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Design of an On/Off Circuit

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Member level 3
Jan 15, 2015
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I need a circuit which will give output continue sly if an input is given only once. The input will be given only once but the output should start continue sly when it receives the input. Please help me.

Thank you.


What voltage? Should it be logic level?

What you descirbe can be done wit a "two input OR gate". one OR input is the START signal. on the other input connect the OR output.
Then output is high when you give a hig to the input. Then it stays HIGH.

But how to reset it?

Thanks for the reply klausST, voltage should be 5V. I'm planning the reset by switching off whole circuit. Could you please suggest OR gate ICs compatible for this circuit.


I have another, similar idea.

Use a capacitor, a resistor and a noninverting logic AND, OR, buffer...
Connect the capacitor to gnd and the gate input. Connect the resistor from output to input of gate

At powerup the capacitor is discharged giving a valid low to the input. The output is also low and with the feedback resistor it keeps the input low. A pusbutton from vcc to gate input pulls the capacitor high. The output becomes high too and thus it keeps this state, even when the pushbutton is released.

I'd use a 74HC1Gxx device, HCT is also OK, even a comparator should work.

Hope this helps

Hi, another way is to make an S/R latch: 4 BJTs or FETS and a few resistors where they become necessary to mainly pull up or down bases/gates.
There is a simple one here in this link at the bottom of the page (all these NPN gates work and can be made with FETS, I made them to learn how and to practice), not CMOS but easier to implement.
Mosaic Industries has schematics for latching/toggle pushbutton switches/high side switches and so on, using standard logic ICS or discrete components.
There are many ways to do this. You should state the application and the required voltage and current it should switch. This will help to find the best solution.

but i want one only, from this many ways, and design circuit
and name of components
thank you
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Hi KlausST, I've uploaded a circuit with this reply. Did u meant in a way I've drawn? If it is, when the switch is open from where do the OR gate will get the continues Vcc? The capacitor discharges only for few moments. Also using 74LS32 would be ok?
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circuit is correct. You may use just one input and connect the other input to GND.

from where do the OR gate will get the continues Vcc
You have to supply VCC. Read datasheet on VCC supply range and supply current.

The capacitor discharges only for few moments.
It should be discharged at powerup. (to ensure this you may add a diode - anode to capacitor, cathode to VCC.)

Because of the feedback resistor it keeps it´s state.

On powerup it is low voltage, low input to gate, low output of gate, feedback keeps low.
On keypress it is high voltage, high input to gate, high output of gate, feedback keeps state.

Also using 74LS32 would be ok?
Yes. but "LS" has some drawbacks:
* needs more VCC current (An AHC1G32 needs only 1uA at 5.5V and 25°C, good for battery powered systems)
* needs more input current
* has a lot of pins. Mind to tie unused input pins to valid levels.
* needs 4.5 ... 5.0V (An AHC1G device may be supplied with 2.0V ... 5.5V)


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