo’s have, one button start/stop the engine. Can you help? Oh, I should mention, it’s a positive ground setup (no idea why).

    Thank you.

    • Jim Keith

      I recommend a single coil latching relay –such is very easy to connect and can be operated via a single momentary contact or either polarity –also the contacts can easily handle the some 300V magneto primary voltage.

  • casini

    !C2 charges up to 12V via R8 and D2! It’s meant to said… C2 charges up to 12V via R8 and D3…

    Excellent approach!

  • KROKKENOSTER

    This is something that I get now GRRRRR! 25 years too late as I needed it to control a high rising forklift truck that when it emerges out of the racking a small toggle switch that did not last long as it get hammered all day Thanks for this I like it to control now my REVOX tape recorders

  • Jim Keith

    Single NO Normally Open Two Wire Pushbutton Push Button Contact Closure On /Off Start /Stop Switch Power Control Circuit Function

    • Chinmoy Mitra

      Jim, can you be more specific please, I cant make head or tail about your comment!

    • Jim Keith

      To effectively use search engines, the proper search terms must be entered. Anyone searching for this obscure circuit function may not use the best search terms, so I added some keywords in the comment. I will attempt to include them in the articles in the future.

      Somehow, the first line “Keyword Search Terms:” got edited out.

      At least I am glad to hear that someone reads the comments.

    • Chinmoy Mitra

      😉
      Thanks for the clarification. I always make it a point to read the comments. The article by itself is just a project…my objective here is to be of help to others (if I can). I have been in the field of electronics for the past 35 years, and at the ripe age of 57 years, I feel that I should pass on whatever little knowledge is stored in my brain cells to others, before the neurons lose all their retentivity and the data gets lost in oblivion! lol

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