Flip flop based touch on touch off switch

• Apr 20, 2018

• #1

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boylesg

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Flip fllop based touch on touch off switch

I tried the right most circuit in Multisim and it worked.

In place of the relay and diode I had a 120R resistor with the gate pf logic FET connected to the collector of the transistor.
Then I had an LED setup in the drain/source circuit of the FET.

But when I tried it on my bread board for real I could not get it to work.

Have I perhaps I wired in the FET incorrectly?

 

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• Apr 20, 2018

• #2

KlausST

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Re: Flip fllop based touch on touch off switch

Hi,

I could not get it to work.

Click to expand...

What does it mean? Did it explode?

--> Give useful information: What did you expect and how did it behave?

Klaus

 

• Apr 20, 2018

• #3

betwixt

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Re: Flip fllop based touch on touch off switch

Certainly an unconventional circuit with some odd values in it!

Brian.

 

• Apr 20, 2018

• #4

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boylesg

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Re: Flip fllop based touch on touch off switch

betwixt said:

Certainly an unconventional circuit with some odd values in it!

Brian.

Click to expand...

Brian can you tell me what values you would use?

Or a similar circuit that you would use to implement a touch on/touch off switch?

I am afraid I am just not knowledgeable enough with transistor circuits to tell if the circuit is unusual or not.

The LED does not come on at all.

 

• Apr 20, 2018

• #5

KlausST

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Re: Flip fllop based touch on touch off switch

Hi,

there are ready to buy solutions like MAX16054.
A lot of IC manufacturers produce similar devices.

A toggle-FF like T-FF or JK-FF will work also.

Klaus

Added:

The LED does not come on at all.

Click to expand...

Did you check gate voltage of the MOSFET?

Klaus

 

• Apr 20, 2018

• #6

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boylesg

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Re: Flip fllop based touch on touch off switch

KlausST said:

Hi,

there are ready to buy solutions like MAX16054.
A lot of IC manufacturers produce similar devices.

A toggle-FF like T-FF or JK-FF will work also.

Klaus

Added:

Did you check gate voltage of the MOSFET?

Klaus

Click to expand...

OK. but as a learning exercise I would like to master the discrete transistor version first.

 

• Apr 20, 2018

• #7

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dick_freebird

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Re: Flip fllop based touch on touch off switch

I see no reason offhand why the cross-coupled pair
should toggle; more like a coin-toss every time. And
switch bounce might make one press, numerous
"events".

 

• Apr 20, 2018

• #8

betwixt

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Re: Flip fllop based touch on touch off switch

There is a better design and a good explanation on this web page:
https://electronics.stackexchange.c...able-multivibrator-using-discrete-transistors

As dick-freebird points out, it might toggle state but there is only a change in current on both sides of the circuit to make it do so and that change is very small anyway. The resistor values are so low that it would draw a fairly steady 120mA while only needing to supply a few nA to the gate of the MOSFET which means it would be incredibly inefficient and probably run quite hot. You could easily scale up the resistor values by 10 or even 100 times or more to reduce the current and it would still work exactly the same.

Brian.

 

• Apr 20, 2018

• #9

d123

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Re: Flip fllop based touch on touch off switch

Hi,

If you have an oscilloscope, enjoy what the output signal "squarewave" really looks like - it's not a nice thing. I was wondering, is the lowest voltage the NMOS sees 2 or 3 volts? If so, it'll never turn off surely?

Maybe a buffer stage between the presumably feeble oscillator output and the presumably more demanding gate charge MOSFET gate, like a push-pull with BJTs or with a comparator with (a lot of) hysteresis or even an NPN configured as an inverter, maybe two stages to not invert the original signal? Don't know if that would help. Mayeb an op amp follower and 20mA would be enough, the MOSFET datasheet will provide the puzzle pieces to figure out actual required VGS and nC gate charge graph for gate current and expected current out (/in)... Good luck with that part of the task.

 

• Apr 21, 2018

• #10

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BradtheRad

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Re: Flip fllop based touch on touch off switch

I got this toggle (bistable) flip-flop to work in simulation. The transistors are cross-coupled which is typical in a basic memory cell.

The transistors are also a long-tail pair. Close the momentary switch to send a pulse of current to the bias of both transistors. Whichever transistor is off, it turns on. The result is to turn off the other transistor.

 

• Apr 21, 2018

• #11

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boylesg

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Re: Flip fllop based touch on touch off switch

Seems like a touch on/touch off switch (with a single switch) is difficult without a MCU.

Looks as though it is only reliable with separate on and off switches.

 

• Apr 21, 2018

• #12

KlausST

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Re: Flip fllop based touch on touch off switch

Hi,

Seems like a touch on/touch off switch (with a single switch) is difficult without a MCU.

A toggle-FF or JK-FF is no MCU and it is not difficult, and it runs safely, while consuming only a microampere ...that's why I mentioned it.

Klaus

 

• Apr 21, 2018

• #13

d123

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Re: Flip fllop based touch on touch off switch

Hi,

See if there isn't something more suitable here:

Latch and Toggle Power Circuits (MOSFET transistor circuits power an embedded system ON and OFF with the push of a button.)

 

• Apr 21, 2018

• #14

betwixt

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Re: Flip fllop based touch on touch off switch

I think the most serious problem with most reliable designs is that the switch isn't grounded. A circuit with a floating switch, especially if it is remote from the rest of the circuit is noise pick up and risk of damage if a voltage is applied to it (ESD for example). There is also an need to 'debounce' the switch or at least create a dead time before it can be operated again.

The simplest solution I can think of is a simple 'D' flip-flop IC with feedback and an LPF at it's clock pin. I'm imagining three or four components and it being able to drive a small LED directly or having enough output voltage to cleanly switch a MOSFET on and off.

Brian.

 

• Apr 21, 2018

• #15

FvM

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Re: Flip fllop based touch on touch off switch

Yes, DFF with debounce definitely works. Here with an additional delayed "forced-off" function.

 

• Apr 21, 2018

• #16

betwixt

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Re: Flip fllop based touch on touch off switch

Boylesg, note that the schematic FvM posted draws about 1uA of current (excluding the load) while in steady state and about 50uA when the switch is closed. This is about 0.000002 times the current your original circuit uses! You get the added benefit of an inverted output if you use pin 5 instead of/as well as pin 6 from the 74HC74. CD series equivalents are available if you need to work from higher supply voltages.

Brian.